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Ophthalmology Viva & Practical - Complete Exam Notes

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1. Bones Forming the Orbit (7 Bones)

• Maxilla (floor + medial wall)
• Zygomatic (lateral wall + floor)
• Ethmoid (medial wall - "paper plate"/lamina papyracea - thinnest part)
• Lacrimal (medial wall)
• Sphenoid (lateral wall, apex)
• Frontal (roof)
• Palatine (small contribution to floor)

The medial wall is thinnest (ethmoid/lamina papyracea) - most vulnerable to blowout fractures. Average orbital volume = 30 mL.

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2. Layers of the Cornea (5 Layers)

1. Epithelium - stratified squamous non-keratinized, 5-6 cell layers thick; regenerates rapidly (24-48 hrs). Innervated by ophthalmic branch of CN V1.
2. Bowman's layer - acellular condensed stroma; does NOT regenerate if damaged (scars form).
3. Stroma - thickest layer (~90% of corneal thickness); collagen lamellae (type I) arranged in highly regular orthogonal pattern - key to transparency.
4. Descemet's membrane - basement membrane of endothelium; regenerates if damaged.
5. Endothelium - single layer of cells that pump fluid out of stroma (maintains dehydration = transparency). Does NOT regenerate.

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3. Factors Affecting Corneal Transparency

1. Avascularity - no blood vessels in normal cornea
2. Regular collagen arrangement - stroma collagen fibrils of uniform diameter and spacing (destructive interference of scattered light)
3. Relative dehydration - maintained by endothelial pump; cornea is kept at ~78% water content (vs ~100% hydration = opaque)
4. Smooth surface - covered by tear film for uniform refracting surface
5. No myelination of nerve fibers within cornea

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4. Nutrition/Derivation of Cornea

• Aqueous humor (primary source - glucose, amino acids via diffusion from anterior chamber)
• Tear film (oxygen from atmosphere)
• Limbal capillaries (peripheral cornea only)

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5. Power of the Eye / Eyeball

• Cornea contributes ~70% of total refracting power
• Lens contributes ~30% (but can change with accommodation - the cornea cannot)
• The lens is a biconvex structure - anterior surface less curved (~10 mm radius), posterior surface more curved (~6 mm radius)

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6. Visual Pathway (Step by Step)

Photoreceptors (rods & cones) in retina
        ↓
Bipolar cells (1st order neuron)
        ↓
Ganglion cells (2nd order neuron) → axons form optic nerve (CN II)
        ↓
Optic chiasm (nasal fibers decussate; temporal fibers stay ipsilateral)
        ↓
Optic tract → lateral geniculate nucleus (LGN) of thalamus
        ↓
Optic radiations (3rd order neuron)
  - Upper fibers → superior lip of calcarine cortex
  - Lower fibers (Meyer's loop, temporal lobe) → inferior lip
        ↓
Primary visual cortex (V1), occipital lobe, area 17 (calcarine cortex)

In many exam contexts: 1st = photoreceptor/bipolar, 2nd = ganglion cell (optic nerve/tract), 3rd = LGN to cortex.

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7. Pupillary Reflex Pathway (Light Reflex)

Afferent limb:

Efferent limb:

• Light in one eye → both pupils constrict
• Direct response = ipsilateral constriction
• Consensual response = contralateral constriction

RAPD (Relative Afferent Pupillary Defect) / Marcus Gunn Pupil:

• Tested with swinging flashlight test
• In optic nerve lesion on one side: when light swings TO the affected eye, both pupils dilate (paradoxical dilation)
• Indicates defective afferent limb (asymmetric optic nerve disease)

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8. Layers of the Retina (10 Layers)

1. Retinal Pigment Epithelium (RPE)
2. Photoreceptor layer (outer segments of rods & cones)
3. External limiting membrane (tight junctions between Müller cells & photoreceptors)
4. Outer nuclear layer (cell bodies of photoreceptors)
5. Outer plexiform layer (synapses between photoreceptors & bipolar cells)
6. Inner nuclear layer (cell bodies of bipolar, horizontal, amacrine, Müller cells)
7. Inner plexiform layer (synapses between bipolar & ganglion cells)
8. Ganglion cell layer (cell bodies of ganglion cells)
9. Nerve fiber layer (axons of ganglion cells forming optic nerve)
10. Internal limiting membrane (inner boundary, Müller cell footplates)

Fovea: Central pit in macula, ~1.5 mm from optic disc; contains only cones; no blood vessels (foveal avascular zone); thinnest retina; highest visual acuity. Inner layers are displaced peripherally.

