1. © 2003 WebMD Inc. All rights reserved. ACS Surgery: Principles and Practice
8 CRITICAL CARE 24 DISORDERS OF WATER AND SODIUM BALANCE — 1
24 DISORDERS OF WATER AND SODIUM
BALANCE
Richard H. Sterns, M.D.
Overview of Body Fluid Homeostasis
mated with reasonable accuracy from the concentrations of the ma-
Life takes place in an aqueous solution. Cells, the blood bringing jor extracellular solutes, as follows:
nutrients and oxygen to them, and the interstitial fluid bathing them
are all mostly water. Each day, water and salt are lost and replaced. [Glucose] BUN
Posm ≅ 2 × plasma [Na+ ] + 18 + 2.8
To maintain stability of the internal milieu, body fluids are processed
by the kidney, guided by intricate physiologic control systems that The multiple of 2 reflects the anions accompanying sodium ions,
regulate fluid volume and composition. and 18 and 2.8 are the corrections required to convert glucose and
urea nitrogen concentration from mg/dl (the units used by most lab-
DISTRIBUTION AND COMPOSITION OF BODY WATER
oratories in the United States) to mmol/L. Exogenous solutes (e.g.,
Water accounts for approximately half of an adult human’s body ethanol, methanol, ethylene glycol, glycine, mannitol) are measured
weight. Because fat contains little water, individuals with more body by osmometers but are not included in the formula shown above. A
fat have less body water. On average, total body water constitutes discrepancy between the measured and the calculated plasma osmo-
60% of lean body weight in young men, 50% in young women and lality values (an osmolar gap) is useful clinically as a way to recognize
older men, and 45% in older women. Two thirds of body water is the presence of an exogenous solute.2
intracellular and the remainder is contained in the extracellular flu-
id compartment, which includes intravascular (plasma) and intersti- Fluid Movement between Body Fluid Compartments
tial fluid. Small amounts of water are also contained in bone, dense The intravascular and interstitial subdivisions of the extracellu-
connective tissue, digestive secretions, and cerebrospinal fluid.1 lar fluid compartment are separated by capillary walls that are
Extracellular solutes are predominantly sodium salts (primarily a freely permeable to small extracellular solutes but relatively imper-
mixture of NaCl and NaHCO3). Thus, extracellular fluid can be meable to plasma proteins. Protein-free saltwater continuously
thought of as saltwater. Except for protein (present at a higher con- moves across the capillary endothelial barrier by filtration, driven
centration in plasma [approximately 1 mmol/L] than in interstitial by a hydrostatic pressure gradient (generated by contractions of
fluid), the compositions of the intravascular and interstitial subdivi- the heart), which forces fluid from the capillary into the intersti-
sions of the extracellular fluid compartment are similar. tium, and an oncotic pressure gradient (the consequence of the os-
The sodium-potassium adenosine triphosphatase (Na+,K+- motic force created by intravascular protein), which draws intersti-
ATPase) pump on cell membranes keeps intracellular sodium at tial fluid into capillaries [see Figure 1]. These so-called Starling
low levels. Potassium, the dominant intracellular cation, is electri- forces, which regulate the disposition of fluid within the extracellu-
cally balanced, in large part, by anionic charges on impermeant lar compartment, determine how much extracellular saltwater is
macromolecules. Stability in the number of intracellular anionic contained in intravascular plasma and how much is in interstitial
charges makes the total solute content of cells much less variable fluid [see Disorder of Saltwater Excess: Edematous States, below].
than that of the extracellular fluid. Sodium salts, urea, glucose, and other small extracellular solutes
freely cross the capillary wall, achieving similar concentrations in
Osmolality the interstitial fluid and plasma.Thus, changes in plasma osmolal-
Extracellular and intracellular fluids contain different types of ity do not influence water movement between the intravascular
solutes, but the concentrations of solutes inside and outside of cells and interstitial fluid compartments.
are equal. Concentration differences exist only transiently because Fluid movement between the extracellular and the intracellu-
they create an extremely strong force for water movement across cell lar fluid compartments is unaffected by Starling forces. Rather,
membranes. Osmotic pressure moves water rapidly to the fluid com- transcellular water movement is driven by osmotic forces, a func-
partment with the higher solute concentration until concentrations tion of the concentration of solutes in the extracellular fluid. A de-
once again become equal.The osmotic pressure responsible for wa- crease in extracellular solute concentration (hypotonicity) drives
ter movement across cell membranes depends on the total number water into cells, causing cell swelling; an increase in the concentra-
of solute particles (osmoles) dissolved in solution, a property known tion of exclusively extracellular solutes (hypertonicity) draws fluid
as osmolality.2 Osmolality is usually expressed as milliosmoles of out of cells, dehydrating them.
solute per kilogram of solvent (mOsm/kg), but it can be thought of Not all solutes contribute to the tonicity of extracellular fluid.2
more simply as the number of millimoles of solute particles per liter Permeant solutes such as urea and ethanol readily cross cell mem-
of solution. A solute particle’s contribution to osmolality is indepen- branes, achieving equal concentrations in the extracellular and intra-
dent of its charge and molecular size. Ionic substances such as sodi- cellular fluid compartments without driving transcellular water
um chloride that dissociate in solution contribute more than one os- movement and without affecting cell volume. Such solutes, which in-
motically active particle. Sodium salts, glucose, and urea, commonly crease plasma osmolality without altering plasma tonicity, are some-
measured as blood urea nitrogen (BUN), are responsible for most of times called ineffective osmoles. Impermeant solutes are excluded
the solute particles normally present in extracellular fluid. Plasma os- from cell water either by active transport (e.g., sodium ions) or be-
molality can be measured directly with an osmometer or can be esti- cause the cell membrane is impermeable to them (e.g., mannitol).

2. © 2003 WebMD Inc. All rights reserved. ACS Surgery: Principles and Practice
8 CRITICAL CARE 24 DISORDERS OF WATER AND SODIUM BALANCE — 2
um is bound to bone and is therefore osmotically inactive.)
Luminal Membrane Thus, the plasma sodium concentration is, in effect, a measure
of the concentration of tonicity of all body fluids.3 In the absence of
an osmolar gap, the plasma sodium concentration is a more valid
measure of body fluid tonicity than plasma osmolality (which in-
Na+
cludes the ineffective osmole, urea). With a few exceptions, most
Tubular Tubular Peritubular notably hyperglycemia, a low plasma sodium concentration indi-
Lumen Cell Capillary
cates hypotonicity and cell swelling, whereas a high plasma sodium
X concentration indicates hypertonicity and cellular dehydration.
3Na+ RENAL PROCESSING OF BODY FLUIDS
Na+, K+ -
ATPase Glomerular Filtration
Approximately 170 L of extracellular saltwater containing over
2K+
25,000 mmol of sodium are filtered by the glomerulus each day.
Although glomerular hydrostatic pressure is considerably higher
K+
than the pressure of other capillary beds, the Starling forces that
– + control fluid movement between intravascular and interstitial fluid
also drive glomerular filtration.The glomerular filtrate contains the
Basolateral same concentrations of sodium and other solutes as interstitial flu-
Membrane id and is nearly protein free.
