ophthalmology residents

1. Corneal Physiology
Through Numbers
Mohammed Bilal Khalil
22/9/2016
90% of the incident light is transmitted

2. Corneal Clarity
1-Physical Factors
2-Hydration

3. PHYSICAL FACTORS
Epithelium
5-7cell layers: flat superficial
deeper winged
columnar basal cells
50 μm
10% of the total corneal thickness
“Surface irregularities caused by epithelial
edema are more damaging to vision than
stromal edema or scarring”

4. PHYSICAL FACTORS
Stroma
5% is keartocytes
0.01% nerves
70% water
25% collagen and GAGs
90% of the total corneal thickness
“When the stroma swells, the diameter of the
collagen fibrils remains constant”

5. PHYSICAL FACTORS
Endothelium
3000 to 3500 cells/mm2 in the young adult
decreases by about two-thirds in elderly
5 μm
usually if <500 cells/mm2,
depending on the health of the
cells, corneal edema ensues.

6. CORNEAL HYDRATION
1. Stromal Swelling Pressure
2. Barrier Function
3. Endothelial Pump
4. Evaporation
5. IOP

7. CORNEAL HYDRATION
1. Stromal Swelling Pressure
Relative dehydrated state
Excised cornea 78%  98% in aqueous
Ability to swell decreases with hydration
Principles and Practices of Ophthalmology. Vol 2

9. CORNEAL HYDRATION
1. Stromal Swelling Pressure
IP = IOP – SP (IP < SP)
=-30-40 mmHg
SP = IOP + 30-40
= 10-20 + 30-40 = 40-60 mmHg
excised cornea IP= -SP = -50 mm Hg
GAG high resistance to water flow
GAG resistance reduced with hydration

10. Barrier Function of Epithelium and
Endothelium
• to diffusion of electrolytes and to the flow
(not diffusion) of water
• The epithelium offers twice the resistance to
flow of water than the endothelium, and
relative resistance to diffusion of electrolytes
is 200 times greater in the epithelium than in
the endothelium.

13. Endothelial Pump
• “temperature reversal” of hydration

15. Evaporation From the Corneal Surface
• only 4% of thinning occurs in humans as a
result of osmotic extraction of fluid due to
tears made hypertonic by evaporation

16. Intraocular Pressure
• when the IP becomes positive—that is, when
the IOP exceeds the SP—epithelial edema
ensues