A 28-year-old woman presents with progressive bilateral pupillary constriction over 6 months, with preserved light and accommodation reflexes. She reports no vision loss or ocular pain. Slit-lamp examination reveals no iris atrophy, transillumination defects, or anterior chamber inflammation. Systemic examination is unremarkable. Which investigation is most appropriate to evaluate the underlying cause of this bilateral pupillary miosis?

Explanation

## Diagnosis: Bilateral Miosis with Preserved Light and Accommodation Reflexes The clinical scenario describes a 28-year-old woman with **progressive bilateral pupillary constriction**, preserved light and accommodation reflexes, no iris structural changes (no atrophy, no transillumination defects), no anterior chamber inflammation, and an unremarkable systemic examination. This presentation points away from a pharmacological or iris-structural cause and toward a **central neurological etiology** affecting sympathetic or parasympathetic pathways. ### Why MRI of the Brain and Brainstem is the Most Appropriate Investigation **Key Point:** Bilateral miosis with preserved reflexes and no local ocular pathology is a neurological red flag requiring structural imaging of the brain and brainstem as the first-line investigation. **High-Yield:** Pharmacological testing (cocaine/apraclonidine) is designed to confirm and localize **unilateral** Horner syndrome. In a bilateral, symmetric presentation with preserved reflexes and no ptosis or anhidrosis described, the pre-test probability of bilateral Horner syndrome is low. The more urgent and appropriate step is to exclude structural CNS pathology (e.g., pontine lesion, tectal lesion, syringomyelia, or mass lesion affecting the Edinger-Westphal nucleus or sympathetic pathways bilaterally). ### Neuroanatomical Basis | Structure | Lesion Effect | |---|---| | **Pontine tegmentum** | Bilateral miosis (pinpoint pupils) — classic in pontine hemorrhage | | **Hypothalamus / descending sympathetics** | Bilateral sympathetic dysfunction → bilateral miosis | | **Tectal/pretectal region** | Affects pupillary light reflex pathways | | **Edinger-Westphal nucleus** | Parasympathetic tone alteration | MRI (with gadolinium, including brainstem sequences) is the gold standard for identifying structural lesions in these regions (Harrison's Principles of Internal Medicine, 21st ed.; Kanski's Clinical Ophthalmology, 9th ed.). ### Why Pharmacological Testing is NOT the First Step Here **Pharmacological testing with cocaine and apraclonidine:** - Designed specifically for **unilateral** Horner syndrome to confirm sympathetic denervation and localize the lesion (pre- vs. postganglionic) - In **bilateral** symmetric miosis with preserved reflexes and no ptosis/anhidrosis, cocaine/apraclonidine testing does not differentiate central structural pathology from other causes - Pharmacological testing is a *follow-up* tool after clinical localization, not a first-line investigation for bilateral neurological miosis ### Why Other Options Are Incorrect **Pupillography (C):** - Quantifies pupillary dynamics and light responses - Useful for documenting abnormalities but does NOT identify the underlying structural or neurological cause - Cannot replace neuroimaging in a progressive bilateral presentation **Anterior segment fluorescein angiography (D):** - Evaluates iris vasculature and anterior segment perfusion - Relevant for iris ischemia or vascular pathology - Not indicated here — no iris structural changes are present **Clinical Pearl:** In any patient with progressive, bilateral neurological pupillary abnormality without a clear pharmacological or local ocular cause, MRI of the brain and brainstem is the most appropriate first investigation to exclude life-threatening structural pathology (e.g., pontine glioma, demyelination, or posterior fossa mass). Pharmacological testing is reserved for confirming and localizing unilateral Horner syndrome after clinical assessment.