Figure 1 Sodium reabsorption by the renal tubules is driven Tubular Reabsorption
by the Na+, K+-ATPase pump in the basolateral (blood side) On a conventional diet, all but 2 L of filtered fluid and all but
membrane, which maintains a low sodium concentration within 175 mmol of filtered sodium is reabsorbed by the renal tubules.
the tubular cell. Filtered sodium in the tubular lumen enters the Regulation of tubular reabsorption of salt and water is the key to re-
cell down its concentration gradient via a transmembrane carri-
nal regulation of body fluid balance.4
er (which may also transport another solute, X) or via a channel
At the end of the proximal tubule, the remaining filtrate has the
in the luminal membrane [see Figure 2].
same sodium concentration as plasma has; as the filtrate passes
through downstream tubular segments, it undergoes major changes
Such solutes, which cause both hyperosmolality and hypertonicity, in composition. In these more distal segments, sodium and water re-
are sometimes called effective osmoles. absorption are uncoupled; salt can be reabsorbed without water,
and water can be reabsorbed without salt.Thus, depending on con-
Body Fluid Tonicity and the Plasma Sodium Concentration ditions, the sodium concentration of the final urine can vary from
Normally, sodium salts are the major effective osmoles in extra- less than 1 mEq/L to nearly 300 mEq/L, and urine osmolality can
cellular fluid, and potassium salts are the major effective osmoles vary from one sixth (50 mOsm/kg) to four times (1,200 mOsm/kg)
in cells. Given that effective osmolality is equal in all fluid com- that of plasma.
partments, body fluid tonicity can be described by the following The process of sodium reabsorption is mediated by carriers or
equation: channels embedded in the tubular cell’s luminal and basolateral
Tonicity ≅ (blood side) membranes. In each nephron segment, sodium reab-
2 (Exchangeable Na+ + Exchangeable K+ ) sorption is powered by the so-called sodium pump, Na+,K+-
2 × Plasma [Na+] ≅ ATPase, which is located on the blood side of the tubular cell [see
Total body water
therefore, Figure 1].This sodium pump exports sodium from the cell, lower-
Exchangeable Na+ + Exchangeable K+ ing the intracellular sodium concentration.The lowered concentra-
Plasma [Na+] ≅ tion of cellular sodium creates an electrochemical gradient driving
Total body water sodium from the tubular lumen into the cell.Tubular segments at
(Only the exchangeable fractions of sodium and potassium are various regions of the nephron utilize different luminal mechanisms
included in the equation, because one third of overall body sodi- for sodium reabsorption [see Table 1 and Figure 2]. Luminal ex-
Table 1—Sodium Transport in Different Nephron Segments
Glomerular Mechanism of Luminal
Nephron Segment Physiologic Regulation Diuretic Site of Action
Filtrate Reabsorbed Sodium Entry
+ +
Na -H exchange; cotrans-
Angiotensin II; renal nerves; Carbonic anhydrase inhibitors (e.g.,
Proximal tubule 60%–70% port with glucose and other
peritubular Starling forces acetazolamide)
organic solutes
Flow dependent; peritubular Loop diuretics (e.g., furosemide,
Na -K -2Cl - cotransport
+ +
Loop of Henle 20%–25%
Starling forces bumetanide, ethacrynic acid)
Na -Cl - cotransport
+
Distal tubule 5% Flow dependent Thiazide diuretics
Collecting tubule 4%
+
Na channels Aldosterone; atrial natriuretic factor Potassium-sparing diuretics (e.g.,
amiloride, triamterene, spironolactone)

3. © 2003 WebMD Inc. All rights reserved. ACS Surgery: Principles and Practice
8 CRITICAL CARE 24 DISORDERS OF WATER AND SODIUM BALANCE — 3
changers, cotransporters, and ion channels along the nephron are nels—called aquaporins—are inserted in the luminal membrane of
subject to physiologic control, and they can be inhibited pharmaco- the distal tubule and collecting duct.7,8 When plasma levels of ADH
logically by specific diuretic agents.5 Mutations in these transport are high, water is attracted osmotically from the tubular lumen to
proteins are responsible for well-defined clinical disorders.6 the hypertonic medullary interstitium, permitting excretion of a
The luminal membrane of the collecting duct is impermeable to small volume (as little as 0.5 L daily) of concentrated urine.
water in the absence of arginine vasopressin, an antidiuretic hor-
REGULATION OF BODY FLUID VOLUMES
mone (ADH).Thus, when plasma ADH levels are low, this segment
progressively reduces the osmolality and sodium concentration of Saltwater (isotonic saline) is confined to the extracellular space.
the final urine and permits the excretion of large volumes (as much Accumulation of saltwater expands extracellular volume; loss of salt-
as 20 L daily) of dilute urine. In the presence of ADH, water chan- water causes volume depletion. In either case, changes in saltwater
balance do not alter the plasma sodium concentration or cell vol-
ume. By contrast, so-called electrolyte-free water, or pure water, is
distributed throughout body fluids, affecting both extracellular and
intracellular fluid compartments. Because only one third of body
water is extracellular, electrolyte-free water has only one third the
impact on extracellular volume that saltwater has; however, unlike
saltwater balance, electrolyte-free water balance has a major impact
on the plasma sodium concentration, body fluid tonicity, and cell
volume.
Extracellular and intracellular fluid volumes are maintained by
separate but interacting control systems [see Table 2]; the extracellu-
Proximal lar system primarily regulates urinary sodium excretion, whereas
Tubule the intracellular system regulates the intake and excretion of water.
Distal Tubule Extracellular fluid volume maintains a proper degree of vascular
Na+ fullness, a variable that is sensed by atrial stretch receptors and arte-
rial baroreceptors. Intracellular volume is regulated by hypothalam-
H+ Na+ ic osmoreceptor cells that swell or shrink in response to changes in
Na+ Cl– plasma tonicity.
Glucose Control of Extracellular Fluid Volume
Phosphate
Amino Acids In a healthy person, the amount of sodium in the extracellular
Citrate space can vary considerably, depending on dietary salt intake. On
the other hand, under normal conditions, the extracellular sodium
concentration remains almost constant because of physiologic con-
Collecting trol systems that tightly regulate water intake and excretion. In
Tubule healthy persons, more salt in the extracellular space means an ex-
Na+ panded extracellular fluid volume; less salt means a smaller extra-
K+ cellular volume. In either case, the extracellular sodium concentra-
2Cl– Na+ tion does not change.
+
Sodium balance and intravascular volume are affected by numer-
K+ ous hormonal and nonhormonal mediators; in addition to aldos-
K
K+
+ - terone and angiotensin—the best known mediators of sodium excre-
- tion—the sympathetic nervous system, natriuretic peptides, and
changes in the renal circulation all play important regulatory roles
[see Table 2].9 Because of redundancy and overlap in the control sys-
tem, failure of a single factor does not cause major, sustained abnor-
malities in intravascular volume.The relative importance of the vari-
ous mediators that affect urinary sodium excretion are incompletely
understood, and it is likely that some sodium regulatory factors re-
Loop of Henle main undiscovered.
Control of Intracellular Fluid Volume
Water balance and cell volume are controlled by a single hor-
monal mediator, arginine vasopressin [see Tubular Reabsorption,
Figure 2 In each tubular segment, sodium enters the tubular above], which is released into the systemic circulation by the neuro-
cell passively, down the favorable electrochemical gradient creat- hypophysis [see Table 2 and Figure 3].The hormone activates V2 re-
ed by the Na+, K+-ATPase pump. Luminal sodium enters by a dif- ceptors on the basolateral membrane of principal cells in the renal
ferent mechanism in each of the nephron segments. The proximal
collecting duct, initiating a cyclic adenosine monophosphate−de-
tubule reabsorbs filtered bicarbonate and other solutes, such as
glucose, phosphate, amino acids, and citrate. An Na+, K+, 2Cl-
pendent (cAMP-dependent) process that culminates in the inser-
transporter mediates Na+ entry into the ascending limb of the tion of water channels (aquaporins) into the cells’ luminal mem-
loop of Henle. The distal tubule has an Na+, Cl- cotransporter. branes.7,8 Modulation of the number of water channels controls
Na+ enters the principal cells of the cortical collecting tubules urine osmolality and the rate of water excretion by the kidney.Vaso-
through Na+ channels in the luminal membrane. pressin’s short half-life in the circulation and continuous shuttling of

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8 CRITICAL CARE 24 DISORDERS OF WATER AND SODIUM BALANCE — 4
Table 2—Control of Body Fluid Volumes
Electrolyte-Free Water Balance
Saltwater Balance
Day to Day Emergency Backup
Extracellular volume
Regulated variable Cell volume Arterial filling
Vascular fullness
Blood pressure Blood pressure
Clinical indicator Plasma sodium concentration
Edema Edema
Sensors Baroreceptors, atrial volume receptors Hypothalamic osmoreceptors Baroreceptors, atrial volume receptors
Renin-angiotensin-aldosterone system
Antidiuretic hormone (arginine Antidiuretic hormone (arginine
Sympathetic nervous system
Mediators vasopressin) vasopressin)
Atrial natriuretic peptide
Thirst Thirst
Starling forces in peritubular capillaries
Urine osmolality Urine osmolality
Affected variable Urinary sodium excretion
Water intake Water intake
aquaporins between the collecting duct’s cell membrane and cy- Normally, water intake and excretion are modulated to maintain
tosol ensure that urinary water excretion responds rapidly to body fluid tonicity within a narrow physiologic range. However, un-
changes in body fluid tonicity. der pathologic conditions, body cells can be exposed to a hypotonic
Vasopressin levels are normally unmeasurable when the plasma or hypertonic milieu.10 The first response to osmotic stress is a com-
sodium concentration falls to 135 mEq/L or lower [see Figure 3]. pensatory adjustment to intracellular electrolytes: loss of potassium
Low levels of the hormone result in the excretion of large volumes in hypotonicity and accumulation of sodium and potassium in hy-
of a maximally dilute urine (50 mOsm/kg). Above a sodium level of pertonicity. With time, changes in organic solutes dominate the
135 mEq/L, plasma vasopressin levels are linearly related to the response.
