Anatomy and Examination of the Eye - High-Yield Notes

1. Basic Anatomy of the Eye

A. Orbit and adnexa

• Orbit: Bony cavity containing the globe, extraocular muscles, optic nerve, vessels, nerves, and orbital fat.
• Eyelids: Protect the eye and spread tear film.
  • Levator palpebrae superioris elevates upper lid, supplied by CN III.
  • Superior tarsal muscle assists lid elevation, supplied by sympathetics.
  • Orbicularis oculi closes eyelids, supplied by CN VII.
• Conjunctiva: Thin mucous membrane lining the inner eyelids (palpebral conjunctiva) and covering anterior sclera (bulbar conjunctiva).

B. Coats of the globe

C. Important structures

• Cornea: Transparent anterior surface. Major refractive surface of the eye.
• Sclera: Tough white outer coat.
• Iris: Coloured diaphragm controlling pupil size.
  • Parasympathetic: constriction, miosis
  • Sympathetic: dilation, mydriasis
• Pupil: Central aperture in iris.
• Ciliary body:
  • Produces aqueous humour.
  • Ciliary muscle changes lens shape during accommodation.
• Lens: Biconvex, transparent structure that fine-focuses light on retina.
• Choroid: Vascular pigmented layer supplying outer retina.
• Retina:
  • Macula: Area for central detailed vision.
  • Fovea: Centre of macula, highest visual acuity.
  • Optic disc: Exit point of optic nerve, physiologic blind spot.
• Optic nerve (CN II): Carries visual information from retina to brain.

D. Chambers and fluids

• Anterior chamber: Between cornea and iris.
• Posterior chamber: Between iris and lens.
• Both contain aqueous humour.
• Vitreous chamber: Behind lens, filled with vitreous gel.

E. Extraocular muscles and nerve supply

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2. Method of Examination of the Eye

General principles

• Examine each eye separately, then both eyes together.
• Compare both sides.
• Use the sequence:
  Visual acuity → visual fields → external exam → ocular movements → pupils → anterior segment → intraocular pressure → fundus.
• Ask about pain, redness, discharge, trauma, photophobia, floaters, flashes, diplopia, loss of vision, spectacles/contact lenses, and systemic disease.

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A. Visual acuity

Method

• Test with a Snellen chart at 6 m or 20 ft.
• Test right eye, left eye, then both eyes, with the other eye completely covered.
• Test using the patient’s usual corrective lenses.
• If acuity is reduced, use a pinhole.

Interpretation

• 6/6 or 20/20: Normal acuity.
• Pinhole improvement suggests a refractive error.
• If the patient cannot read the chart, record:
  • CF: Counts fingers
  • HM: Hand movements
  • PL: Perception of light
  • NPL: No perception of light

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B. Visual fields

Confrontation test

1. Sit opposite the patient at eye level.
2. Patient covers one eye and examiner covers the opposite eye.
3. Bring fingers from the periphery toward the centre in all four quadrants.
4. Compare the patient's field with your own.

Defects

• Monocular visual-field loss: Eye, retina, or optic nerve lesion.
• Bitemporal hemianopia: Optic chiasm lesion.
• Homonymous hemianopia: Retrochiasmal lesion.
• Central scotoma: Macular disease or optic neuritis.

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C. External examination

• Eyelid position: ptosis, lid lag, entropion, ectropion.
• Eyelid swelling, masses, crusting.
• Conjunctival injection, chemosis, haemorrhage, discharge.
• Corneal clarity, opacity, ulcer, foreign body.
• Globe position:
  • Proptosis/exophthalmos: Forward displacement.
  • Enophthalmos: Posterior displacement.
• Facial sensation around the orbit, especially after trauma.

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D. Eye movements and alignment

Inspection

• Strabismus or squint
• Nystagmus
• Head tilt
• Diplopia
• Limitation of movement

H-test

Key movements

• Abduction: Lateral rectus, CN VI
• Adduction: Medial rectus, CN III
• Elevation in adduction: Inferior oblique, CN III
• Depression in adduction: Superior oblique, CN IV
• Elevation in abduction: Superior rectus, CN III
• Depression in abduction: Inferior rectus, CN III

Cover tests

• Cover-uncover test: Detects manifest squint, or tropia.
• Alternate cover test: Detects latent squint, or phoria.

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E. Pupillary examination

1. Size
2. Shape
3. Equality
4. Direct light reflex
5. Consensual light reflex
6. Near response
7. Relative afferent pupillary defect, RAPD

Pupillary reflex pathway

• Afferent limb: Retina → optic nerve, CN II
• Efferent limb: Oculomotor nerve, CN III → sphincter pupillae

Swinging flashlight test

• Normally, both pupils constrict equally when either eye is illuminated.
• In an RAPD / Marcus Gunn pupil, both pupils dilate relatively when the light is moved to the affected eye.
• This suggests significant unilateral or asymmetric optic nerve disease or severe retinal disease.

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F. Examination of the anterior segment

Torchlight examination

• Lids and lashes
• Conjunctiva
• Cornea
• Anterior chamber depth
• Iris
• Pupil
• Lens, if visible

Slit-lamp examination

• Eyelids and conjunctiva
• Corneal epithelium and stroma
• Anterior chamber cells/flare
• Iris
• Lens
• Anterior vitreous

Fluorescein staining

• Corneal abrasion
• Corneal ulcer
• Foreign body-related epithelial defect
• Dendritic ulcer in herpes simplex keratitis
• Seidel sign, indicating aqueous leak from corneal perforation

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G. Intraocular pressure

Tonometry

• Applanation tonometry: most accurate standard method.
• Non-contact tonometry.
• Tono-Pen.

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H. Fundus examination

Direct ophthalmoscopy

1. Red reflex
2. Optic disc
3. Retinal vessels
4. Macula
5. Peripheral retina, as far as visible

Normal findings

• Red reflex: Uniform orange-red glow.
• Optic disc: Pink, well-defined margins.
• Cup-disc ratio: Usually less than 0.5 and similar in both eyes.
• Macula: Darker area temporal to disc, with foveal light reflex in young people.
• Vessels: Arteries narrower and brighter than veins.

Important abnormal findings

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3. Rapid Examination Checklist

• Visual acuity
• Visual fields
• External examination and extraocular movements
• Pupils
• Slit lamp and staining
• Fundus examination

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4. High-Yield Clinical Correlations

• Painful red eye + reduced vision + halos + nausea/vomiting: Think acute angle-closure glaucoma.
• Painful red eye + photophobia + corneal fluorescein uptake: Consider keratitis/corneal ulcer.
• Painless sudden visual loss + RAPD: Consider optic nerve or retinal pathology.
• Flashes, floaters, curtain-like loss of vision: Suspect retinal detachment.
• Ptosis + dilated pupil + ophthalmoplegia: CN III palsy, potentially compressive and urgent.
• Diplopia worse on looking laterally: CN VI palsy.
• Diplopia worse on looking down and in: CN IV palsy.
• Papilloedema: Raised intracranial pressure until proven otherwise.