3. history
• 1826: William Bowman used digital tonometry as a routine examination
test.
• 1863: Albrecht von Grafe designed the first instrument to attempt to
measure intraocular pressure.
• Further instruments followed, notably by Donders in 1865 and Preistly-
Smith in 1880.
These instruments were all of the indentation type and rested on the sclera (no
anaesthetic was used until 1884).
• 1885: Maklakov designed an applanation tonometer. This was refined in
1892. Used for a number of years in Russia and Eastern Europe. This was
used till 1959.
•
1905: Hjalmar Schiotz produced his indentation tonometer. This made
tonometry a simple and routine clinical test.
6. Ideal tonometer
• Should give accurate and reasonable IOP
measurement
• Convenient to use
• Simple to calibrate
• Stable from day to day
• Easier to standardise
• Free of maintenance problems
7. Types of tonometry
• Tonometry (IOP)
• Direct – manometry
• Indirect –
• A) Static tonometry – a)contact, b)noncontact
• a) contact tonometers
• 1)Indentation
• Schiotz ,Mercurial ,Electronic ,Scleral tonometry
• 2)Applanation
• Variable force
• GAT, Perkins, MMT, Tonopen, Pneum.Tonometer
• Constant force
• Maklakov, Glucotest, Applanometer
• b)Non Contact tonometer – Pulsair
8. B) Dynamic instruments
• Ballistic tonometers
• Impact acceleration
Impact duration
Rebound velocity
• Vibration tonometers (Krakau)
• Vogelsang 1927 – Ballistic Tonometer .The rebound of a
small metal ball from the eye is measured and this depends
to a large extent on the physical properties of the coats of
the eye.
• Roth & Blake 1963- Vibration tonometer cause minimal
deformation by oscillating force by a probe, which also
functions as a sensor and measures the resonant frequency
of the eye.
9. Types of tonometers
• In a normal eye IOP becomes more during Schiøtz
tonometry.
• low-displacement tonometers.
• Tonometers in which the IOP is negligibly raised during
tonometry (less than 5%) are termed as low-displacement
tonometers.
• Eg. Goldmann Applanation Tonometer.Mackay-Marg
tonometer.
• The Goldmann tonometer displaces only 0.5 μl of aqueous
humor and raises IOP by only 3%.
• high-displacement tonometers
• Tonometers that displace a large volume of fluid and
consequently raise IOP significantly are termed as high-
displacement tonometers.
11. Direct method
• Manometry
• Canula inserted into eye.
• Not practical clinically.
• Intraocular pressure is higher than atmospheric pressure; therefore, if
a small hollow needle is inserted into the anterior chamber, aqueous
humor flows out through the needle.
• If the needle is attached to a
reservoir of fluid that is raised just
high enough to prevent any loss of
aqueous, the height of the column
of fluid, usually calibrated in cm of
water or mm of mercury, reflects
the intraocular pressure
12. Indirect method
• Palpation Method/ digital tonometry
• Intraocular pressure (IOP) is estimated by
response of eye to pressure applied by
finger pulp.
• indents easily – low IOP
• Firm to touch – normal IOP
• Hard to touch – high IOP
14. Parts of schiotz tonometer
Tonometer weight = 11g
scale
needle
Additional weights
7.5,10,15g
lever
Weight 5.5g
3mm diameter plunger
holder
ROC 15mm Foot plate
15. Schiotz tonometry - characteristics
• The extent to which cornea is indented by plunger
is measured as the distance from the foot plate
curve to the plunger base and a lever system
moves a needle on calibrated scale.
• The indicated scale reading and the plunger
weight are converted to an IOP measurement.
• More the plunger indents the cornea, higher
the scale reading and lower the IOP
• Each scale unit represents 0.05 mm protrusion
of the plunger.
16. PRINCIPLE
• The weight of tonometer on the eye increases the actual
IOP (Po) to a higher level (Pt).
• The change in pressure from Po to Pt is an expression of
the resistance of the eye (scleral rigidity) to the
displacement of fluid.
• P(t) = P(o) + E
• IOP with Tonometer in position Pt =
Actual IOP Po + Scleral Rigidity E
• Determination of Po from a scale reading Pt requires
conversion which is done according to Friedenwald
conversion tables.