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9. Tear Film (3 Layers)

1. Lipid layer (outermost) - secreted by Meibomian glands (tarsal glands); prevents evaporation
2. Aqueous layer (middle, thickest ~98% of tear film) - secreted by lacrimal gland (main) and accessory lacrimal glands (Krause, Wolfring); contains antibodies (IgA), lysozyme, lactoferrin
3. Mucin layer (innermost) - secreted by goblet cells of conjunctiva; allows tear film to spread over hydrophobic corneal epithelium

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10. Lacrimal Gland and Sac - Location

• Located in the lacrimal fossa, in the superolateral angle of the orbit (frontal bone)
• Divided by levator aponeurosis into orbital (larger, upper) and palpebral (smaller, lower) lobes
• Nerve supply: Parasympathetic from CN VII (facial) → greater petrosal nerve → pterygopalatine ganglion → zygomatic nerve → lacrimal gland

• Lies in the lacrimal groove (between anterior and posterior lacrimal crests) of the medial orbital wall
• Extends slightly above medial canthal tendon (upper part is the fundus)

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11. Anterior Chamber

• Bounded anteriorly by cornea, posteriorly by iris and pupil (and lens in the pupil area)
• Filled with aqueous humor
• Angle (iridocorneal angle) contains trabecular meshwork → Canal of Schlemm → aqueous drainage
• Depth: ~3 mm centrally
• Aqueous humor is produced by ciliary body (ciliary processes) by active secretion + ultrafiltration; drains via trabecular meshwork (conventional, 90%) and uveoscleral route (unconventional, 10%)

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12. Visual Acuity Testing

Snellen Chart (standard):

• Tested at 6 metres (or 20 feet)
• Normal vision = 6/6 (20/20)
• Format: 6/X means patient reads at 6 m what a normal eye can read at X metres
• Each letter subtends 5 minutes of arc at designated distance

Procedure:

1. Test each eye separately (cover the other)
2. Use pinhole to differentiate refractive error from pathology (pinhole improves acuity if refractive)
3. Note best-corrected visual acuity

Near vision: Jaeger's chart or N-notation tested at 33 cm

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13. Confrontation Test

1. Examiner sits ~1 metre facing patient, at same level
2. Patient covers one eye; examiner covers opposite eye
3. Patient fixes on examiner's eye/nose
4. Examiner moves finger/target from periphery inward in all quadrants
5. Patient reports when they first see the target; compared to examiner's own field (assumed normal)

• Not quantitative / cannot map precise field defects
• Depends on examiner having normal visual fields (inherent flaw)
• Cannot detect subtle field losses (e.g., early glaucoma)
• Poor at detecting small central scotomas
• Inter-examiner variability
• Not sensitive for small defects

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14. Functions of Vitreous Humor

• Maintains shape of the posterior eyeball (fills vitreous chamber)
• Optical clarity - allows light transmission to retina (99% water, gel-like collagen/hyaluronic acid matrix)
• Metabolic support - slight nutritive function for lens and retina
• Shock absorption - protects retina and lens from trauma
• Holds retina in place against RPE
• Supports lens (anteriorly)

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15. Optic Nerve

• Formed by axons of retinal ganglion cells
• Has 4 parts: intraocular (optic disc, 1 mm), intraorbital (25-30 mm), intracanalicular (6-10 mm in optic canal), intracranial (10 mm to chiasm)
• Covered by dural, arachnoid, and pial sheaths (continuous with brain meninges) - hence raised ICP causes papilloedema
• Exits orbit via optic canal (in lesser wing of sphenoid)
• Blood supply: Posterior ciliary arteries + ophthalmic artery branches

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16. Cross-Section of Eyeball (Drawing Points)

1. Fibrous coat - Sclera (posterior 5/6) + Cornea (anterior 1/6)
2. Vascular coat (Uvea) - Choroid + Ciliary body + Iris
3. Neural coat - Retina

• Cornea, lens (biconvex), iris, pupil
• Anterior chamber (between cornea and iris/lens)
• Posterior chamber (between iris and lens)
• Vitreous chamber (posterior to lens)
• Ciliary body, suspensory ligaments (zonules) of lens
• Optic disc, optic nerve
• Fovea/macula
• Retina, choroid, sclera

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Quick Summary Table for Viva