34. 60 years old man presented with sudden onset ataxia. CT scan shows cerebellar ataxia. One day after the admission he become drowsy. Examination shows one pupil of 5mm & other pupil of 3mm. blood pressure is 200/100mmHg. What can be the cause for his drowsiness? A. Hydrocephalus B. Hypertensive encephalopathy C. Hypoglycemia D. Hyponatremia E. Seizures 34. 60 years old man presented with sudden onset ataxia. CT scan shows cerebellar ataxia. One day after the admission he become drowsy. Examination shows one pupil of 5mm & other pupil of 3mm. blood pressure is 200/100mmHg. What can be the cause for his drowsiness? A. Hydrocephalus B. Hypertensive encephalopathy C. Hypoglycemia D. Hyponatremia E. Seizures
Answer: A. Hydrocephalus
Reasoning:
This patient has a cerebellar hemorrhage/infarct (sudden onset ataxia, CT showing cerebellar pathology). The key sequence is:
- Day 1: Cerebellar lesion causes ataxia
- Day 2: Drowsiness develops + anisocoria (one pupil 5mm, other 3mm)
The anisocoria is the critical clue. The 5mm (dilated, fixed) pupil indicates CN III compression - this is a sign of transtentorial (uncal) herniation, where the brainstem is being compressed.
Why Hydrocephalus?
The cerebellum sits in the posterior fossa, a tight compartment. A cerebellar lesion (hemorrhage or large infarct) can:
- Swell over 24-48 hours (cerebral edema peaks at ~24-72 hours)
- Obstruct the 4th ventricle and the cerebral aqueduct (aqueduct of Sylvius)
- This causes obstructive (non-communicating) hydrocephalus
- Raised ICP causes transtentorial herniation → CN III compression → unequal pupils
- Herniation also compresses the reticular activating system (RAS) in the midbrain → drowsiness/coma
Why not the others?
| Option | Why excluded |
|---|---|
| B. Hypertensive encephalopathy | BP 200/100 is elevated but HE causes bilateral symmetric findings (papilledema, diffuse encephalopathy, bilateral small pupils). It does NOT explain anisocoria. |
| C. Hypoglycemia | Causes global encephalopathy. Does NOT cause anisocoria. No metabolic history given. |
| D. Hyponatremia | Causes diffuse cerebral edema/seizures. Does NOT cause unilateral pupil dilation. |
| E. Seizures | Post-ictal drowsiness is possible, but seizures do NOT cause fixed anisocoria with unilateral dilation. |
The unilateral dilated pupil (5mm) is the anchor finding - it points to focal mass effect with herniation, and the mechanism in posterior fossa pathology is obstructive hydrocephalus from 4th ventricle compression.
Clinical pearl: Cerebellar lesions are neurosurgical emergencies precisely because the posterior fossa has no room to expand - even moderate edema causes rapid herniation. Treatment is urgent external ventricular drain (EVD) or suboccipital decompression.
Answer: A. Hydrocephalus
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