17. Friedenwald formula
• Friedenwald generated formula for linear relationship
between the log function of IOP and the ocular distension.
• Pt = log Po + C ΔV
• This formula has ‘C’ a numerical constant, the coefficient
of ocular rigidity which is an expression of distensibility
of eye. Its average value is 0.025
• ΔV is the change in volume
19. TECHNIQUE
• Patient should be anasthetised with 4%lignocaine or 0.5%
proparacaine
• With the patient in supine position, looking up at a
fixation target while examiner separates the lids and
lowers the tonometer plate to rest on the anesthetized
cornea so that plunger is free to move vertically .
• Scale reading is measured.
• The 5.5 gm weight is initially used.
• If scale reading is 4 or less, additional weight is added to
plunger.
• Conversion table is used to derive IOP in mm Hg from
scale reading and plunger weight.
20. SOURCES OF ERROR
• Accuracy is limited as ocular rigidity varies from eye to
eye.
• As conversion tables are based on an average coefficient
of ocular rigidity; eye that varies significantly from this
value gives erroneous IOP.
• Repeated measurements lower IOP.
• steeper or a thicker cornea causes greater displacement of
fluid during tonometry and gives a falsely high IOP
measurement.
• Schiøtz reads lower than GAT
21. Factors Affecting Scleral Rigidity
• High Scleral Rigidity
• hyperopia
• long standing glaucoma
• ARMD
• vasoconstrictors
22. Factors Affecting Scleral Rigidity
•Low Scleral Rigidity
•increasing age
• high myopia
•miotics
•vasodilators
•Postoperative after RD surgery (vitrectomy,
cryopexy, scleral band)
•intravitreal injection of compressible gas.
•keratoconus (?).
•Low ocular rigidity ----- falsely high scale reading
----- falsely low IOP.
23. LIMITATIONS
• Instrumental errors
• Standardisation - testing labs for certification
• Mechanical obstruction to plunger etc.
• Muscular contractions
• Of extra ocular muscles increase IOP
• Accomodation decreases IOP
• Variations in volume of globe
• Microphthalmos
• High Myopia
• Buphthalmos
• It can be recorded in supine position only
24. Advantages of schiotz tonometer
• Simple technique
• Elegant design
• Portable
• No need for SlitLamp or power supply
• Reasonably priced
• Anodized scale mount which is highly
resistant to sterilizing water.
• Schiotz tonometer is still most widely
tonometer.
25. calibration
• The instrument should be calibrated before
each use by placing it on a polished metal
sphere and checking to be sure that the
scale reading is zero.
• If the reading is not zero, the instrument
must be repaired.
26. sterilization
• The tonometer is disassembled between each use
and the barrel is cleaned with 2 pipe cleaners, the
first soaked in isopropyl alcohol 70 % or
methylated spirit and the second dry.
• The foot plate is cleaned with alcohol swab.
• All surfaces must be dried before reassembling.
• The instrument can be sterilized with ultraviolet
radiation, steam, ethylene oxide.
• As with other tonometer tips, the Schiotz can be
damaged by some disinfecting solutions such as
hydrogen peroxide and bleach.
27. Differential tonometry
• It is done to get rid from ocular rigidity.
• A reading is taken with one weight on the Plunger and then a second
reading' in taken with a different weight.
• Making a diagnosis of glaucoma in a pt. with myopia presents
unusual difficulties. The low ocular rigidity in these eyes result in
Schiotz readings within normal limits.
5.5g 10g Ocular IOP
rigidity
18 mm Hg 15 mm Hg lower >18
18 mm Hg 21 mm Hg higher <18
18 mm Hg 18 mm Hg equal 18
28. APPLANATION TONOMETER
Connects to the slit
lamp
Biprism
(measuring prism)
Feeder arm
Control weight insert
Housing
Adjusting knob
29. PRINCIPLE
• Applanation tonometry is based on the
Imbert-Fick principle, which states that
the pressure (p) inside an ideal dry, thin-
walled sphere equals the force (F)
necessary to flatten its surface divided by
the area of the flattening (A).
