Orbit and extraocular muscles

Bony orbit

• Blowout fracture: numb cheek and upper lip (infraorbital nerve, V2), diplopia on upgaze (inferior rectus trapped), enophthalmos.
• The eyeball is slung in a hammock: the suspensory ligament of Lockwood, a thickening of Tenon's capsule between the medial and lateral check ligaments. After maxillectomy removing the orbital floor, it keeps the globe almost level (late enophthalmos still needs reconstruction).

Eyelids and the orbital septum

• Upper lid from front to back: skin, orbicularis oculi, orbital septum, preaponeurotic fat pad, levator aponeurosis, Müller muscle, tarsal plate, conjunctiva. The fat lies behind the septum and in front of the levator, a key surgical landmark.
• The orbital septum runs from the orbital rim to the tarsal plates. Infection in front of it is preseptal cellulitis (red swollen lid, normal vision, pupils and movements, no proptosis); behind it, orbital cellulitis (proptosis, painful restricted movement, reduced vision).
• Ptosis: levator palpebrae superioris (III) lifts the lid; Müller muscle (sympathetic) adds 1–2 mm. Aponeurotic ptosis in the elderly or after cataract surgery: the aponeurosis disinserts, so the crease is high and the lid thin, but levator function is normal. A new ptosis with a dilated pupil is a third nerve palsy until proved otherwise.

Extraocular muscles

• Four recti arise from the common tendinous ring, forming the muscle cone. III, VI and nasociliary pass inside the cone; IV enters above the ring, outside the cone, to reach superior oblique.
• Retrobulbar (intraconal) block paralyses III and VI but may spare IV: a small down-and-in (intorting) movement remains. The lids still close because orbicularis oculi is supplied by VII.
• Sub-Tenon block: a blunt cannula through a conjunctival incision delivers anaesthetic between Tenon's capsule and the sclera; chemosis is common.

Nerve palsies

Pupil pathways

• Parasympathetic: Edinger–Westphal nucleus, III, ciliary ganglion, short ciliary nerves to sphincter pupillae and ciliary muscle.
• Sympathetic: hypothalamus, cord T1, cervical sympathetic chain, superior cervical ganglion, plexus on the internal carotid, nasociliary and long ciliary nerves to dilator pupillae and Müller muscle; sweat fibres follow the external carotid.
• Horner syndrome: mild ptosis, miosis, anhidrosis with a warm flushed dry half-face (loss of vasoconstriction). Causes include Pancoast tumour and cervical sympathetic injury.
• Argyll Robertson pupil (neurosyphilis): small irregular pupils that accommodate but do not react to light.
• Acute angle-closure glaucoma: painful red eye, haloes, vomiting, fixed mid-dilated pupil. Avoid mydriatics such as atropine, which crowd the iris into the angle.

Corneal reflex

• Afferent: nasociliary nerve (V1); efferent: VII to orbicularis oculi, on both sides.
• Touching the right cornea gives no blink in either eye, but touching the left gives both: the right V1 is damaged. If one eye fails to blink whichever cornea is touched, that side's VII is damaged. Loss of the corneal reflex is an early sign of acoustic neuroma.

Optic nerve and retina

• Optic nerve passes through the optic canal with the ophthalmic artery; the central retinal artery (end artery) enters it. Occlusion: sudden painless monocular blindness, pale retina, cherry-red spot.
• The optic nerve is wrapped in all three meningeal layers, and the subarachnoid space extends along it to the globe. Raised intracranial pressure compresses the central retinal vein and axoplasmic flow: bilateral papilloedema. Early vision is normal; only the blind spot enlarges.

Lacrimal apparatus

• Lacrimal gland secretomotor path: superior salivatory nucleus, VII, greater petrosal nerve, pterygopalatine ganglion, zygomatic and lacrimal nerves. A lesion here gives a dry eye.
• Tears drain through puncta, canaliculi, lacrimal sac, nasolacrimal duct into the inferior meatus. Congenital blockage is usually at the valve of Hasner and settles with massage.
• Dacryocystorhinostomy joins the lacrimal sac to the middle meatus, bypassing the blocked duct.

Eyeball

• Fibrous coat: sclera and cornea (avascular, most refractive power). Uvea: choroid, ciliary body (aqueous, accommodation), iris. Retina: macula and fovea (cones only), optic disc (blind spot).
• Aqueous: ciliary body, posterior chamber, pupil, anterior chamber, trabecular meshwork, canal of Schlemm.
• Sympathetic ophthalmia: penetrating injury exposes sequestered uveal antigens, triggering autoimmune inflammation in both eyes weeks later.
• Uveal melanoma is the commonest primary adult intraocular tumour; it escapes through scleral emissary channels.

Remember LR6, SO4, all the rest III; IV alone runs outside the muscle cone.

Exam trap Superior oblique is tested by looking down and in, not down and out.

Exam trap The nasolacrimal duct opens into the inferior meatus; a DCR drains into the middle meatus.

How FCPS asks about this

• Vertical diplopia going downstairs, head tilt away: trochlear nerve palsy; worst in down-and-in gaze.
• Down-and-out eye with ptosis and dilated pupil: compressive III palsy (PCom aneurysm).
• Residual intorsion after retrobulbar block: IV lies outside the muscle cone.
• Infection kept out of the orbit: orbital septum (preseptal cellulitis).
• Ptosis, miosis and dry flushed face: Horner syndrome from cervical sympathetic damage.
• Why papilloedema occurs: subarachnoid space extends along the optic nerve.
• Drug to avoid in acute angle closure: atropine.