plasma sodium concentration and increase measurably in response Most cells maintain relatively high concentrations of small, os-
to changes in the plasma sodium concentration of as little as 1 motically active organic molecules known as organic osmolytes. Or-
mEq/L. Once the plasma sodium concentration reaches approxi-
mately 142 mEq/L, plasma vasopressin levels are high enough to
promote the excretion of maximally concentrated urine (1,200
mOsm/kg). A rising plasma sodium concentration also causes hypo-
thalamic cell volume receptors to relay signals to nearby thirst cen-
ters. Mediated by thirst and changes in vasopressin secretion, the
Plasma Vasopressin (pg/ml)
Urine Osmolality (mOsm/kg)
5 1,200
plasma sodium concentration is normally prevented from rising
above 142 mEq/L or falling below 135 mEq/L.
Under day-to-day conditions, water intake, vasopressin secretion,
and urinary free water excretion primarily respond to changes in the
plasma sodium concentration created by variations in electrolyte- Thirst
free water balance. Unlike sodium excretion, which is affected only
by changes in intravascular volume, free water excretion can be af-
fected by two types of stimuli—intravascular volume and tonicity.
Under pathologic conditions, osmotic control of vasopressin secre- 0 50
tion and thirst can be overridden by hemodynamic stimuli.The hy-
pothalamic neurons that secrete vasopressin receive neural input
from baroreceptors in the great vessels and volume receptors in the 135 142
atria.When these receptors are stimulated by hypotension or by a Serum Sodium (mEq/L)
major reduction in plasma volume, impulses are carried via cranial
nerves IX and X.8 Vasopressin and thirst responses to hypovolemia Figure 3 Graph depicts the normal relation between plasma vaso-
and hypotension can be regarded as back-up systems that serve to pressin levels and urine osmolality (black line) and the plasma
maintain arterial blood volume under emergency conditions, sacri- sodium concentration (red line). Plasma vasopressin levels
ficing tonicity to tissue perfusion. change within minutes in response to changes in plasma sodium,
and urine osmolality changes within minutes in response to changes
CELL VOLUME REGULATION IN HYPOTONICITY AND in vasopressin levels. When hydration reduces the plasma sodium
HYPERTONICITY level below 135 mEq/L, plasma vasopressin becomes undetectable
Cell volume is determined by the amount and concentration of and the urine becomes maximally dilute (osmolality, 50
mOsm/kg). Between sodium concentrations of 135 and 142
intracellular solute. Because intracellular and extracellular solute
mEq/L, vasopressin levels are linearly related to the plasma sodi-
concentrations must be equal, the relation between cell water and um, causing nearly a 100 mOsm/kg increase in urine osmolality for
extracellular osmolality can be described by the following equation: every 0.5 mEq/L increase in sodium concentration. Above a plasma
sodium concentration of 142 mEq/L, the urine is maximally con-
Cell water centrated; increased water intake, mediated by thirst, then
Cell volume =
Extracellular osmolality becomes the major defense against progressive hypernatremia.

5. © 2003 WebMD Inc. All rights reserved. ACS Surgery: Principles and Practice
8 CRITICAL CARE 24 DISORDERS OF WATER AND SODIUM BALANCE — 5
ganic osmolytes are nonperturbing solutes; unlike sodium and whose normal kidneys are unable to excrete maximally dilute urine.
potassium, their intracellular concentrations may vary widely with- A pathologically low plasma sodium concentration occurs when
out affecting tertiary protein structure. Cells accumulate organic os- water is taken in at a time when renal diluting mechanisms are not
molytes under hypertonic conditions and lose them when confront- functioning maximally because either (1) diuretics or tubular trans-
ed with hypotonicity. port defects are blocking sodium reabsorption in the renal diluting
The need for cell volume regulation is most imperative in the segments or (2) antidiuretic hormone levels are elevated.
brain, where the rigid calvaria places sharp limits on the degree of tis-
sue expansion or contraction that can be tolerated.10-13 An increase Nonosmotic Release of Vasopressin
in brain water content of more than about 5% to 10% is incompati- Vasopressin is a water-retaining hormone that is released when
ble with life.Variations in the intracellular concentration of organic water is needed. Because hypotonic hyponatremia normally inhibits
osmolytes provide the brain with an astonishing ability to adapt to vasopressin secretion, detectable vasopressin in a patient who is hy-
chronic osmotic disturbances. However, because changes in the os- ponatremic indicates that a nonosmotic stimulus for vasopressin re-
molyte content of brain cells require a few days to develop fully, the lease must be present.Vasopressin action increases the urine osmo-
brain is imperiled by rapid osmotic changes. Thus, acute hypona- lality, which can be thought of as a bioassay for the hormone.
tremia or hypernatremia may be fatal at plasma sodium concentra-
tions that are well tolerated chronically. Hemodynamic stimuli for vasopressin Hypovolemia, heart
With sustained osmotic disturbances, adaptations that protect failure, and cirrhosis are the most common nonosmotic stimuli for
against brain swelling and shrinkage also predispose to injury when antidiuretic hormone secretion.16-18 The hemodynamic abnormali-
the osmotic disturbance is suddenly corrected. In chronic hypona- ties that stimulate vasopressin release also promote sodium reab-
tremia, cellular solutes lost in the adaptive phase must be recovered sorption by the renal tubules; thus, these conditions result in both
when the plasma sodium concentration returns to normal—a process sodium and water retention.
that may require several days. Unless solute recovery keeps pace
with the rising extracellular osmolality, brain cells will become dehy- Inappropriate antidiuretic hormone secretion Nonos-
drated.11,12 This phenomenon may cause clinical complications [see motic release of vasopressin without a hemodynamic stimulus to
Disorder of Water Excess: Hyponatremia, Chronic Hyponatremia, account for it is considered “inappropriate.” Patients with the syn-
Complications of Therapy: Myelinolysis and Osmotic Demyelina- drome of inappropriate antidiuretic hormone secretion (SIADH)
tion Syndrome, below]. Similarly, in chronic hypernatremia, accu- retain water because of nonosmotic release of vasopressin but have
mulated solutes must be shed during correction of the electrolyte no abnormality in sodium balance, evidence of volume depletion,
disturbance.10,13 Cells that have become acclimated to a hypertonic or tendency to form edema; in the steady state, sodium excretion
environment lose organic osmolytes slowly because of slow turnover matches intake.8,12,19 Because of water retention, SIADH causes
of the efflux mechanism, slow downregulation of hypertonically mild, subclinical volume expansion. Any additional volume expan-
stimulated uptake pathways, or both.Thus, when chronic hyperna- sion is met by a brisk increase in urinary sodium excretion.
tremia is corrected rapidly, brain cells swell to a greater than normal
volume. Reset osmostat Reset osmostat is a variant of SIADH, com-
monly seen in patients with chronic, debilitating illness; it is also a
characteristic of normal pregnancy. Patients with this condition are
Disorder of Water Excess: Hyponatremia able to dilute their urine normally but at a lower set-point than in
Hyponatremia simply means a low plasma sodium concentra- normal individuals. Such patients are thus mildly hyponatremic, but
tion. In most cases, hyponatremia is associated with a low plasma unlike other patients with SIADH, they are not predisposed to pro-
osmolality level and body fluids that are too dilute (hypotonic hy- gressive water retention and do not require dietary water restriction
ponatremia). However, there are exceptions to this rule [see Differ- or other measures used to treat chronic hyponatremia.11 Reset os-
ential Diagnosis for Hyponatremia, below]. mostat can, however, be seen in malignancies, and like other causes
of SIADH, it requires a diagnostic evaluation to determine its cause.
