A 68-year-old man with hypertension presents to the emergency department with acute onset severe headache and neck stiffness. Non-contrast CT brain is performed. The structure marked **B** (septum pellucidum) is clearly visualized at the midline, and the lateral ventricles appear symmetrically enlarged with compression of the sulci. Which of the following is the most likely diagnosis?

Explanation

## Why Communicating hydrocephalus from impaired CSF malabsorption is right The clinical presentation of acute severe headache with neck stiffness suggests subarachnoid hemorrhage (SAH), which is a classic cause of **communicating hydrocephalus** from impaired CSF malabsorption. The septum pellucidum (**B**) remains at the midline (not shifted), indicating no focal mass effect, but the symmetrically enlarged ventricles with sulcal compression indicate hydrocephalus. In communicating hydrocephalus, CSF flow through the ventricular system is patent, but reabsorption is impaired—typically post-SAH or post-meningitis. The visibility and midline position of the septum pellucidum is a key landmark confirming the ventricles are enlarged symmetrically without mass effect, which is the hallmark of communicating hydrocephalus. (Sutton Radiology; Harrison 21e) ## Why each distractor is wrong - **Acute obstructive hydrocephalus from fourth ventricle outflow obstruction**: While obstructive hydrocephalus also presents with enlarged ventricles, the clinical context of acute headache + neck stiffness is far more consistent with SAH causing communicating hydrocephalus. Obstructive hydrocephalus typically results from tumor or aqueductal stenosis, not acute SAH. - **Diffuse cerebral edema with ventricular compression**: Diffuse edema causes sulcal effacement and ventricular *compression* (narrowing), not enlargement. The imaging here shows enlarged ventricles, which is opposite to the expected finding in cerebral edema. - **Normal pressure hydrocephalus with gait disturbance**: NPH is a chronic condition presenting with the classic triad of gait disturbance, dementia, and incontinence. It does not present acutely with severe headache and neck stiffness, and the imaging findings are typically more subtle with only mild ventricular enlargement. **High-Yield:** Symmetric ventricular enlargement with midline septum pellucidum preserved = communicating hydrocephalus; shifted midline = mass effect; compressed ventricles = cerebral edema. [cite: Sutton Radiology; Harrison 21e]