F
• P=
A
30. • Cornea being aspherical, wet, and slightly inflexible fails
to follow the law.
• Moisture creates surface tension (S) or capillary
attraction of tear film for tonometry head.
• Lack of flexibility requires force to bend the cornea (B)
which is independent of internal pressure.
• The central thickness of cornea is about 0.55 mm and the
outer area of corneal flattening differs from the inner area
of flattening (A1). It is this inner area which is of
importance.
32. • Modified Imbert-Fick Law is
• W + S = PA1 + B
• When A1 = 7.35 mm2, S balances B and W =P.
• This internal area of applanation is achieved when
the diameter of the external area of corneal
applanation is around 3.06 mm.
• Grams of force applied to flatten 3.06 diameter of
the cornea multiplied by 10 is directly converted
to mmHg.
33. cont…..
The two beam-splitting prism within the
applanating unit optically convert the circular
area of corneal contact in to semicircles
34. cont….
The instrument is mounted
on a standard slit lamp in such
a way that the examiners view
is directed through the centre
of a plastic Biprism.
Biprism is attached by a rod
to a housing which contains a
coil spring and series of levers
that are used to adjust the
force of the biprism against
the cornea.
Two beam splitting prisms
within applanating unit
optically convert circular area
of corneal contact in 2
semicircles.
35. procedure
• The patient is asked not to drink alcoholic beverages as it
will lower IOP and not to take large amounts of fluid (e.g.,
500 ml or more) for 2 hours before the test, as it may raise
the IOP.
• The angle between the illumination and the microscope
should be approximately 60°.
• The room illumination is reduced.
• A fixation light may be placed in front of the fellow eye.
• The tension knob is set at 1 g. If the knob is set at 0, the
prism head may vibrate when it touches the eye and
damage the corneal epithelium.
• The 1 g position is used before each measurement.
36. Procedure cont..
• The palpebral fissure is a little wider if the patient looks
up. However, the gaze should be no more than 15° above
the horizontal to prevent an elevation of IOP.
• After instilling topical anaestheia, Edge of corneal contact
is made apparent by instilling fluorescein while viewing
in cobalt blue light.
• The biprism should not touch the lids or lashes because
this stimulates blinking and squeezing.
• The patient should blink the eyes once or twice to spread
the fluorescein-stained tear film over the cornea, and then
should keep the eyes open wide.
Do not to place any pressure on the globe because this raises
IOP.
37. Procedure cont..
• In some patients, it is necessary for the examiner to hold
the eyelids open with
the thumb and forefinger
of one hand against the
orbital rim.
• By manually rotating a dial calibrated in grams, the force
is adjusted by changing the length of a spring within the
device.
• The prisms are calibrated in such a fashion that inner
margin of semicircles touch when 3.06 mm of the cornea
is applanated.
• The Intra ocular pressure is then read directly from a scale
on the tonometry housing.
38. cont….
The fluorescein rings should be The fluorescent semicircles are viewed
approximately 0.25–0.3 mm in through the biprism and the force against
thickness – or about one-tenth the the cornea is adjusted until the inner
diameter of the flattened area. edges overlap.
40. Effect of central corneal thickness (CCT):
• A thinner cornea may require less force to applanate it,
leading to underestimation of true IOP while a thicker
cornea would need more force to applanate it, giving an
artificially higher IOP.
• The Goldmann applanation tonometer was designed to
give accurate readings when the CCT was 520 μm.
• The deviation of CCT from 520 μm yields a change in
applanation readings of 0.7 mm Hg per 10 μm.
• IOP measurements are
also modified after PRK and
LASIK.
• Thinning of the central
cornea is gives lower readings
on applanation.
41. • Wider meniscus or improper vertical alignment gives
higher IOP readings
• If the two semicircles are not equal in size, IOP is
overestimated.
• For every 3D increase in corneal curvature, IOP raises
about 1 mm Hg as more fluid is displaced under steeper
corneas causing increase in ocular rigidity
• More than 6 D astigmatism produces an elliptical area
on applanation that gives erroneous IOP. 4D with-the-rule
astigmatism underestimate IOP and 4D against-the-rule
astigmatism overestimate IOP.