PATHOGENESIS OF HYPONATREMIA
Hypotonic hyponatremia results from two basic mechanisms in- Urinary Electrolyte Losses: Desalination and Hyponatremia
dividually or together: (1) massive water intake, exceeding the ca- If the urine is concentrated, urinary sodium and potassium loss-
pacity to excrete electrolyte-free water, or (2) impaired water excre- es can contribute to the pathogenesis of hyponatremia.The plasma
tion. Normally, the capacity for water excretion is rather large. In sodium concentration can be reduced either by loss of sodium or
the absence of vasopressin, urine osmolality falls to approximately potassium or by water gain. However, to lower the plasma sodium
50 mOsm/kg. A typical American diet provides 600 to 900 mOsm concentration, electrolytes must be lost in urine that has a higher
of electrolytes and urea that must be excreted each day. At this rate electrolyte concentration than plasma. The combination of high
of solute excretion, the volume of maximally dilute urine equals 12 vasopressin levels (which concentrate the urine) and a high rate of
to 18 L.Water intake can occasionally exceed the normal excretory sodium and potassium excretion can yield hypertonic urine capable
capacity, primarily in psychotic patients who frantically ingest gal- of generating free water, which in essence desalinates the plasma.20
lons of water over a few hours14 and in very heavy beer drinkers who
DIFFERENTIAL DIAGNOSIS FOR HYPONATREMIA
ingest large volumes of fluid but take in small amounts of salt and
protein.15 More commonly, hyponatremia occurs in patients with a Several conditions can lower the plasma sodium concentration
diminished ability to excrete free water.8,12 without causing hypotonicity and are referred to as nonhypotonic
hyponatremia [see Table 3].The diagnostic and therapeutic approach
Impaired Water Excretion to these conditions differs fundamentally from the approach to hy-
Water excretion is obviously compromised in severe renal failure; potonic hyponatremia.Thus, it is important that nonhypotonic hy-
oliguric patients become hyponatremic if they are given too much ponatremia be excluded whenever a low plasma sodium concentra-
water. However, most cases of hyponatremia occur in patients tion is encountered.

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8 CRITICAL CARE 24 DISORDERS OF WATER AND SODIUM BALANCE — 6
Table 3—Causes of Nonhypotonic Hyponatremia
Condition Plasma Osmolality Pathogenesis Therapeutic Implications
During treatment of hyperglycemia, anticipate
Hyperglycemia High Extracellular glucose osmotically draws water
3 mEq/L increase in serum sodium for every
into the ECF, diluting extracellular sodium
200 mg/dl reduction in blood sugar
Intravenous hypertonic Water shift from ICF to ECF as with Mannitol is rapidly excreted when renal func-
mannitol therapy High hyperglycemia tion is normal
Intravenous γ-globulin therapy High Maltose present in solution acts like Measure plasma osmolality when hyponatrem-
mannitol ia is suspected
Mannitol is rapidly excreted; sorbitol is metab-
Normal or low Absorbed solute—mannitol, sorbitol, olized, causing late-onset hypotonic
Irrigant absorption (prostatectomy (when hypoosmo- or glycine (most common)—initially con- hyponatremia; glycine is neurotoxic and
or intrauterine surgery) lar irrigants are fined to ECF, causing severe hyponatremia causes transient blindness and is metabo-
used) but little change in plasma osmolality lized to ammonia, causing encephalopathy;
consider hemodialysis
Pseudohyponatremia (severe Laboratory artifact; plasma water constitutes Suspect when serum is lactescent; compare
hyperlipidemia, multiple a smaller fraction of the plasma sample, measured plasma osmolality with calculated
Normal
myeloma, macroglobulinemia) causing a more serious underestimate of osmolality or measure plasma sodium with
the true sodium concentration direct-reading sodium electrode
ECF—extracellular fluid—–ICF—intracellular fluid
Hyperglycemic Hyponatremia eventually to urea and glucose. Hyperammonemia may be responsi-
Hyperglycemia lowers the plasma sodium concentration; in the ble for most of the symptomatology, and glycine itself has direct neu-
absence of insulin, glucose is an effective osmole that attracts wa- roinhibitory effects and may cause hypotension, bradycardia, and vi-
ter from cells and thereby dilutes extracellular sodium.Therefore, sual disturbances.
the blood glucose level should always be examined when a low Pseudohyponatremia
plasma sodium concentration is being evaluated. The plasma
sodium concentration falls by approximately 3 mEq for every 200 High plasma concentrations of lipid or protein cause mild non-
mg/dl (10 mmol) increase in blood glucose and will increase by hypotonic hyponatremia because of an artifact of laboratory mea-
this amount when hyperglycemia is corrected with insulin. To surement [see Table 3].2,11 With extremely high concentrations of
evaluate hyponatremia in the presence of hyperglycemia, the lipid (enough to cause lactescent serum) or protein (from multiple
serum sodium concentration must be “corrected” for the osmotic myeloma or Waldenstrom macroglobulinemia), plasma water may
effect of glucose. In effect, the clinician must ask the question, constitute a smaller fraction of the plasma sample than normal,
What would the serum sodium concentration be if the excess glu- which can result in an underestimate of the “true” sodium concen-
cose were no longer present? The correction factor most com- tration. The plasma osmolality and the sodium concentration in
monly used today is a 1.6 mEq/L decrease in serum sodium con- plasma water (as measured by a sodium-sensitive electrode) are un-
centration for every 100 mg/dl increase in blood glucose. A small affected.There are no symptoms, and no therapy is required.
study has suggested that the true correction factor is approximate-
ACUTE HYPONATREMIA (WATER INTOXICATION)
ly 2.4 mEq/L for every 100 mg/dl increase in blood glucose.21 A
precise correction factor is probably unobtainable because in The term water intoxication was coined in the early 1920s to de-
practice, hyperglycemia develops, in part, from the ingestion of scribe a neurologic syndrome that develops when large volumes of
glucose with water and resolves, in part, from the urinary excre- water are retained within a relatively short period of time (< 48
tion of glucose with water.22 hours).The syndrome is often called acute hyponatremia.12,24
Exogenous solutes such as mannitol and maltose (a sugar con-
tained in intravenous immunoglobulin preparations) are confined Etiology
to the extracellular space and have an effect on the plasma sodium Acute hyponatremia develops when water intake is large and
concentration similar to that of hyperglycemia. When the clinical electrolyte-free water excretion is impaired. Potentially, hypona-
setting suggests that these solutes might be responsible for hypona- tremia can develop rapidly in any patient predisposed to water re-
tremia, their presence can be confirmed by measuring the plasma tention who takes in a large volume of water in a short period of
osmolality and comparing it with the calculated value to identify time. However, this is likely to occur in a limited number of settings
an osmolar gap [see Overview of Body Fluid Homeostasis, Distrib- [see Table 4], and such instances account for most cases of severe
ution and Composition of Body Water, Osmolality, above].2,11 symptomatic hyponatremia and for most of the recorded fatalities.
