A 58-year-old man from Delhi presents to the emergency department with sudden-onset severe headache and neck stiffness. Blood pressure is 180/110 mmHg. Non-contrast CT head shows hyperdense material in the subarachnoid space, predominantly in the basal cisterns and sylvian fissures bilaterally. No intraparenchymal hemorrhage or hydrocephalus is noted. What is the most likely source of bleeding?

Explanation

## Clinical Presentation & Imaging Pattern **Key Point:** The distribution of blood in the basal cisterns and sylvian fissures on non-contrast CT is pathognomonic for **subarachnoid hemorrhage (SAH)** from a ruptured saccular aneurysm. ### Why This Distribution? Saccular aneurysms arise at branch points of major intracranial arteries, most commonly at the anterior communicating artery (30%), posterior communicating artery (30%), and middle cerebral artery bifurcation (20%). When they rupture, blood flows directly into the subarachnoid space and distributes along the major arterial territories and cisterns. ### Imaging Features of Aneurysmal SAH | Feature | Finding | Significance | |---------|---------|---------------| | **Location of blood** | Basal cisterns, sylvian fissures, interhemispheric space | Follows subarachnoid distribution | | **Intraparenchymal hemorrhage** | Absent (unless secondary rupture into brain) | Suggests pure SAH | | **Hydrocephalus** | May develop (acute or delayed) | From blood in ventricular system | | **Timing of CT** | Hyperdense in first 5–7 days | Becomes isodense then hypodense | **High-Yield:** The **absence of intraparenchymal hemorrhage** and the **basal cistern distribution** rule out hypertensive hemorrhage, which typically presents as a focal parenchymal hematoma (putamen, thalamus, pons, cerebellum) with secondary SAH extension. ### Clinical Pearl The classic triad of aneurysmal SAH is: 1. Sudden severe "thunderclap" headache (worst headache of life) 2. Neck stiffness (meningeal irritation from blood) 3. Hypertension (sympathetic surge) All three are present in this case, strongly supporting aneurysmal rupture. ### Next Steps in Management - **CTA or DSA** to identify and localize the aneurysm - **Lumbar puncture** (if CT negative but clinical suspicion high) to detect xanthochromia - **Vasospasm monitoring** (peak at days 4–14) with transcranial Doppler - **Aneurysm repair** (endovascular coiling or surgical clipping) within 24–48 hours [cite:Harrison 21e Ch 297]