A 42-year-old woman presents with a 3-day history of progressive blurred vision in the right eye and mild headache. On examination, the right pupil is 6 mm and reacts sluggishly to light with light-near dissociation (pupil constricts better to accommodation than to light). The left pupil is 3 mm and reacts normally. Visual acuity is 6/9 in both eyes. MRI brain and orbits is normal. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Adie's Tonic Pupil The presentation of a dilated pupil with sluggish light reaction but better constriction to accommodation (light-near dissociation) in a young woman with normal neuroimaging is classic for Adie's tonic pupil (acute adie syndrome). ## Pathophysiology **Key Point:** Adie's tonic pupil results from denervation of the ciliary ganglion due to inflammation or degeneration of postganglionic parasympathetic fibers. The pupil is dilated because the parasympathetic supply to the iris sphincter is damaged, but accommodation is initially preserved because some ciliary muscle fibers remain innervated. **High-Yield:** Light-near dissociation (better accommodation response than light response) is the hallmark of Adie's tonic pupil and distinguishes it from other causes of a dilated pupil. ## Distinguishing Features of Adie's Tonic Pupil | Feature | Adie's | RAPD | Dorsal Midbrain | 3rd Nerve Palsy | |---------|--------|------|-----------------|------------------| | Pupil size | Dilated (5–6 mm) | Normal or dilated | Normal or dilated | Dilated (5–6 mm) | | Light reaction | Sluggish | Abnormal (afferent) | Normal | Fixed | | Accommodation | Better than light | Normal | Impaired | Impaired | | Light-near dissociation | **Yes** | No | Yes | No | | Eye movements | Normal | Normal | Normal | **Impaired** | | Ophthalmoplegia | Absent | Absent | Absent | **Present** | | Headache | Absent | May occur | Often present | Often present | | Neuroimaging | Normal | Normal | Abnormal | Abnormal | **Clinical Pearl:** Adie's tonic pupil is a benign condition that typically affects young women (female:male ratio 2:1) and is often unilateral. Over time, the pupil becomes smaller and the accommodation symptoms improve, though the pupil may remain dilated. ## Mnemonic for Light-Near Dissociation **Mnemonic:** **LNDS = Light-Near Dissociation Syndromes** - **L**ues (neurosyphilis/Argyll Robertson pupil) - **N**eurological (dorsal midbrain syndrome, Parinaud syndrome) - **D**iabetes (autonomic neuropathy) - **S**yphilis (Argyll Robertson pupil) However, in this case, the context (young woman, acute onset, normal imaging, no systemic disease) points to **Adie's tonic pupil** as the most likely diagnosis. ## Why This Is NOT Other Diagnoses **Relative Afferent Pupillary Defect (RAPD):** While optic neuritis can cause visual symptoms and headache, RAPD is characterized by a paradoxical dilation of the affected pupil when light is swung to it (positive swinging flashlight test). The pupil size is typically normal, not dilated. Additionally, RAPD affects the light reflex symmetrically, not light-near dissociation. **Dorsal Midbrain Syndrome:** This causes light-near dissociation but is associated with convergence insufficiency, upward gaze palsy, and lid retraction. Neuroimaging would show a midbrain lesion, which is normal in this patient. **Third Nerve Palsy:** This causes a dilated pupil but with complete loss of accommodation and ophthalmoplegia (ptosis, impaired eye movements). The pupil is fixed, not tonic/sluggish. Neuroimaging would show a midbrain or nerve lesion. ## Management 1. **Reassurance:** Adie's tonic pupil is benign and does not indicate serious neurological disease. 2. **Pharmacological testing:** Instillation of 0.125% pilocarpine (dilute cholinergic agonist) causes pupillary constriction in Adie's (denervation supersensitivity) but not in other causes of a dilated pupil. 3. **Monitoring:** Most patients improve spontaneously over months to years. 4. **Symptomatic relief:** Glasses for near vision if accommodation is impaired; sunglasses for photophobia. [cite:Khurana 6e Ch 9]