A 72-year-old woman with a history of hypertension and previous stroke presents with a 3-day history of right-sided headache and diplopia. On examination, the right pupil is 5 mm and reacts sluggishly to light, while the left pupil is 3 mm and reacts briskly. The right eye is adducted and depressed at rest. Visual acuity is 6/6 in both eyes. Fundoscopy is normal. MRI brain with contrast shows an acute infarct in the right midbrain. What is the most likely pupillary abnormality?

Explanation

## Clinical Diagnosis: Weber's Syndrome ### Anatomical Basis **Key Point:** Weber's syndrome is a midbrain stroke syndrome characterized by ipsilateral oculomotor nerve (CN III) palsy and contralateral hemiparesis due to involvement of the oculomotor nerve fascicles and cerebral peduncle. ### Pathophysiology of Weber's Syndrome ```mermaid flowchart TD A[Midbrain Infarction]:::outcome --> B[Involvement of CN III fascicles]:::outcome A --> C[Involvement of cerebral peduncle]:::outcome B --> D[Ipsilateral CN III palsy]:::outcome C --> E[Contralateral hemiparesis]:::outcome D --> F[Pupillary dilation<br/>Eye adduction/depression loss<br/>Ptosis]:::action E --> G[Weakness of opposite arm/leg]:::action ``` ### Pupillary Findings in Weber's Syndrome 1. **Ipsilateral pupillary dilation** (5 mm on right) — due to parasympathetic fibre damage in CN III. 2. **Sluggish light reflex** — parasympathetic pupillomotor fibres run in the periphery of CN III and are affected in Weber's syndrome. 3. **Contralateral pupil normal** — the left pupil is unaffected (3 mm, brisk reflex). **High-Yield:** The key distinguishing feature of Weber's syndrome is the combination of: - Ipsilateral CN III palsy (pupillary dilation + ophthalmoplegia) - Contralateral hemiparesis (from cerebral peduncle involvement) ### Why This Patient Has Weber's Syndrome - **MRI confirms midbrain infarction** — the anatomical location of Weber's syndrome. - **Right pupil dilated and sluggish** — CN III parasympathetic involvement on the right. - **Right eye adducted and depressed** — CN III motor nerve palsy (loss of medial rectus, superior rectus, inferior rectus, and inferior oblique). - **Contralateral hemiparesis expected** — though not explicitly mentioned, it would be present on the left side due to cerebral peduncle involvement. ### Differential Diagnosis: Midbrain Syndromes | Syndrome | Pupillary Finding | CN III Involvement | Other Features | Lesion Location | |----------|-------------------|-------------------|-----------------|------------------| | **Weber's** | Ipsilateral dilation, sluggish | Yes (ipsilateral) | Contralateral hemiparesis | Midbrain ventral (CN III fascicles + cerebral peduncle) | | **Benedikt's** | Ipsilateral dilation, sluggish | Yes (ipsilateral) | Contralateral tremor/ataxia (red nucleus) | Midbrain ventral (CN III + red nucleus) | | **Parinaud's** | Pupils dilated, light-near dissociation | No | Upward gaze palsy, convergence-retraction nystagmus | Midbrain dorsal (superior colliculus) | | **Horner's** | Ipsilateral miosis, normal reflex | No | Ptosis, anhidrosis, enophthalmos | Sympathetic pathway (not CN III) | **Clinical Pearl:** Weber's syndrome and Benedikt's syndrome both present with ipsilateral CN III palsy and pupillary dilation. The key difference is that Benedikt's also involves the red nucleus, causing contralateral tremor and cerebellar signs, whereas Weber's causes contralateral hemiparesis from cerebral peduncle involvement. ### Examination Findings Summary - **Right eye:** Pupil dilated (5 mm), sluggish light reflex, adducted and depressed at rest (CN III palsy). - **Left eye:** Pupil normal (3 mm), brisk light reflex. - **Visual acuity:** Preserved (6/6) because the lesion does not affect the optic pathways. - **Fundoscopy:** Normal, ruling out posterior segment pathology. **Mnemonic:** **"Weber = oculomotor + motor"** — ipsilateral oculomotor nerve palsy + contralateral motor (hemiparesis). [cite:Walsh & Hoyt's Clinical Neuro-Ophthalmology 6e Ch 9; Harrison's Principles of Internal Medicine 21e Ch 451]