A 35-year-old woman presents with a dilated pupil on the left side that reacts sluggishly to light but shows near-light dissociation (brisk accommodation despite poor light reaction). She has no ptosis or ophthalmoplegia. What is the most common cause of this pupillary abnormality?

Explanation

## Most Common Cause of Light-Near Dissociation **Key Point:** Adie's tonic pupil (also called Holmes–Adie syndrome when bilateral with areflexia) is the most common cause of light-near dissociation in young to middle-aged patients, characterized by a dilated pupil that reacts sluggishly to light but shows brisk accommodation. ### Clinical Features of Adie's Tonic Pupil | Feature | Adie's Tonic Pupil | Argyll Robertson | Posterior Communicating Aneurysm | |---------|-------------------|------------------|--------------------------------| | **Age of onset** | 20–50 years | Older, syphilis history | Any age, acute | | **Pupil size** | Dilated (5–6 mm) | Small, irregular | Dilated (5–6 mm) | | **Light reaction** | Sluggish/absent | Absent | Absent | | **Accommodation** | Brisk | Brisk | Affected | | **Near-light dissociation** | Yes (classic) | Yes | No | | **Laterality** | Unilateral (90%) | Bilateral | Unilateral | | **Associated findings** | Areflexia (Holmes–Adie) | Argyll Robertson sign | Ptosis, ophthalmoplegia, pain | | **Prevalence** | Most common cause | Rare in modern era | Rare | **High-Yield:** Light-near dissociation = Adie's tonic pupil until proven otherwise. The combination of unilateral presentation, sluggish light reaction, and brisk accommodation is pathognomonic. ### Pathophysiology of Adie's Tonic Pupil 1. **Lesion site:** Ciliary ganglion and postganglionic parasympathetic fibres 2. **Mechanism:** Denervation of sphincter muscle fibres with aberrant reinnervation 3. **Result:** Slow, tonic contraction to accommodation; sluggish, incomplete dilation in darkness 4. **Progression:** Pupil gradually constricts over years ("redilation lag") ### Diagnostic Features **Clinical Pearl:** - **Vermiform movements:** Fine, worm-like movements of the iris margin visible on slit lamp in bright light — highly specific for Adie's - **Segmental iris constriction:** Iris sphincter contracts in segments rather than uniformly - **Redilation lag:** Pupil takes 15–20 seconds to dilate in darkness (vs. normal 20 ms) - **Accommodation amplitude:** Reduced over time due to ciliary muscle denervation ### Why Adie's is Most Common - **Prevalence:** 0.4–1.2% of population; most frequent cause of light-near dissociation in young patients - **Benign course:** No systemic disease association; self-limited, slowly progressive - **Diagnosis of exclusion:** Once serious causes (aneurysm, syphilis) are ruled out, Adie's is most likely **Mnemonic:** **ADIE** = **A**bnormal **D**ilation, **I**ris **E**xploration (vermiform movements) ### Associated Conditions (Holmes–Adie Syndrome) When Adie's tonic pupil occurs with areflexia or hyporeflexia: - Tendon reflexes absent or diminished (especially ankle jerks) - Segmental hyperhidrosis - Accommodation amplitude gradually declines - Autoimmune basis suspected (anti-neuronal antibodies) [cite:Neuro-ophthalmology, Kaufman & Gross; Harrison 21e Ch 428]