• Mires may be distorted on applanating on irregular
corneas .
42. • Elevating the eyes more than 15° above the horizontal
causes an overestimation of IOP.
• Widening the lid fissure excessively causes an
overestimation of IOP
• Repeated tonometry reduces IOP, causing an
underestimation of the true level.This effect is greatest
between the first and second readings, but the trend
continues through a number of repetitions.
• A natural bias for even numbers may cause slight errors in
readings.
43. Applanation - Possible Errors
• Falsely low IOP • Falsely high IOP
• too little flouroscein • too much flouroscein
• thin cornea • thick cornea
• steep cornea
• corneal edema • against the rule
• with the rule astigmatism astigmatism
1mm Hg per 3D
• 1mm Hg per 4 D • wider meniscus
• Widening the lid fissure
• prolonged contact excessively
• Repeated tonometry • Elevating the eyes more
than 15°
44. Potential Sources of Error – During Measurement
If the fluorescein rings are too wide, the patient’s eyelids should be blotted
carefully with a tissue, and the front surface of the prism should be dried with
lint-free material.
An excessively wide fluorescein ring can cause IOP to be overestimated
45. Potential Sources of Error – During Measurement
If the rings are too narrow, the patient should blink two or three times to
replenish the fluorescein; additional fluorescein may be added if necessary.
If the fluorescein rings are too narrow,IOP is underestimated.
54. CALIBRATION
• GAT should be calibrated periodically, at least monthly. If
the GAT is not within 0.1 g (1 mmHg) of the correct
calibration, the instrument should be repaired; however,
calibration errors of up to 2.5 mmHg may still be tolerated
clinically.
55. • Following checks are necessary:
• • Check position 0: Turn the zero calibration on the
measuring drum downwards by the width of one
calibration marking, against the index marker.
• When the feeler arm is in the free movement zone, it
should then move itself against the stop piece in the
direction of the examiner.
• • Check position 0.05: Turn the zero calibration on the
measuring drum upwards by the width of one calibration
marking, against the index marker.
• When the feeler arm is in the free movement zone, it
should then move itself against the stop piece in the
direction of the patient.
56. • • Check position at drum setting 2: For checking this
position, check weight is used.
• Five circles are engraved on the weight bar.
• The middle one corresponds to drum position 0, the two
immediately to the left and right to position 2 and the
outer ones to position 6.
• One of the marks on the weight corresponding to drum
position 2 is set precisely on the index mark of the weight
holder.
• Holder and weight are then fitted over the axis of the
tonometer so that the longer part of the weight points
towards the examiner.
57. • Check position 1.95: The feeler arm should move towards
the examiner.
• Check position 2.05.The feeler arm should move in the
direction of the patient.
• • Check at measuring drum setting 6: Turn the weight
bar to scale calibration 6, the longer part shows in the
direction of the examiner.
• • Check position 5.9/6.1 as performed for drum setting 2.
58. sterilization
• Applanation tip should be soaked for 5-15 min in
diluted sodium hypochlorite, 3% H2O2 or 70%
isopropyl alcohol or by wiping with alcohol, H2O2,
povidone iodine or 1: 1000 merthiolate.
• Other methods of sterilization include: 10 min of
rinsing in running tap water, wash with soap and
water, cover the tip with a disposable film, and
exposure to UV light.
• Disposable tonometer tips may also be used
59. When using disposable tips, they have a smooth
applanating surface.
The acrylic disposable tips seem to be somewhat more
accurate than the silicone ones.
While disposable shields or tips may be safer than
disinfection solutions, they are not 100% protective against
prion disease.
60. • It is possible to transfer bacteria, viruses, and other
infectious agents with the tonometer head, including such
potentially serious infections as epidemic
keratoconjunctivitis, hepatitis B, Jacob-Kreutzfeld and,
theoretically, acquired immunodeficiency syndrome.
• Care must be taken to be sure any sterilizing solution has
been completely rinsed off the tonometer tip, as some of
these solutions may be toxic to the corneal epithelium,
especially after LASIK or other corneal procedures.
