Q (1) Applied physiology (explain the following observations):
a- Improvement in lamellar corneal surgery is based on peculiar corneal physiological characteristics. (5 marks)b- Recently aphakics or pseudophakics may experience a slight change/tinge in their color perception. (5marks)
(1a) Improvement in lamellar corneal surgery is based on peculiar corneal physiological characteristics. (5 marks)
The cornea is avascular and immune privileged, so graft rejection is low.
The patient's healthy endothelium is preserved.
The corneal endothelium has very limited regenerative capacity.
Preserving the endothelium maintains corneal deturgescence.
This maintains corneal transparency and clear vision.
(1b) Recently aphakic or pseudophakic patients may experience a slight change in color perception. (5 marks)
The natural crystalline lens is yellow and absorbs some blue light.
After lens removal, more blue light reaches the retina.
Colors may initially appear slightly bluish or brighter.
The change is usually mild and temporary.
The brain gradually adapts to the new color perception.
a- Panum`s area and mention its clinical value (5 marks)
b- Black out index and its significance (5marks)
a) Panum's area and its clinical value.
Panum's area is the region around the horopter where images are perceived as single.
It allows sensory fusion and stereopsis.
Outside Panum's area, physiological diplopia occurs.
It provides the fusional range.
It is clinically important in strabismus (squint).
Panum's area is the region around the horopter where images are perceived as single.
It allows sensory fusion and stereopsis.
Outside Panum's area, physiological diplopia occurs.
It provides the fusional range.
It is clinically important in strabismus (squint).
b) Blackout index and its significance.
Blackout index is the percentage of time vision is interrupted by blinking.
It depends on the blink duration and interblink interval.
Blink duration is about 0.1–0.3 seconds.
The visual cortex suppresses the brief interruption.
Therefore, continuous vision is normally perceived despite blinking.
(5) What are the causes of anisocoria. How can different levels of illumination guide your diagnosis? (10 marks).
Blackout index is the percentage of time vision is interrupted by blinking.
It depends on the blink duration and interblink interval.
Blink duration is about 0.1–0.3 seconds.
The visual cortex suppresses the brief interruption.
Therefore, continuous vision is normally perceived despite blinking.
Causes:
Physiological anisocoria.
Horner syndrome.
Third nerve palsy.
Adie tonic pupil.
Drugs.
Iris damage.
Illumination:
Dark: Small pupil abnormal → Horner syndrome.
Bright: Large pupil abnormal → 3rd nerve palsy, Adie, drugs, iris damage.
Same in dark and light: Physiological anisocoria.
Q(6) What are the factors affecting dark adaptation? (10 marks).
- Duration in the dark: cones adapt first (5 to 10 minutes), then rods (up to 30 minutes).
- Pre-exposure light: brighter and longer previous light means slower adaptation.
- Vitamin A: needed for rhodopsin regeneration, so deficiency slows adaptation.
- Retinal area: rods in the periphery adapt best.
- Wavelength: red light does not affect rods.
- Pupil size and age: older age is slower.
- Oxygen shortage and retinal diseases (retinitis pigmentosa).
Q(7) Describe the types of visual acuity criteria? (10 marks)
- Minimum visible: the smallest object that can be detected.
- Minimum separable (resolution): the smallest gap between two objects that can be seen as separate (Landolt C, grating).
- Minimum legible (recognition): the smallest letter or symbol that can be identified (Snellen chart).
- Minimum discriminable (hyperacuity): the smallest misalignment or displacement detected (vernier acuity).
(8) Define and describe types of phosphenes
Definition: Phosphene is a sensation of light without external light stimulation.
Types:
Pressure (mechanical): Pressure on the eyeball produces a light sensation, usually opposite the pressure.
Electrical: Produced by electrical stimulation of the retina or visual pathway.
Traction: Vitreoretinal traction causes flashes of light (photopsia).
Blow: “Seeing stars” after trauma to the eye or head.
Movement (saccadic): Light sensation during sudden eye movements in darkness.
Cortical: Stimulation of the visual cortex, e.g. migraine aura.
Q(9) Mention the functions of retinal pigment epithelium (10 marks)
- Absorbs scattered light (melanin), giving a sharper image.
- Visual cycle: regenerates 11-cis retinal and stores vitamin A.
- Phagocytosis: eats the shed outer segments of photoreceptors.
- Outer blood-retinal barrier (tight junctions).
- Transport: brings nutrients to photoreceptors and removes waste, ions, and water.
- Secretes growth factors (such as vascular endothelial growth factor).
- Keeps the retina attached and protects against oxidative damage.
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