Postprostatectomy Syndrome and Hysteroscopic Hyponatremia Postoperative hyponatremia Vasopressin is released imme-
Irrigants containing mannitol, sorbitol, or glycine are used for en- diately after surgical procedures in what appears to be a stress re-
doscopic transurethral and intrauterine procedures [see Table 3].2,23 sponse [see Table 4].20,25 Particularly during the first 24 hours, the
Occasionally, several liters of irrigant may be absorbed systemically, concentration of urinary cations (sodium plus potassium) may
reducing the plasma sodium in a matter of minutes. Immediately af- greatly exceed the plasma sodium concentration. As a result, even
ter surgery, the serum sodium concentration is much lower than isotonic fluids may be “desalinated” and can lower the plasma sodi-
would be anticipated, because the electrolyte-free solution is initially um concentration.20 Thus, all hypotonic fluids and excessive
confined to the extracellular space. Glycine, the most commonly amounts of isotonic fluids should be avoided after surgery. As dis-
used irrigant in the United States, is metabolized to ammonia and cussed earlier, endoscopic prostatectomy and intrauterine proce-

7. © 2003 WebMD Inc. All rights reserved. ACS Surgery: Principles and Practice
8 CRITICAL CARE 24 DISORDERS OF WATER AND SODIUM BALANCE — 7
Table 4—Causes and Treatment of Acute Hyponatremia
Causes Pathogenesis Effect of Treatment Recommendations
Vasopressin is secreted in response Avoid hypotonic fluid (e.g., D5W,
to surgical stress for 2 or more Normal saline ineffective for correc- 0.45% saline) and excessive vol-
days; free water from hypotonic I.V. tion—administered sodium excret- umes of isotonic fluid (lactated
Postoperative stress* fluids is retained and sodium and ed in concentrated urine, “desali- Ringer solution or 0.9% saline) after
potassium are excreted in urine at nating” isotonic fluid and causing surgery; treat symptomatic hypona-
high concentrations water retention tremia with 3% saline and
furosemide
Used in obstetrics to induce labor;
Avoid administration of oxytocin in or
direct antidiuretic effect of drug Urine becomes dilute when oxytocin
Oxytocin with hypotonic fluids; treat hypona-
mimics SIADH; free water from I.V. is discontinued
tremia by discontinuing drug
fluids retained
Drug has antidiuretic effect that per-
sists for as long as 12 hours; Normal saline ineffective for correc- Treat symptomatic hyponatremia with
Cyclophosphamide patients are encouraged to drink tion as in other causes of persis- 3% saline and furosemide
large volumes of water to prevent tent SIADH
chemically induced cystitis
Normal ability to dilute urine in most Monitor diurnal weight in institutional-
Extreme polydipsia (> 1 L/hr) com- patients so hyponatremia self-cor- ized patients for early detection;
mon among patients with severe rects when water intake stops; avoid antidiuretic medications; treat
Psychotic self-induced water psychosis; retained water causes some patients have vasopressin hyponatremia with water restriction;
intoxication hyponatremia by late afternoon or release (often transient) from use hypertonic saline and
evening, and water diuresis re- stress, smoking, or medications furosemide for occasional patient
stores normonatremia by morning (e.g., carbamazepine) with SIADH
Extracellular volume depletion
caused by saltwater losses from
3% saline without furosemide for
sweating and possibly stress are Isotonic saline restores ability to seizures; isotonic saline and water
Marathon running nonosmotic stimuli for vasopressin dilute urine restriction for more moderate
secretion; large volumes of sugar
symptoms
water consumed during race are
retained
Excessive fluid intake and inappropri- Isotonic saline ineffective; self-
Ecstasy (MDMA) use ate antidiuretic hormone secretion, correction typical but may be Hypertonic saline for severe symptoms
induced by MDMA, is implicated delayed
*Excluding irrigant absorption syndromes [see Table 3].
D5W—5% dextrose in water MDMA—methylenedioxymethamphetamine SIADH—syndrome of inappropriate antidiuretic hormone
dures can cause hyponatremia if irrigant used in the procedures is evening; however, water diuresis typically restores normonatremia
absorbed systemically.The management of irrigant absorption syn- by the following morning. Occasionally, individuals drink enough
dromes differs from that of other causes of postoperative hypona- water to produce seizures. By monitoring diurnal changes in body
tremia [see Postprostatectomy Syndrome and Hysteroscopic Hy- weight, water intoxication can be recognized before the onset of se-
ponatremia, above].25 vere neurologic symptoms. Transient release of vasopressin (most
commonly provoked by nausea and vomiting) may contribute to
Oxytocin infusions Oxytocin, which is used in obstetrics to in- water retention.There is little evidence that any of the major tran-
duce labor, has a direct antidiuretic effect. If the drug is adminis- quilizers has a significant antidiuretic effect; however, carbamaze-
tered in 5% dextrose in water (formerly a common practice), symp- pine, an anticonvulsant, enhances sensitivity to vasopressin.
tomatic hyponatremia may emerge after the infusion of less than 3
L of fluid [see Table 4].11 Termination of the infusion permits a water Water intoxication during exercise During a race, runners
diuresis and correction of hyponatremia; however, the syndrome is sweat profusely, losing large volumes of saltwater. Extracellular vol-
best avoided by using isotonic saline as a vehicle for the drug. ume contraction and possibly the stress of exertion cause the release
of vasopressin.When saltwater losses are replaced by the intake of
Cyclophosphamide infusion Intravenous cyclophosphamide sugar water, water retention and symptomatic hyponatremia may
impairs water excretion by an unknown mechanism.26 The anti- occur.27,28 Most severe cases have been reported after marathons or
diuretic effect of the drug begins 4 to 12 hours after injection and ultramarathons. However, symptomatic hyponatremia may occur
persists for as long as 12 hours. Patients receiving cyclophosphamide after recreational running and military fitness training.
are particularly susceptible to hyponatremia, because they are en-
couraged to drink large volumes of water to prevent chemically in- Water intoxication from “ecstasy” During the 1990s, 3,4-
duced cystitis [see Table 4]. methylenedioxymethamphetamine (MDMA, or ecstasy) gained
widespread popularity as a recreational drug taken at dances.29
Psychotic self-induced water intoxication Extreme poly- When malignant hyperthermia was recognized as a complication as-
dipsia is relatively common in patients with psychiatric illnesses, sociated with this drug, MDMA users were advised in underground
particularly schizophrenia, and it may lead to symptomatic hypona- magazines and the lay press to drink plenty of fluids. Subsequently,
tremia [see Table 4].14 Daily intake of 10 to 15 L has been docu- acute water intoxication emerged as a potentially lethal complication
mented, and much of the intake may take place over a few hours. of the drug [see Table 4]. Excessive fluid intake and inappropriate
Many patients become hyponatremic in the late afternoon and ADH secretion, induced by MDMA, have been implicated.