• If the tonometer tip is not mechanically wiped after each
use, epithelial cells may stick to the tip with the small but
serious risk of transmitting Jacob-Kreutzfeld virus.
61. SAFETY REGULATIONS
• No examination should be undertaken in case of eye
infections (or) injured corneas.
• Only clean and disinfected measuring prism should be
used.
• No damaged prisms should be used.
• If the measuring prism come in to contact with the
cornea without the drum having previously been
correctly set, vibration can occur in the feeler arm,
which will produce unpleasant feeling for the patient.
• The tonometer tips should be examined periodically
under magnification as the antiseptic solutions and
mechanical wiping may cause irregularities in the
surface of the tip that can, in turn, injure the cornea.
62. Perkins tonometer
• It uses same prisms as Goldmann
• It is counterbalanced so that tonometry is
performed in any position
• The prism is illuminated by battery
powered bulbs.
• Being portable it is practical when
measuring IOP in infants / children, bed
ridden patients and for use in operating
rooms.
63.
64. Draeger Tonometer
• Draeger tonometer is similar to Perkins
• It has a different set of prisms
• It operates with a motor.
66. Mackay-Marg Tonometer
• 1.5 mm diameter plunger
• rigid spring
• rubber sleeve.
• Movement of plunger is electronically monitored by a
transducer and recorded on a moving paper strip.
• This instrument is useful for measuring IOP in eyes with
scarred, irregular, or edematous corneas because the end
point does not depend on the evaluation of a light reflex
sensitive to optical irregularity, as does the Goldmann
tonometer.
• It is accurate when used over therapeutic soft contact
lenses.
67. At 1.5 mm of corneal area applanation, tracing reaches a
peak and the force applied = IOP + force required to deform
the cornea.
At 3 mm flattening, force required to deform cornea is
transferred from plunger to surrounding sleeve, creating a
dip in tracing corresponding to IOP.
Flattening of >3 mm of area gives artificial elevation of
IOP.
69. Tono pen
• Portable
• battery operated .
• same principle as that of Mackay-Marg tonometer.
• It is particularly useful in community health fairs, on
ward rounds ,children, irregular surfaces, measuring
through an amniotic membrance patch graft, to read from
the sclera .
• Tono-Pen tends to overestimate the IOP in infants so its
usefulness in congenital glaucoma screening and
monitoring is somewhat limited.
• In band keratopathy where the surface of the pathology is
harder than normal cornea, the Tono-Pen tends to
overestimate the IOP
• A disposable latex cover which is discarded after each use
provides infection control.
70. Pneumatonometer or pneumatic tonometer
• It is like Mackay-Marg tonometer.
• The sensor is a air pressure like electronically controlled
plunger in Mackay-Marg tonometer.
• It can also be used for continuous monitoring of IOP.
71. • It gives significantly higher IOP estimates.
• It has a sensing device that consists of a gas chamber
covered by a polymeric silicone diaphragm.
• A transducer converts the gas pressure in the chamber into
an electrical signal that is recorded on a paper strip.
• The gas in the chamber escapes through an exhaust vent
between the diaphragm and the tip of the support nozzle.
• As the diaphragm touches the cornea, the gas vent is
reduced in size, and the pressure in the chamber rises.
72. Maklakov tonometer
•Indentation
•Pt supine
•wire holder
Dumb-bell-shaped
metal cylinders with
flat end plates of
polished glass
Diameter of 10 mm
The surface of the
weight is painted
with a dye, such as
mild silver protein
(Argyrol) mixed
with glycerin.
1 sec contact
imprint on end plate
73. • IOP = W / π (d/2) 2
• weight (W) diameter of the area of applanation (d)
• Intraocular pressure is measured in grams per square
centimeter and is converted to millimeters of mercury by
dividing by 1.36.
• widely in Russia and China
• This instrument displaces a greater volume of aqueous
humor and thus IOP readings are more influenced by
ocular rigidity.
• It does not correct for corneal bending, capillary
attraction, or tear encroachment on the layer of dye.
• Many instruments similar to the Maklakow device have
been described,like the Applanometer, Tonomat, Halberg
tonometer, and GlaucoTest.
