What is the most likely cause of coma in a 60-year-old (sixty, not six) man with a history of lung cancer, presenting with a two-day history of headache, lethargy, and progressive decline in level of consciousness, who is comatose and unresponsive to stimuli, with CT scan findings of enlarged lateral and third ventricles, small cerebral sulci, normal fourth ventricle, and multiple metastatic lesions?

Obstructive Hydrocephalus from Metastatic Brain Lesions

The most likely cause of this patient's coma is obstructive hydrocephalus resulting from metastatic brain lesions blocking cerebrospinal fluid (CSF) flow at the level of the third ventricle or aqueduct of Sylvius. 1

Clinical Reasoning

The CT findings are pathognomonic for obstructive hydrocephalus:

• Enlarged lateral and third ventricles with a normal-sized fourth ventricle indicates obstruction at or near the cerebral aqueduct (aqueduct of Sylvius), which connects the third and fourth ventricles 1
• Small cerebral sulci reflect increased intracranial pressure compressing the brain surface 2
• Multiple metastatic lesions from lung cancer are strategically positioned to obstruct CSF flow 3, 4

This pattern is distinct from communicating hydrocephalus (where all ventricles would be enlarged) or mass effect alone (which would show midline shift and focal compression rather than symmetric ventricular enlargement).

Pathophysiology

Obstructive hydrocephalus from cerebellar or periventricular metastases blocking CSF flow is a neurosurgical emergency that can rapidly progress to coma and death if untreated. 1 The mechanism involves:

• Metastatic lesions physically obstructing the narrow CSF pathways (aqueduct, foramina of Monro, or fourth ventricular outflow) 1
• Progressive CSF accumulation proximal to the obstruction causing ventricular dilation 2
• Rising intracranial pressure leading to decreased cerebral perfusion, brainstem compression, and ultimately coma 3, 2

Lung cancer accounts for approximately 50% of all brain metastases, making this scenario highly characteristic. 3, 4 The two-day progression from headache to coma reflects the acute nature of obstructive hydrocephalus, which can deteriorate much faster than mass effect from tumor growth alone.

Immediate Management Priorities

High-dose dexamethasone 16 mg/day should be initiated immediately to reduce perilesional edema, though this will not resolve the mechanical obstruction. 3

Emergent neurosurgical consultation is mandatory for consideration of:

• External ventricular drain (EVD) placement to acutely decompress the ventricles 1
• Ventriculoperitoneal shunt if longer-term CSF diversion is needed 1
• Possible surgical resection if a single large metastasis is causing the obstruction 3

Alternative Considerations (Less Likely)

While cerebral edema from multiple metastases contributes to the clinical picture, the specific CT pattern of selective proximal ventricular enlargement with normal fourth ventricle definitively indicates mechanical CSF obstruction rather than diffuse mass effect alone. 1, 2

Hemorrhage into metastases was explicitly excluded by CT. 1 Leptomeningeal disease could impair CSF resorption but would typically cause communicating hydrocephalus with all ventricles enlarged. 1

Prognostic Context

Without urgent intervention to relieve the hydrocephalus, the natural history is progression to herniation and death. 3, 4 Even with optimal management, patients presenting comatose from metastatic disease have extremely poor prognosis, typically measured in weeks. 3, 5