Pupillary Disorders — Clinical Syndromes

Horner Syndrome

Horner syndrome results from a lesion anywhere along the three-neuron oculosympathetic pathway. Damage at any level produces the same ipsilateral triad: miosis from loss of dilator muscle tone, mild ptosis from loss of Müller's muscle (the sympathetically innervated eyelid retractor), and apparent enophthalmos with lower lid elevation, a "reverse ptosis" caused by loss of the analogous lower-lid tarsal muscle. Anhidrosis, loss of facial sweating, occurs only with first- or second-order lesions proximal to the superior cervical ganglion; the sweat-gland innervation to the face branches off before the third-order neuron. The pupil itself is not fixed or non-reactive. It constricts normally to light but shows dilation lag in dim light, redilating more slowly than the normal eye once the light is removed.

Localizing Horner Syndrome: Three Pharmacologic Tests

Three diagnostic drops distinguish and localize Horner syndrome along that three-neuron chain.

• Cocaine 4–10% confirms the diagnosis. It blocks norepinephrine reuptake at sympathetic nerve terminals, so a normal pupil dilates as stored norepinephrine accumulates at the synapse. In Horner syndrome, reduced norepinephrine release at any level of the pathway means the affected pupil fails to dilate, confirming the diagnosis regardless of where the lesion sits.
• Apraclonidine 0.5–1% also confirms the diagnosis, through a different mechanism and a striking sign: reversal of anisocoria. This alpha-2 agonist carries weak alpha-1 activity that only becomes clinically relevant where alpha-1 receptors have upregulated from denervation supersensitivity, exactly what happens on the iris dilator of a Horner pupil. It dilates the Horner pupil while mildly constricting the normal one, flipping which eye looks larger.
• Hydroxyamphetamine 1% localizes the lesion once the diagnosis is already confirmed. It releases stored norepinephrine from intact postganglionic nerve terminals. If the third-order (postganglionic) neuron is the damaged one, there is no terminal left to release from, and the pupil fails to dilate, which localizes the lesion there. If the lesion is instead first-order (central, e.g. brainstem or spinal cord) or second-order (preganglionic, e.g. an apical lung tumor compressing the sympathetic chain), the postganglionic terminal is intact and the pupil dilates normally.

Adie Tonic Pupil

Adie's tonic pupil results from postganglionic parasympathetic denervation, classically at the ciliary ganglion. The pupil is large and reacts poorly to light, but it shows a slow, tonic constriction to sustained near effort (a form of light-near dissociation), sluggish redilation afterward, and, on slit-lamp exam, vermiform (worm-like) movements of the iris sphincter. Holmes-Adie syndrome pairs this pupil finding with an absent patellar reflex.

The pharmacologic hallmark is denervation supersensitivity. The affected sphincter constricts to dilute pilocarpine (0.1%), a concentration too weak to affect a normal pupil, and that single test distinguishes Adie's from a pharmacologically dilated pupil (accidental anticholinergic exposure), which fails to constrict even with full-strength 1% pilocarpine.

Adie's tonic pupil is also the classic bedside mimic of a third nerve palsy dilated pupil, and bedside exam distinguishes the two as reliably as pharmacology does. Near-response sparing points to Adie's. A CN III palsy instead presents with ptosis and extraocular muscle restriction, the eye deviated down and out, alongside a pupil that is completely fixed and non-reactive, with no near response spared at all.

Argyll Robertson Pupils

Argyll Robertson pupils are small, irregular, and bilateral, and they show light-near dissociation, as Adie's does, but bilaterally and with a brisk near response: they constrict to a near target but do not react to light. The classic cause is neurosyphilis, though it is not the only one; diabetes mellitus can produce a similar picture. The underlying lesion selectively damages the pretectal pupillary light-reflex fibers while sparing the separate, more ventrally situated near-reflex pathway, the same anatomical divergence covered in the pupillary pathway article. Here it is the light-reflex side that is knocked out, while near constriction stays brisk.

Third Nerve (CN III) Palsy

A unilateral, poorly reactive, mid-dilated or fixed pupil accompanied by ptosis and an eye deviated "down and out" suggests a compressive oculomotor nerve palsy, classically from a posterior communicating artery aneurysm. Compressive lesions characteristically affect the peripherally located pupillary fibers of CN III early, which is why a compressive third nerve palsy typically presents with pupil involvement. Microvascular (ischemic) third nerve palsies more often spare the pupil, because the ischemia preferentially affects the centrally located motor fibers instead. Any pupil-involving third nerve palsy mandates urgent neuroimaging to rule out an aneurysm.

When a dilated pupil appears without ptosis or motility restriction, pharmacologic mydriasis from an anticholinergic exposure is the main mimic to rule out. A drop of 1% pilocarpine settles the question. A compressive third nerve palsy pupil constricts briskly, since its iris sphincter is anatomically intact and simply lacks its normal parasympathetic drive; a pharmacologically blocked pupil stays fixed even at full strength, because its muscarinic receptors are chemically occupied rather than nerve-starved.

A third cause of a fixed, dilated pupil is direct mechanical injury to the iris sphincter after blunt trauma. Traumatic mydriasis behaves just like pharmacologic mydriasis on pilocarpine testing: the torn sphincter cannot constrict, so the pupil stays fixed even at full strength, the same result seen with anticholinergic exposure. Pilocarpine alone cannot tell the two apart. A slit-lamp exam settles it, looking for pupil-margin irregularity, visible sphincter tears, or iris transillumination defects, together with a history of recent blunt trauma.

Parinaud (Dorsal Midbrain) Syndrome

Parinaud syndrome results from dorsal midbrain compression, classically a pinealoma, and produces a specific pupillary abnormality: light-near dissociation with mid-dilated pupils, resembling Argyll Robertson pupils except that the pupils are mid-dilated rather than small. The lesion affects pretectal pupillary light fibers while sparing the more ventrally located tegmental accommodation and convergence pathway, so near constriction stays relatively preserved even as the light reflex is reduced or absent.

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