74. The Ocuton tonometer
• The Ocuton™ tonometer
• hand-held tonometer
• works on the applanation principle
• probe is so light that it is barely felt
• needs no anesthetic in most patients.
• It has been marketed in Europe for home tonometry
• useful to get some idea of the relative diurnal variation in IOP if the patient
or spouse (etc.) can learn to use it.
76. • It is a new and updated version of an indentation tonometer
• Portable
• can be used without anesthetizing the eye.
• A very light, disposable, sterile probe is propelled forward
into the cornea .
• The time taken for the probe to return to its resting position
and the characteristics of the rebound motion are indicative
of the IOP.
• The time taken for the probe to return to its resting position
is longer in eyes with lower IOP and faster in eyes with
higher IOP.
77. • It is comparable to the GAT.
• It correlates with central corneal thickness like the
Goldmann, .
• used in screening situations, when patients are unable to
be seated or measured at the slit lamp, or when topical
anesthetics are not feasible or usable.
• Not useful in scarred corneas (as does the Goldmann).
78. Trans palpebral tonometry
used in situations where other, more accurate, devices are not practical,
such as in young children, demented patients and severely
developmentally-challenged patients.
In addition to all the problems facing indentation tonometry, such as
scleral rigidity, transpalpebral tonometry adds variables such as the
thickness of the eyelids, orbicularis muscle tone and potential Intra
palpebral scarring.
79. • Portable. patients can measure their own IOP at home,
DVT
• pressure on the eyelid in most eyes produces retinal
phosphenes.
• The pressure on the eyelid required to induce these
phosphenes is proportional to the intraocular pressure.
• It is not accurate always. inter observer and intra observer
variability was large.subsequent studies failed to confirm
the accuracy of this device.
80. Non contact tonometer
• Noncontact tonometer (NCT) was introduced by Grolman.
• Original NCT has 3 subsystems:
• 1. Alignment system: It aligns patient’s eye in 3 dimensions.
• 2. Optoelectronic applanation monitoring system:
• It comprises transmitter, receiver and detector, and timer.
• a. Transmitter directs a collimated beam of light at corneal apex.
• b. Receiver and detector accept only parallel coaxial rays of light
reflected from cornea.
• c. Timer measures from an internal reference to the point of peak light
intensity.
• 3. Pneumatic system: It generates a
puff of room air directed against cornea
81. PRINCIPLE
• A puff of room air creates a constant force that
momentarily flattens the cornea. The corneal apex is
deformed by a jet of air
• The force of air jet which is generated by a solenoid
activated piston increases linearly over time.
• When the reflected light is at peak intensity, the cornea is
presumed to be flattened.
• The time elapsed is directly related to the force of jet
necessary to flatten the cornea and correspondingly to
IOP.
• The time from an internal reference point to the moment
of flattening is measured and converted to IOP.
82. • A puff of air of known area is generated against cornea (B).
• At the moment of corneal applanation,a light (T), which is
usually reflected from the normal cornea into space,
suddenly is reflected (R) into an optical sensor (A).
• When the sensor is activated by the reflected light, the air
generator is switched off. The level of force at which the
generator stops is recorded, and a computer calculates and
displays the intraocular pressure.
83. • NCT is accurate if IOP is nearly normal, accuracy
decreases with increase in IOP and in eyes with abnormal
cornea or poor fixation.
• It is useful for screening programs because it can be
operated by non-medical personnel
• It does not absolutely require topical anesthesia .
• There is no direct contact between instrument and the eye.
• The patient should be warned that the air puff can be
startling.
• The non-contact tonometer measures IOP over very short
intervals, so it is important to average a series of readings.
• New NCT, Pulsair is a portable hand held tonometer.
84.
85. Ocular Response Analyzer
• It is an adaptation of the non-contact tonometer.
• It directs the air jet against the cornea and measures not one but two
pressures at which applanation occurs
• 1) when the air jet flattens the cornea as the cornea is bent inward and
2) as the air jet lessens in force and the cornea recovers.
86. Ocular response analyser
• The first is the resting intraocular pressure.
• The difference between the first and the second
applanation pressure is called corneal hysteresis
• corneal hysteresis is a measure of the viscous
dampening and, hence, the biomechanical properties of
the cornea.