8. © 2003 WebMD Inc. All rights reserved. ACS Surgery: Principles and Practice
8 CRITICAL CARE 24 DISORDERS OF WATER AND SODIUM BALANCE — 8
Diagnosis Table 5—Causes of the Syndrome of
Symptoms of water intoxication include headaches, weakness, Inappropriate Antidiuretic Hormone
nervousness, and vomiting, progressing to disorientation, delirium,
Bronchogenic (small cell)
tremulousness, and ultimately convulsions and coma.12,24 The
Pancreatic
pupils are often dilated, and bilateral Babinski signs may be present. Duodenal
On occasion, patients may present with hemiparesis, mimicking a Urethral
cerebrovascular accident. The syndrome reflects cerebral edema, Tumors
Nasopharyngeal
which can lead to herniation of the brain and death. Clinical find- Leukemia
ings may emerge explosively. Complaints of headache and mild Hodgkin disease
confusion may be followed within hours by respiratory arrest and, Thymoma
in some cases, neurogenic pulmonary edema. For reasons that re- Psychosis
main obscure, almost all reported fatalities from acute postoperative Trauma
hyponatremia have been in women (usually of childbearing age) Neoplasms (primary and metastatic)
Neurologic
and young children. Fatal cases of acute hyponatremia from other disorders Vascular (hemorrhage, infarction, and vasculitis)
etiologies have been recorded in men and women. Infection (meningitis, brain abscess, and encephalitis)
Acute hyponatremia should be suspected in any patient who Miscellaneous (Guillain-Barré syndrome, multiple sclero-
sis, hydrocephalus, Shy-Drager syndrome)
has unexplained neurologic symptoms, especially in psychiatric
patients, marathon runners, users of ecstasy, and patients receiv- Infectious (bacterial, viral, and fungal pneumonia and
Pulmonary tuberculosis)
ing hypotonic fluids intravenously (e.g., after surgery). Serum disorders Functional (asthma, acute respiratory failure, and
electrolytes should be obtained immediately. In the proper set- mechanical ventilation)
ting, a tentative diagnosis of water intoxication is advisable when
Endocrine Glucocorticoid deficiency (hypopituitarism)
symptoms develop in a patient whose serum sodium concentra- diseases Hypothyroidism
tion is lower than 130 mEq/L (provided that causes of nonhypo-
tonic hyponatremia have been excluded). Although severe neuro- Antidiuretic hormones (vasopressin, DDAVP, and oxytocin)
logic symptoms do not usually appear until the sodium level has Psychotropic agents (tricyclic antidepressants, serotonin
reuptake inhibitors, monoamine oxidase inhibitors, and
fallen below 120 mEq/L, some patients may be unusually suscep- carbamazepine)
tible to brain edema when they become acutely hyponatremic; Ecstasy (MDMA)
rare fatalities have been reported at plasma sodium concentrations Drugs Antineoplastic agents (cyclophosphamide, vincristine,
between 120 and 128 mEq/L.20,25 and vinblastine)
Nonsteroidal anti-inflammatory drugs
Computed axial tomography demonstrates cerebral edema in se-
Diabetic agents (chlorpropamide and tolbutamide)
vere cases of water intoxication, and it rules out other potential ex- Miscellaneous (bromocriptine and nicotine)
planations for neurologic findings. However, when symptoms are
severe, therapy should not be delayed while imaging studies are be- Postoperative stress
ing obtained. Alcohol withdrawal
Other causes
AIDS
Treatment Nausea
Free-water intake should be stopped immediately whenever wa- DDAVP—1-desamino-8-d-arginine vasopressin MDMA—methylenedioxymethamphetamine
ter intoxication is suspected. Hypertonic saline is the treatment of
choice for water-intoxicated patients who cannot autocorrect their
electrolyte disturbance.24,25 Each 1 ml of 3% saline contains 0.5 to gradually return the plasma sodium concentration to normal.To
mEq of sodium. Because there are approximately 0.5 L of body avoid complications from excessive correction of hyponatremia, the
water for every 1 kg of body weight, 1 ml of 3% saline per 1 kg of plasma sodium concentration should not be intentionally increased
body weight can be expected to increase the plasma sodium con- by more than 12 mEq/L during the first day of therapy or by more
centration by 1 mEq/L. For patients with severe neurologic symp- than 6 mEq/L/day thereafter.
toms, an infusion of 3% saline at 1 to 2 ml/kg/hr will increase the CHRONIC HYPONATREMIA
plasma sodium concentration by approximately 1 to 2 mEq/L/hr, a
rate that is considered appropriate for initial therapy. Hypertonic The distinction between acute and chronic hyponatremia is some-
saline is best infused in 100 ml containers to avoid inadvertently what arbitrary.We consider hyponatremia to be chronic when it has
giving an excessive dose. Except when volume depletion is suspect- evolved over the course of 48 hours or more.11,12,24 Although the pre-
ed (as in marathon runners), concurrent administration of a loop cise duration of an electrolyte disturbance cannot be known when it
diuretic (furosemide, bumetanide, or torsemide) is advisable.The develops outside the hospital (except for psychotic water drinkers,
diuretic prevents volume overload and, by blocking sodium reab- marathon runners, and users of ecstasy) outpatients can be assumed
sorption in the loop of Henle, impedes the formation of concen- to have chronic hyponatremia.30 Prolonged hyponatremia cannot
trated urine. occur unless there is a sustained defect in water excretion. Except for
The goal of therapy in acute hyponatremia is to decrease the patients with renal failure, virtually all chronically hyponatremic pa-
severity of cerebral edema and to stop seizures. A 4 to 6 mEq/L in- tients have some abnormality in vasopressin secretion.
crease in plasma sodium concentration is usually sufficient to ac-
complish these goals. Thus, the plasma sodium concentration Etiology
should be monitored frequently during therapy and emergency Advanced renal failure A low glomerular filtration rate lim-
treatment with hypertonic saline should be stopped after 2 to 3 its the ability to excrete electrolyte-free water. Many patients with
hours. Once initial therapy with high-dose hypertonic saline has advanced renal failure excrete urine that has the same osmolality as
been completed, more conservative measures should be substituted plasma regardless of physiologic conditions (fixed isosthenuria). In

9. © 2003 WebMD Inc. All rights reserved. ACS Surgery: Principles and Practice
8 CRITICAL CARE 24 DISORDERS OF WATER AND SODIUM BALANCE — 9
acute oliguric renal failure, the ability to excrete free water is virtual- a common complication of chest infection. Antidiuretic activity has
ly nil; administration of hypotonic fluids must be scrupulously been demonstrated by bioassay in patients with tuberculous lung
avoided to avoid hyponatremia. tissue, and tuberculosis causes SIADH.11 In pneumonia, vaso-
pressin levels are increased during the acute phase of the disease
Diuretics Thiazide diuretics are commonly the sole cause or a and return to baseline within a few days. Isolated glucocorticoid de-
major contributing factor of hyponatremia requiring hospital admis- ficiency caused by anterior pituitary dysfunction also causes hy-
sion.30,31 For unknown reasons, severe hyponatremia caused by thi- ponatremia; patients with hypopituitarism develop inappropriate
azides affects elderly women much more often than other groups. ADH secretion, but unlike patients with Addison disease, they have
By blocking the reabsorption of sodium and chloride in the distal normal levels of mineralocorticoid and do not become hypovolemic
tubule, thiazides and metolazone prevent the generation of maxi- or hyperkalemic. Hyponatremia caused by glucocorticoid deficien-
mally dilute urine.32 Because sodium reabsorption in the ascending cy promptly resolves when cortisol is replaced. Hypothyroidism also
limb of the loop of Henle is left unaffected by these agents, they per- causes inappropriate ADH secretion; hyponatremia gradually re-
mit excretion of maximally concentrated, hypertonic urine and can solves when thyroid hormone is replaced.36 A number of therapeu-
lead to simultaneous retention of water and depletion of sodium tic agents can induce SIADH.12,37 Nonsteroidal anti-inflammatory
and potassium. Extraordinarily severe hyponatremia can result from drugs (NSAIDs) decrease water excretion because they inhibit for-
thiazides, with plasma sodium levels as low as 100 mEq/L. Vaso- mation of prostaglandin E2, which modulates vasopressin action.8
pressin levels are usually elevated in patients who present with thi- Rare cases of hyponatremia solely attributable to NSAIDs have
azide-induced hyponatremia, sometimes because of diuretic- been reported, but these commonly used agents may exacerbate
induced volume depletion but more often because of the stress of other causes of hyponatremia.
minor intercurrent illnesses. Patients with thiazide-induced hypona-
tremia do not usually appear clinically volume depleted, presumably Hyponatremia in AIDS Hyponatremia is an extremely com-
because retained water partially sustains extracellular fluid volume. mon finding in AIDS.38 Many AIDS patients have features of
Patients who have become hyponatremic on thiazides should not be SIADH associated with opportunistic infections that cause pneu-
given these agents again; recurrent episodes of severe hyponatremia monia and meningitis. Others have clinical signs of volume deple-
are common. tion without low urine sodium values, a finding that may indicate
coexistent renal disease or adrenal insufficiency.39 Hyponatremia of-
Hypovolemia Hypovolemic hyponatremia is most often associ- ten occurs when antibiotics are administered in hypotonic intra-
ated with gastrointestinal fluid losses caused by vomiting, diarrhea, venous solutions.
or laxative abuse. Surprisingly, particularly in alcoholics, patients
who continue to drink while vomiting repeatedly can still absorb Diagnosis
enough ingested water to become hyponatremic. Electrolyte losses Hyponatremia should be approached in a systematic fashion.
in the vomitus, combined with urinary sodium and potassium loss- First, the various disorders that can lower the plasma sodium con-
es that result from metabolic alkalosis, lower the plasma sodium centration without causing hypotonicity should be excluded [see
concentration. Differential Diagnosis for Hyponatremia, above]. Once it has been
established that hypotonic hyponatremia is present, the mechanism
Beer potomania Patients who subsist on beer (a practice for impaired water excretion is identified (hypovolemia versus an
known as beer potomonia) are susceptible to hyponatremia because edematous condition versus SIADH), and the differential diagnosis
of their low rates of solute excretion (beer contains little protein or that applies to that mechanism is considered.The most challenging
electrolyte). Nonosmotic stimuli to vasopressin secretion caused by goals of the diagnostic process are to determine whether chronic
nausea or gastrointestinal fluid losses or by treatment with thiazide SIADH is present and, if it is, to define the specific disease responsi-
diuretics are often contributing factors.15 ble for the syndrome.
