Pupil Assessment 7 Flashcards

A patient presents with right-sided ptosis, miosis, and anhidrosis consistent with Horner syndrome. Apraclonidine 1% is instilled bilaterally. Forty-five minutes later, the right (affected) pupil has dilated and the left pupil has slightly constricted, reversing the original anisocoria. Which mechanism best explains the dilation of the affected pupil?

Answer: Chronic sympathetic denervation leads to upregulation of alpha-1 adrenergic receptors on the iris dilator, causing supersensitivity to apraclonidine

Apraclonidine is primarily an alpha-2 agonist with some alpha-1 activity. In Horner syndrome, chronic loss of sympathetic input causes denervation supersensitivity — the iris dilator upregulates alpha-1 adrenergic receptors. This makes the affected pupil hypersensitive to even low alpha-1 stimulation from apraclonidine, causing it to dilate. The normal eye, lacking this upregulation, constricts slightly due to alpha-2 mediated suppression of norepinephrine release. Reversal of anisocoria is the positive test result.

A patient suffers an ischemic stroke affecting the right optic tract. On swinging flashlight test, which eye is most likely to demonstrate a relative afferent pupillary defect (RAPD), and why?

Answer: Left eye — nasal retinal fibers (which cross at the chiasm into the right tract) outnumber temporal fibers, so the left eye sustains greater total fiber loss

The right optic tract carries temporal fibers from the right eye and nasal fibers from the left eye. Because nasal retinal fibers (approximately 53% of total) outnumber temporal fibers (approximately 47%), a right optic tract lesion destroys more fibers originating from the left eye than from the right. This asymmetry results in a RAPD in the left eye — contralateral to the lesion and ipsilateral to the hemianopic field loss. This is one of the rare situations where RAPD and the field defect localize to the same eye.

On swinging flashlight testing, a patient's right pupil dilates briskly when light moves to it but then slowly re-constricts over 8–10 seconds, ultimately reaching a smaller size than the starting diameter. The near reflex is intact bilaterally. You suspect a ciliary ganglion lesion. Which pharmacological test and expected result would confirm this diagnosis?

Answer: Instill 0.1% pilocarpine bilaterally — the affected pupil will constrict while the normal fellow eye does not

The clinical picture — tonic dilation with slow re-constriction and preserved near response (light-near dissociation) — describes Adie's tonic pupil, caused by ciliary ganglion or short posterior ciliary nerve damage. Denervation supersensitivity causes the iris sphincter to upregulate muscarinic receptors. Dilute 0.1% pilocarpine (insufficient to affect a normally innervated sphincter) will constrict the supersensitive Adie's pupil but leave the normal fellow eye unaffected. This is the confirmatory test. Cocaine and hydroxyamphetamine test the sympathetic pathway and are used for Horner syndrome.

A patient is referred for evaluation of bilateral small (2mm), irregular pupils that show no reaction to bright light in a darkened room, but constrict briskly and symmetrically when asked to focus on a near target. There is no upgaze limitation, no convergence-retraction nystagmus, and the pupils are equal in size. Which diagnosis best fits this complete constellation of findings?

Answer: Argyll Robertson pupils — a midbrain lesion (classically neurosyphilis) produces small, irregular pupils with light-near dissociation and intact accommodation

Argyll Robertson pupils are characteristically small and irregular — unlike the large pupils of Parinaud syndrome or the unilateral/segmental involvement of Adie's. Both Argyll Robertson and Parinaud syndrome cause light-near dissociation, but Parinaud's has large pupils (pretectal suppression of both sympathetic and light-reflex fibers) and typically presents with upgaze palsy and convergence-retraction nystagmus. Bilateral Adie's would show a history of anisocoria and the pupils would be larger, not small. Optic neuropathy causes an afferent defect — the near reflex would also be reduced if afferent input were absent.

A 48-year-old patient presents with a fixed, dilated left pupil (9mm) with no reaction to any stimulus. After instilling 1% pilocarpine in both eyes, the right pupil constricts to 2mm, but the left pupil fails to constrict, remaining at 9mm. What does this pharmacological result most strongly indicate?

Answer: Pharmacological mydriasis from an anticholinergic agent — muscarinic blockade at the iris sphincter prevents pilocarpine binding

One percent pilocarpine directly stimulates muscarinic (M3) receptors on the iris sphincter, bypassing any neural pathway. In CN III palsy, the sphincter muscle itself is intact — it is simply denervated — so it WILL constrict to 1% pilocarpine. Failure to constrict to 1% pilocarpine indicates that the muscarinic receptors themselves are blocked (anticholinergic pharmacological mydriasis), since no topical cholinomimetic can overcome a competitive or near-irreversible receptor blockade at this concentration. Traumatic sphincter tears cause irregular, mechanical dilation rather than a perfectly round, fixed, 9mm pupil.

During examination of a young child with reduced visual acuity, pendular nystagmus, and severe photophobia, you perform a swinging flashlight test in a dimly lit room. Rather than the expected constriction, both pupils consistently DILATE when the light is shone directly on them. Which condition is most consistent with this paradoxical pupillary response?

Answer: Congenital achromatopsia — dysfunction of cone photoreceptors combined with rod-mediated pathway signaling disruption produces paradoxical dilation

Paradoxical pupillary dilation to light (pupils dilate rather than constrict when illuminated) is a well-documented but uncommon finding seen in conditions with primary dysfunction of rod photoreceptors or abnormal rod-cone interaction, including congenital achromatopsia, congenital stationary night blindness (CSNB), and fundus albipunctatus. The proposed mechanism involves rod photoreceptors tonically inhibiting the pupillary light reflex in the dark; when rods are dysfunctional, this inhibitory signal is disordered, causing dilation when light activates the abnormal pathway. Optic neuritis produces an RAPD with reduced (not paradoxical) response. CN III palsy causes a fixed, dilated pupil, not a responsive paradoxical one. The triad of nystagmus, photophobia, and reduced acuity is classic for achromatopsia.