Pupillary Disorders: Efferent/Autonomic

Overview

Where the afferent pupillary page covers whether light input is reaching the brain symmetrically, this page covers the opposite half of the exam: whether the motor output driving pupil size and reactivity is working normally. The dilator pupillae muscle is under sympathetic control (fibers traveling from the superior cervical ganglion via the long ciliary nerves), while the sphincter pupillae is under parasympathetic control (fibers traveling with cranial nerve III via the ciliary ganglion). Efferent problems typically present as anisocoria (pupils of unequal size) rather than as the isolated afferent deficit RAPD represents.

Anisocoria Evaluation

About one in five people have physiologic anisocoria: a benign pupil size difference, generally under 1 mm, that stays constant regardless of room lighting. Pathologic anisocoria behaves differently. The size difference changes with illumination, and that change is what points to a specific efferent lesion.

The key diagnostic maneuver is comparing the anisocoria in bright light against dim light. Worse in bright light means the larger pupil is abnormal: it fails to constrict normally, pointing to a parasympathetic problem such as an Adie pupil, a pharmacologically dilated pupil, or a compressive third nerve palsy. Worse in dim light means the smaller pupil is abnormal: it fails to dilate normally, pointing to a sympathetic problem, namely Horner syndrome. That single comparison turns "the pupils look unequal" into an actual localization and sets the direction of the rest of the workup.

Adie Tonic Pupil

An Adie tonic pupil results from damage to the ciliary ganglion or short ciliary nerves, classically post-viral or post-surgical, which carry the postganglionic parasympathetic fibers to the iris sphincter and ciliary muscle. The pupil is tonically dilated at baseline and reacts poorly to light, if at all, but shows a slow, tonic (vermiform) constriction to a sustained near target. This is light-near dissociation (defined below), here from a lesion in the efferent pathway.

Denervation leaves the iris sphincter's muscarinic receptors upregulated, a phenomenon called denervation supersensitivity, so an Adie pupil constricts briskly to very dilute pilocarpine (around 0.1%), a concentration far too weak to affect a normal pupil. This pharmacologic test confirms the diagnosis and distinguishes a true Adie pupil from a pharmacologically blocked pupil: the blocked pupil fails to constrict even to full-strength pilocarpine, because its receptors are directly antagonized rather than denervated.

Argyll Robertson Pupil & Light-Near Dissociation

Light-near dissociation describes any pupil that reacts poorly or not at all to direct light but constricts normally to a near or accommodative target. The light reflex and near reflex pathways diverge anatomically within the midbrain, so one can be knocked out while the other survives. The classic cause is the Argyll Robertson pupil, classically associated with tertiary neurosyphilis: a dorsal midbrain (pretectal) lesion disrupts the light reflex arc while the separate near-reflex pathway, which does not route through the same pretectal relay, stays intact. Argyll Robertson pupils are also classically small, irregular, and poorly reactive to mydriatics.

Other causes of light-near dissociation are distinguished from Argyll Robertson pupils by context. An Adie tonic pupil produces it unilaterally, with the tonic near response and denervation supersensitivity described above. Dorsal midbrain (Parinaud) syndrome produces it alongside other signs of dorsal midbrain compression, rather than in isolation.

Pharmacologic Mydriasis & Miosis

A mydriatic dilates the pupil. Tropicamide and atropine are the classic clinical examples, used routinely to facilitate fundus examination by blocking parasympathetic tone to the sphincter. A miotic does the opposite: pilocarpine constricts the pupil and is used therapeutically in certain forms of glaucoma to increase aqueous outflow. Both drug classes also matter diagnostically. Accidental or unrecognized topical exposure to a mydriatic is a common real-world cause of an isolated, fixed, dilated pupil that can mimic a compressive third nerve palsy, and distinguishing the two is exactly what formal pharmacologic pupil testing, such as a pilocarpine challenge, is designed to do.

One presentation is a true emergency rather than an outpatient workup. A new, unilateral, fixed and dilated ("blown") pupil in a comatose or rapidly deteriorating patient suggests uncal herniation: rising intracranial pressure pushing the temporal lobe uncus through the tentorial notch and compressing the third nerve. The parasympathetic fibers of CN III sit superficially and are compressed before the more central motor fibers, so the dilated, poorly reactive pupil is often the earliest sign, appearing before frank ophthalmoplegia. Recognizing it immediately matters more than completing a routine anisocoria workup.

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