exam opthalmic physiology cairo 9 april 2025

 

QUESTION 1 (10 marks)

Define:
a) Abnormal retinal correspondence. (5 marks)
b) Purkinje shift phenomenon. (5 marks)

Q1. Define (10 marks)

a) Abnormal Retinal Correspondence (ARC) — 5 marks

• Definition: Sensory adaptation to strabismus where the fovea of the fixing eye corresponds with an extrafoveal point of the deviating eye, producing anomalous binocular single vision.

• Harmonious ARC: Angle of anomaly = angle of squint.

• Unharmonious ARC: Angle of anomaly < angle of squint.

• Common in early-onset, small-angle strabismus.

• Tests: Bagolini glasses, Worth 4-dot, after-image test, synoptophore.

• Significance: May cause paradoxical diplopia after surgery.

b) Purkinje shift — 5 marks

• Definition: Shift of peak spectral sensitivity toward the blue end from photopic to scotopic vision.

• Photopic: Cones, about 555 nm.

• Scotopic: Rods, about 507 nm.

• In dim light, blue/green appear brighter and red darker.

• Absent at the fovea.

(Q2)“Describe the physiological basis of corneal deturgescence.” (10 marks)

Definition: Relative dehydration of cornea needed for transparency.

GAGs: Cause imbibition pressure and water retention.

Epithelium: Barrier to fluid entry.

Endothelial pump: Na⁺/K⁺-ATPase removes ions; water follows by osmosis.

Evaporation + IOP: Help prevent corneal swelling.

Result: Maintains corneal transparency.

(3) What are the theories of presbyopia? (10 marks)

Presbyopia: Age-related loss of accommodation.

1. Helmholtz: Lens loses elasticity.

2. Donders: Lens sclerosis.

3. Duane: Ciliary muscle weakness.

4. Hess-Gullstrand: Lens sclerosis despite strong ciliary muscle.

5. Schachar: Reduced zonular tension.

6. Strenk: Lens growth changes zonular geometry.

Q(6)Define critical fusion frequency? Describe the factors affecting it. (10 mark)

Definition: Lowest flicker frequency at which light appears steady.

Factors:

1. Luminance: brighter light → CFF increases.

2. Stimulus size: larger area → CFF increases.

3. Retina: fovea/periphery affect CFF.

4. Receptors: cones > rods.

5. Adaptation: light adaptation increases CFF.

6. Age: CFF decreases with age.

7. Fatigue/drugs: CFF decreases.

8. Disease: optic nerve disease decreases CFF.

Normal: about 50–60 Hz.

(7)  Define and describe phototransduction. (10 marks)

Definition: Conversion of light into an electrical signal in rods and cones.

1. Dark: cGMP high → channels open → depolarization.

2. Light: photon activates rhodopsin.

3. Rhodopsin → transducin.

4. Transducin → PDE6.

5. PDE6 decreases cGMP.

6. Channels close → hyperpolarization.

7. Glutamate decreases.

8. Signal passes to bipolar cells.

9. Rhodopsin kinase + arrestin stop the response.

10. RPE regenerates 11-cis retinal.

(8)   What are the theories of color vision? (10 marks)

1. Young–Helmholtz: 3 cones — S, M, L.

2. Hering: red-green, blue-yellow, black-white.

3. Hurvich–Jameson: trichromatic + opponent.

4. Land (Retinex): color constancy.

5. Ladd-Franklin: evolution of color vision.

6. Granit: dominators + modulators.

Q (9) Describe the physiological basis of autoregulation of retinal circulation. (10 mark)

1. Keeps retinal blood flow constant.
2. Regulation is mainly local.
3. Myogenic: pressure ↑ → constriction.
4. Metabolic: low O₂ / high CO₂ → dilation.
5. NO: dilation.
6. Endothelin: constriction.
7. Neurovascular coupling: neural activity → increased flow.
8. Pericytes/glia regulate vessels.
9. Works within a limited pressure range.
10. Impaired in diabetes, hypertension, glaucoma, ischemia.

END EXAM 

GOOD LUCK