• The biomechanical properties of the cornea are related to
corneal thickness and include elastic and viscous
dampening attributes.
87. • IOP correlate well with Goldmann tonometry but, on
average, measure a few millimeters higher.
• Further , while IOP varies over the 24-hour day,
hysteresis seems to be stable.
• Congdon et al found that a ‘low’ hysteresis reading with
the ORA correlates with progression of glaucoma,
whereas thin central corneal thickness correlates with
glaucoma damage.
• It has practical value in the management of glaucoma.
89. • Introduced by Kanngiesser
• It is based on a totally different concept other than
indentation or applanation tonometry.
• Principle : By surrounding and matching the contour of a
sphere (or a portion thereof ), the pressure on the outside
equals the pressure on the inside.
• The tip of the probe matches the contour of the cornea.
• A pressure transducer built into the center of the probe
measures the outside pressure, which should equal the
inside pressure, and the IOP is recorded digitally on the
liquid crystal display (LCD).
90. • The concept developed from a previous contact lens
tonometer called the ‘Smart Lens”.
• It superior in accuracy to Goldmann tonometry and
pneumotonometry .
• IOP is not affected by corneal thickness.
• IOP is not altered by corneal refractive surgery that thins
the cornea.
91. • Because the DCT measures IOP in real time, the actual
measurement, like the IOP, is pulsed. The internal
electronics ‘call’ the IOP as the bottom of the pulsed
curve and indicate it digitally on the LCD ..
• IOP readings with the DCT are generally lower than GAT
because, when properly done, indicates the average
difference between the maximum and minimum pressures
whereas the DCT reads the minimum.
92. Ocular pulse amplitude
• The DCT indicates the magnitude of the difference
between maximum and minimum IOP as the ocular
pulse amplitude.
• OPA may be indicative of the status of ocular blood flow
and be differentially affected in different types of
glaucoma.
• ocular pulse amplitude is
increased over normals in
most forms of glaucoma and
may be related to the level
of IOP.
93. Continuous monitoring of intraocular
pressure
• Applanation instruments inside contact lenses or suction
cups or strain gauges in encircling bands that resemble
scleral buckling elements.
• None of these instruments has achieved widespread use.
• resonance applanation tonometry measuring the sonic
resonance of the eye when a continuous force over a fixed
area is applied.
• use of infrared spectroscopy to measure IOP.
• To build a miniature pressure sensor that can reside
inside the eye; one such device is part of an intraocular
lens.
94. Tonometry for Special Clinical Circumstances
• Tonometry on Irregular Corneas
• The accuracy of Goldmann and Tono-Pen tonometers and
the noncontact tonometers is limited in eyes with irregular
corneas.
• The pneumatic tonometer has been shown to be useful in
eyes with diseased or irregular corneas .
• Tonometry over Soft Contact Lenses
• Pneumo tonometry and the Tono-Pen can measure with
reasonable accuracy the IOP through bandage contact
lenses .
• pneumotonometer correlates well with manometrically
determined IOP, whereas the Tono-Pen consistently
underestimates the pressure.
95. • Tonometry with Gas-Filled Eyes
• Intraocular gas affects scleral rigidity, rendering
indentation tonometry unsatisfactory.
• pneumatic tonometer and Tono-Pen used.
• A pneumatic tonometer underestimates Goldmann IOP
measurements in eyes with intravitreal gas
• Tono-Pen compares favorably with Goldmann readings.
• Both instruments significantly underestimated the IOP at
pressures greater than 30 mm Hg .
96. • Tonometry with Flat Anterior Chamber
• IOP readings from the Goldmann applanation tonometer,
pneumotonometer, and Tono-Pen do not correlate well
with manometrically determined pressures.
• Tonometry in Eyes with Keratoprostheses
• In patients at high risk for corneal transplant rejection,
implantation of a keratoprosthesis is now a viable option
for vision rehabilitation .
• Most keratoprostheses have a rigid, clear surface, it is
impossible to measure IOP by using applanation or
indentation instruments.
• In such eyes, tactile assessment appears to be the most
widely used method to estimate IOP.
