Pupil Assessment 9 Flashcards

A patient with confirmed left Horner syndrome undergoes hydroxyamphetamine (Paredrine) 1% testing. After 45 minutes, the left (Horner) pupil fails to dilate while the right pupil dilates normally. Which localization of the lesion does this result indicate?

Answer: Third-order neuron (superior cervical ganglion to iris dilator)

Hydroxyamphetamine works by triggering release of stored norepinephrine from the pre-synaptic terminal of the third-order (post-ganglionic) neuron. If the third-order neuron is damaged, no norepinephrine is available to release, and the Horner pupil fails to dilate. A dilation response would indicate an intact third-order neuron, placing the lesion in the first- or second-order neuron instead. The afferent pathway does not influence this pharmacological test.

A patient with advanced bilateral glaucoma and severely cupped optic nerves undergoes the swinging flashlight test. The examiner detects minimal to no RAPD despite profound bilateral optic nerve damage. What is the most accurate interpretation?

Answer: Symmetric bilateral optic nerve damage produces no detectable relative afferent difference between the two eyes

RAPD is a 'relative' test that compares afferent input between the two eyes. When both optic nerves are equally and symmetrically damaged, the relative difference is minimal, and no detectable RAPD emerges despite severe bilateral disease. A negative RAPD therefore cannot rule out significant bilateral optic nerve pathology — it only rules out asymmetric afferent loss. This is a critical clinical pitfall.

A 30-year-old woman has a unilaterally dilated right pupil with poor light reaction but slow tonic constriction to near. Dilute pilocarpine 0.1% is instilled bilaterally. After 20 minutes, the right pupil constricts markedly while the left shows no response. What explains this pattern?

Answer: The right iris sphincter exhibits denervation supersensitivity following ciliary ganglion damage

Adie's tonic pupil results from damage to the ciliary ganglion and short ciliary nerves (parasympathetic efferent pathway). Denervation causes the iris sphincter to develop supersensitivity to cholinergic agents. Dilute pilocarpine 0.1% — too weak to affect a normal iris — will cause marked constriction in an Adie's pupil. A pharmacologically dilated pupil would show no response even to full-strength pilocarpine, distinguishing it from this scenario. Afferent lesions do not cause anisocoria or altered pharmacological response.

Which combination of findings is MOST characteristic of Argyll Robertson pupils in neurosyphilis?

Answer: Bilateral small irregular pupils with intact near response, absent light reaction, and poor dilation with mydriatics

Argyll Robertson pupils are classically bilateral, small, and irregular. They exhibit light-near dissociation — no or severely reduced reaction to light, but intact constriction to near targets. Critically, they also dilate poorly in response to mydriatics such as tropicamide, which differentiates them from Adie's and other causes of light-near dissociation. The lesion is in the dorsal midbrain (pretectal area), selectively disrupting the light reflex pathway while sparing the near reflex pathway.

A 64-year-old diabetic presents with acute right ptosis, complete ophthalmoplegia, and a dilated, non-reactive right pupil. Why does pupillary involvement carry critical diagnostic significance in this clinical context?

Answer: Parasympathetic fibers travel on the outer surface of CN III and are preferentially compromised by extrinsic compression, raising concern for a posterior communicating artery aneurysm

Parasympathetic pupillomotor fibers run on the outer (superficial) surface of the oculomotor nerve, making them the first fibers affected by extrinsic compression — such as from a posterior communicating artery aneurysm — before motor fibers are disrupted. In contrast, ischemic CN III palsy (common in diabetics) infarcts the nerve's central core, sparing the peripheral parasympathetic fibers and thus the pupil. Pupil involvement therefore mandates urgent neuroimaging to exclude a life-threatening aneurysm.

A patient has anisocoria measuring 1.5mm in bright light and 3.0mm in dim light. The smaller of the two pupils is the abnormal one. Which diagnosis is most consistent with this lighting-dependent pattern?

Answer: Horner syndrome

In Horner syndrome, the affected (smaller) pupil lacks sympathetic innervation to the iris dilator. In dim light, a normal pupil dilates while the Horner pupil cannot fully dilate, so the anisocoria increases — a hallmark sign called 'dilation lag.' In bright light, both pupils constrict and the difference narrows. Adie's and CN III palsy produce a larger abnormal pupil, with anisocoria greatest in bright light when the normal eye constricts but the affected eye cannot. Physiologic anisocoria remains stable across lighting conditions.