Edematous conditions Any disease that can cause edema Clinical manifestations Because cerebral edema is usually
also predisposes to water retention and hyponatremia.The same he- not severe, the symptoms of chronic hyponatremia are much more
modynamic factors that promote sodium retention are nonosmotic subtle, vague, and nonspecific than those of acute water intoxica-
stimuli for vasopressin release.8,16-18 Elevated vasopressin levels have tion.12,24 The electrolyte disturbance is often asymptomatic at sodi-
been reported in hyponatremic patients with congestive heart fail- um levels that may be lethal to a patient with acute water intoxica-
ure, cirrhosis, and nephrotic syndrome. In heart failure, hypona- tion. As the plasma sodium concentration falls below 115 to 120
tremia is associated with a low cardiac output and a poor prognosis. mEq/L, patients often experience anorexia, nausea, vomiting, mus-
cle weakness, and muscle cramps.They may be irritable and show
SIADH Nonosmotic release of vasopressin that has no hemo- personality changes, becoming uncooperative, confused, hostile, or
dynamic explanation is termed inappropriate [see Table 5].12,19 A simply slow to respond. At plasma sodium concentrations below
number of tumors (most commonly small cell carcinoma of the 110 mEq/L, gait disturbances, falling, stupor, tremulousness, and,
lung) ectopically synthesize and secrete vasopressin.33 Unexplained, more rarely, seizures may occur.
persistent hyponatremia should be considered a marker for an un- Chronic hyponatremia itself is rarely, if ever, fatal. However, be-
derlying malignancy. cause hyponatremia can be a marker for severe underlying illness,
SIADH may also complicate the course of a wide variety of con- hospitalized patients with hyponatremia often have a high mortality
ditions in which there is damage to or inflammation of the central rate, dying with but not of chronic hyponatremia.There is little evi-
nervous system.8,12 In patients with subarachnoid hemorrhage, na- dence that chronic hyponatremia itself leads to permanent sequelae,
triuretic peptides released by the brain may directly promote uri- even when the plasma sodium concentration falls below 105
nary sodium loss, regardless of extracellular volume (cerebral salt mEq/L.30,40 However, patients with prolonged severe hyponatremia
wasting).34,35 Urinary salt losses combined with vasopressin-in- are susceptible to iatrogenic injury if their electrolyte disturbance is
duced water retention are responsible for hyponatremia. SIADH is corrected too rapidly [seeTreatment, below].

10. © 2003 WebMD Inc. All rights reserved. ACS Surgery: Principles and Practice
8 CRITICAL CARE 24 DISORDERS OF WATER AND SODIUM BALANCE — 10
History and physical examination The history of patients um, developing clinically obvious edema.Volume-depleted patients
with chronic hyponatremia should include information about diet, initially retain the administered sodium, but as soon as hypo-
fluid intake, gastrointestinal fluid losses, and use of diuretics, antide- volemia is corrected, the urine becomes dilute, the rate of urinary
pressants, or other antidiuretic drugs. During the physical examina- sodium excretion increases to match intake, and hyponatremia im-
tion, physicians should look for clinical signs of volume depletion or proves as water is excreted in the urine. Urinary sodium excretion
an edematous condition. Evidence of volume depletion may not al- promptly increases in patients with SIADH, but the urine remains
ways be definitive, however. For example, vomiting may be a symp- concentrated and hyponatremia persists. Isotonic saline should be
tom rather than the cause of hyponatremia; extreme hyponatremia given with extreme caution to patients with very low plasma sodi-
may occasionally impair baroreceptor reflexes causing postural hy- um concentrations; in SIADH, saline can exacerbate hyponatrem-
potension and a false impression of volume depletion; and retained ia, whereas in volume depletion, hyponatremia may correct too
water may mask underlying volume depletion.When the distinction rapidly.
between hyponatremia caused by hypovolemia and hyponatremia
caused by inappropriate antidiuretic hormone secretion is not obvi- Identifying a specific cause for SIADH SIADH is a mech-
ous, laboratory clues may helpful. anism for developing hyponatremia, not a diagnosis. In all patients
with SIADH, a specific etiology for inappropriate vasopressin secre-
Laboratory tests Measurement of the urinary sodium, tion should be sought.When hyponatremia develops in the hospital,
chloride concentration, or both is often the most helpful test.41 the cause is sometimes obvious (e.g., pneumonia, meningitis, acute
Water retention caused by hypovolemia or by an edematous con- respiratory failure) and no further testing is indicated. In a patient
dition is usually associated with a urinary sodium concentration with clinical features of SIADH but no obvious cause for it, a more
lower than 20 mEq/L. Hypovolemia caused by upper gastroin- extensive evaluation is indicated.The workup should include a care-
testinal fluid losses is an important exception. Loss of gastric fluid ful search for malignancy and central nervous system pathology and
causes a metabolic alkalosis that may increase urinary sodium ex- an endocrine evaluation to exclude hypothyroidism and hypocorti-
cretion despite volume depletion; the diagnosis can be made by solism. Sometimes, no cause for SIADH is found, especially in el-
measuring the urine chloride concentration, which is reduced in derly patients and patients with psychiatric disorders, mental retar-
this condition. In SIADH, urinary sodium matches intake; as the dation, or alcoholism.42 Careful follow-up is important, because
urine is usually concentrated, the urinary sodium concentration malignancies may become clinically apparent after several years in
exceeds 40 mEq/L unless dietary sodium intake is very low. Mea- so-called idiopathic SIADH.
surements of the BUN and serum uric acid complement these
measurements.When a hemodynamic abnormality is responsible Treatment
for hyponatremia, the kidney is underperfused, urea and uric acid Patients with very low plasma sodium concentrations usually
clearances are diminished, and the BUN and serum uric acid lev- have some neurologic symptoms, and they are at risk of injuries
els are usually elevated. Conversely, SIADH is a volume-expand- from falls. However, unlike acute water intoxication, there is little
ed state, and BUN and uric acid levels are usually low. Uric acid is risk of an explosive onset of seizures or a fatal outcome in chronic
a more reliable indicator of volume status than the BUN, because hyponatremia. On the other hand, patients with chronic hypona-
the latter value is affected by dietary protein intake as well as renal tremia are at considerable risk of neurologic injury caused by over-
clearance. Assessment of acid-base and potassium balance may aggressive correction.Thus, there are four major goals in managing
provide helpful clues to the diagnosis.The serum potassium and chronic hyponatremia: (1) prevention of a progressive decrease in
bicarbonate levels are normal in SIADH. Hypokalemia and meta- plasma sodium concentration; (2) amelioration of symptoms
bolic alkalosis suggest diuretic therapy or vomiting, which can be caused by hyponatremia; (3) avoidance of excessive correction; and
surreptitious. Hyperkalemia and metabolic acidosis suggest the (4) gradual restoration and maintenance of a normal plasma sodi-
possibility of adrenal insufficiency. Hypokalemia and acidosis are um concentration.
found in diarrhea and raise the possibility of surreptitious laxative Free-water restriction should be instituted in all patients until the
abuse. plasma sodium concentration has begun to increase. Intravenous
fluids should be at least isotonic, and oral fluid intake should be lim-
Withdrawal of hyponatremic drugs When a patient is tak- ited to 500 to 1,000 ml/day, depending on the severity of the elec-
ing a drug that can cause hyponatremia, it is important to exclude trolyte disturbance. In patients with reversible defects in water ex-
another underlying cause for hyponatremia before attributing the cretion, limitations on free-water intake should be lifted once the
electrolyte disturbance to the medication. For example, thiazide di- plasma sodium concentration has begun to increase.
