The eye: anatomy, eye movements and palsies

1. The coats of the eye

The globe has three coats: the tough white sclera, continuous in front with the clear cornea; the vascular middle layer, the uvea (iris, ciliary body, choroid); and the retina, the neural layer lining the back. The vitreous gel fills the posterior segment; aqueous humour fills the anterior segment in front of the lens.

Surgeon’s corner: Penetrating eye injury: a teardrop-shaped pupil or a soft eye. Never press on the globe: shield it and refer — and no MRI if a metallic foreign body is possible.

2. Cornea, iris and the drainage angle

The cornea provides about two thirds of the eye’s focusing power. Aqueous humour, made by the ciliary body behind the iris, flows through the pupil into the anterior chamber and drains at the angle between iris and cornea, through the trabecular meshwork into Schlemm’s canal.

Surgeon’s corner: Acute angle-closure glaucoma: the iris blocks the angle and the pressure rises fast — red painful eye, halos, a fixed mid-dilated pupil, nausea. An emergency: lower the pressure medically, then laser iridotomy to both eyes (the other eye is at high risk).

3. Lens and accommodation

The lens hangs on the zonular fibres from the ciliary body. Looking near, the ciliary muscle contracts, the zonules slacken and the elastic lens rounds up (accommodation, together with pupil constriction and convergence: the near triad). With age the lens stiffens (presbyopia) and clouds (cataract).

Surgeon’s corner: Cataract surgery — removing the clouded lens and implanting an artificial one — is one of the commonest operations in the world. A lens dislocated upwards and outwards suggests Marfan syndrome.

4. Retina, optic disc and the central retinal artery

The photoreceptors of the retina convert light into signals; the macula, temporal to the optic disc, gives sharp central vision. The nerve fibres converge on the optic disc (the blind spot) and leave as the optic nerve. The central retinal artery, a branch of the ophthalmic artery, enters within the nerve and feeds the inner retina; the choroid feeds the outer layers.

Surgeon’s corner: Central retinal artery occlusion: sudden painless loss of vision, a pale retina with a cherry-red spot at the macula — an eye stroke: urgent stroke work-up and, in older patients, exclude giant cell arteritis. Papilloedema: swollen discs from raised intracranial pressure.

5. Six muscles move the eye

Four recti arise from the tendinous ring at the orbital apex; the superior oblique turns through its pulley (trochlea) and the inferior oblique arises from the orbital floor. Clinically each muscle is tested where it acts most purely: lateral rectus looking out, medial rectus looking in, superior and inferior rectus looking out and up or down, the obliques looking in and down (superior) or in and up (inferior). Tap a muscle in the panel.

Surgeon’s corner: LR6 SO4, the rest 3: the lateral rectus is supplied by the abducens nerve, the superior oblique by the trochlear nerve, all the others (and levator palpebrae) by the oculomotor nerve.

6. Oculomotor (third) nerve palsy

A complete third nerve palsy: ptosis, the eye down and out, and a dilated pupil that does not react. The pupil fibres run on the outside of the nerve: compression (posterior communicating artery aneurysm, uncal herniation) affects the pupil early, while small-vessel (diabetic) palsies usually spare it — but a spared pupil does not rule out an aneurysm, especially when the palsy is partial.

Surgeon’s corner: A new third nerve palsy that involves the pupil, painful or not, is a posterior communicating artery aneurysm until proven otherwise: same-day CT or MR angiography. Small-vessel palsies are often painful too. Image urgently any partial palsy, any patient under 50 and anyone without vascular risk factors, even with a spared pupil.

7. Trochlear and abducens palsies

Fourth nerve: the affected eye drifts up; vertical double vision is worst looking down and in (reading, stairs) and the head tilts away from the affected side. Head trauma is the commonest acquired cause; decompensated congenital and small-vessel palsies are also common. Sixth nerve: the eye turns in and cannot look out; horizontal double vision is worse towards the affected side. Compare both in the panel.

Surgeon’s corner: The abducens nerve has a long subarachnoid course over the petrous apex: raised intracranial pressure can stretch it on either side, a false localising sign. A new sixth nerve palsy in a child needs imaging.

8. Tears: the lacrimal apparatus

The lacrimal gland, in the upper outer orbit, secretes tears (its parasympathetic fibres come from the facial nerve via the pterygopalatine ganglion). Blinking sweeps the tears inwards to the puncta; they drain through the canaliculi into the lacrimal sac and down the nasolacrimal duct into the inferior meatus of the nose.

Surgeon’s corner: Congenital nasolacrimal duct obstruction (a watery, sticky eye in a baby) usually resolves by the first birthday; a swollen, tender sac at the inner corner of the eye is dacryocystitis.