uretics can exacerbate hyponatremia caused by SIADH. The best Attempts to calculate the dose of sodium chloride needed to cor-
way to make a diagnosis of drug-induced hyponatremia is to elimi- rect hyponatremia are doomed to failure. The increase in plasma
nate the offending agent and be sure that water excretion returns to sodium concentration depends on the amounts of administered
normal when the patient is off the drug. Full resolution of hypona- sodium and potassium that have been retained without being ex-
tremia and full recovery of diluting function may be delayed for a creted, as well as on the amount of electrolyte-free water that is
week or two in patients with thiazide-induced hyponatremia. Dur- eliminated in the urine. Indeed, in some cases, the plasma sodium
ing repair of sodium and potassium deficits, transient resetting of concentration will return to normal solely because of a water diure-
the osmostat is common and should not necessarily prompt an ex- sis, with no sodium given. The measures required to increase the
tensive search for an underlying cause. plasma sodium concentration, along with the likelihood of inadver-
tent rapid correction, vary depending on the cause of hyponatrem-
Response to therapy On occasion, evidence regarding the ia. For therapeutic purposes, the causes can be divided into re-
cause of hyponatremia can be equivocal. In such cases, the pa- versible and persistent defects in water excretion.
tient’s response to isotonic saline (or a generous oral salt intake and
the passage of time) is the best clue to the diagnosis. Patients with Reversible defects in water excretion Hyponatremia cor-
subclinical edematous conditions will retain the administered sodi- rects easily when the cause for defective water excretion can be

11. © 2003 WebMD Inc. All rights reserved. ACS Surgery: Principles and Practice
8 CRITICAL CARE 24 DISORDERS OF WATER AND SODIUM BALANCE — 11
eliminated by volume expansion, by withdrawal of a therapeutic are the mainstay of treatment of hyponatremia for patients with
agent, or by treatment of an underlying illness [see Table 4]. In pa- edematous conditions because they increase free-water excretion
tients with reversible defects in water excretion, prevention of ex- and improve hyponatremia, particularly when dietary salt intake is
cessive correction may become a major challenge. increased.There is a natural inclination to discontinue loop diuret-
Hypovolemic hyponatremia responds readily to 0.9% sodium ics when severely edematous patients develop hyponatremia. The
chloride because the sodium concentration of isotonic saline is usual problem, however, is oliguria and diuretic resistance rather
higher than the cation concentration of the excreted urine. Once than overdiuresis; the proper response is to increase the dose of
the volume deficit is repaired and the hemodynamic stimulus to loop diuretics and restrict water intake.The combination of a loop
vasopressin secretion is removed, the urine becomes dilute and a diuretic and an angiotensin converting enzyme (ACE) inhibitor is
water diuresis may rapidly return the plasma sodium concentration particularly effective in patients with congestive heart failure.The
to normal. Similarly, patients with diuretic-induced hyponatremia beneficial effect of an ACE inhibitor can be explained by reduced
are extremely susceptible to rapid correction; restoration of the re- thirst and vasopressin secretion attributable to angiotensin II and
nal diluting mechanism when the diuretic is discontinued and re- by a direct effect on the hydro-osmotic effect of vasopressin, medi-
placement of sodium and potassium deficits contribute to the in- ated by prostaglandins.11
crease in plasma sodium concentration. Hyponatremia in edematous conditions is mediated by vaso-
Intravenous saline should be discontinued once clinically appar- pressin. Clinical trials have shown that vasopressin receptor antago-
ent hypovolemia has been corrected and the plasma sodium con- nists can be effective in managing patients with hyponatremia and
centration has begun to increase. Saline should be given cautiously, edema.43,44
if at all, to hypokalemic patients who require potassium replace-
ment. During repair of a potassium deficit, potassium enters cells, Treatment of hyponatremic seizures A small percentage
displacing sodium, which then returns to the extracellular fluid; ad- of chronically hyponatremic patients with very low plasma sodium
ministered potassium is therefore as effective as sodium in raising concentrations present with seizures. Regardless of the suspected
the plasma sodium concentration. Diuretic-induced hyponatremia duration or cause of the electrolyte disturbance, active seizures
does not usually necessitate use of intravenous saline; for most pa- may be resistant to anticonvulsants alone and should be treated
tients, an adequate diet, replacement of potassium deficits, and dis- with hypertonic saline.The therapeutic approach is similar to that
continuance of thiazide diuretics are sufficient. In severely hypona- used for patients with acute water intoxication, except that even
tremic patients, the plasma sodium concentration should be more vigilance is required to prevent an excessive increase in plas-
monitored every 6 to 8 hours for the first 2 to 3 days of therapy. If it ma sodium concentration once emergency measures have been
appears that a water diuresis is going to increase the plasma sodium discontinued.12,24
by more than the desired amount, replacement of fluid losses with
oral water or 5% dextrose in water may become necessary. Complications of Therapy: Myelinolysis and Osmotic
Demyelination Syndrome
Persistent defects in water excretion: SIADH Patients Excessive correction of chronic hyponatremia may be compli-
with SIADH tend to be resistant to rapid changes in plasma sodi- cated by neurologic injury.40,45 Typically, the patient’s hypona-
um concentration (unless the cause of SIADH is short-lived).Wa- tremic symptoms improve as the plasma sodium concentration
ter restriction is the cornerstone of therapy, but if used alone, water increases, but after a delay of one to several days, new findings
restriction often leads to an extremely slow resolution of hypona- emerge.The patient may become confused and may exhibit psy-
tremia. Isotonic saline is ineffective and may even be counterpro- chotic or catatonic behavior, pathologic crying, or a movement
ductive. Furosemide and loop diuretics are often useful therapeu- disorder. Swallowing dysfunction, progressive unresponsiveness,
tic adjuncts because by blocking sodium reabsorption in the and a spastic quadriparesis may develop. In severe cases, locked-
ascending limb of the loop of Henle, they interfere with the renal- in syndrome occurs—that is, the patient is awake but unable to
concentrating mechanism, partially blocking the effect of vaso- move or respond.The stereotypical pattern of delayed neurologic
pressin. Loop diuretics can be combined with oral salt or a slow deterioration after rapid correction of hyponatremia has been
infusion (approximately 15 ml/hr) of 3% saline. Oral and intra- named the osmotic demyelination syndrome,45 because these
venous urea have been used extensively to treat SIADH in some clinical features are associated with brain lesions (myelinolysis)
parts of Europe, but experience with this agent in the United characterized by disruption of myelin and sparing of neurons and
States is very limited. Demeclocycline, a tetracycline that blocks axons.46,47 Lesions, which are best identified by magnetic reso-
the effect of vasopressin on the collecting duct, is another thera- nance imaging, are typically found in the center of the basal pons
peutic option in chronic SIADH; however, its expense and long (central pontine myelinolysis [CPM]), but histologically similar
duration of action limit its effectiveness. Several orally active vaso- lesions may also occur in a symmetrical distribution in extrapon-
pressin receptor blockers have been developed and are currently in tine areas of the brain where there is a close admixture of gray
clinical trials.43,44 and white matter.The osmotic demyelination syndrome has been
reproduced in animal studies46; these experiments have shown
Persistent defects in water excretion: edematous condi- that the disorder is a complication of rapid correction of hypona-
tions and renal failure Saline should rarely, if ever, be given to tremia rather than the electrolyte disturbance itself. Observation-
correct hyponatremia in edematous patients or patients with renal al studies in severely hyponatremic patients suggest that this ther-
failure (except for those with prerenal azotemia). Because it has no apeutic complication can be avoided if rates of correction are
effect on water excretion, 1 L of 0.9% saline will increase the plasma maintained below 10 to 12 mEq/L/day and 18 mEq/L/48 hr. Be-
sodium concentration by only 1 mEq/L.12 In addition, saline exac- cause large increases in the serum sodium concentration are sel-
erbates edema and ascites in patients with cirrhosis and may cause dom required to relieve hyponatremic symptoms and because un-
pulmonary edema in patients with congestive heart failure or renal intentional excessive correction is common, the goal of therapy
failure. should be to increase serum sodium concentration by 8
Although thiazide diuretics are contraindicated, loop diuretics mEq/L/day or less.12