A 58-year-old man from Mumbai presents to the emergency department with sudden-onset severe headache, neck stiffness, and photophobia. He was hypertensive (BP 180/110 mmHg) and had a brief episode of vomiting. Non-contrast CT head performed within 2 hours of symptom onset shows hyperdense material in the basal cisterns and sylvian fissures bilaterally, with no intraparenchymal hematoma. What is the most likely diagnosis and what is the expected finding on CT angiography of the head and neck?

Explanation

## Clinical Presentation and Imaging Correlation **Key Point:** The classic triad of sudden severe headache, neck stiffness, and photophobia combined with hyperdense material in the basal cisterns and sylvian fissures on non-contrast CT is pathognomonic for subarachnoid hemorrhage (SAH). ### Why the Basal Cistern Distribution Matters The distribution of blood in the subarachnoid space follows gravity and CSF flow patterns: - **Basal cisterns** (interpeduncular, ambient, suprasellar) — most common site for aneurysmal SAH - **Sylvian fissures** — typical in anterior circulation aneurysms (AComm, ACA, MCA) - This pattern is highly specific for ruptured aneurysm rather than traumatic or hypertensive hemorrhage ### Imaging Findings in SAH | Feature | Non-contrast CT | CT Angiography (CTA) | MRI | |---------|-----------------|----------------------|-----| | **Acute blood** | Hyperdense in cisterns | Identifies aneurysm location | FLAIR hyperintensity | | **Aneurysm detection** | Not visible | 95–98% sensitivity | Less sensitive than CTA | | **Vasospasm** | Not shown | Can show narrowing (delayed) | Better for parenchymal ischemia | | **Timing** | Best within 6 hrs | Immediately after diagnosis | Days 4–14 for vasospasm | **High-Yield:** CTA of head and neck is now the gold standard for aneurysm detection in SAH — it replaces conventional angiography in most centers. Expected finding: saccular outpouching from the anterior communicating artery (AComm aneurysms account for ~30% of aneurysmal SAH). ### Why Anterior Communicating Artery? AComm aneurysms are the most common site of rupture in aneurysmal SAH (30–35%), followed by posterior communicating artery (30–35%) and middle cerebral artery bifurcation (20%). The basal and sylvian distribution fits AComm rupture. **Clinical Pearl:** The "sentinel headache" (brief, severe headache preceding major SAH by hours to days) occurs in ~50% of patients and represents a small warning leak. This patient's acute presentation suggests major rupture. **Mnemonic:** **SAH-CT** — **S**ubarachnoid **A**neurysm **H**emorrhage on **C**T shows **T**hunder-clap headache with **C**istern blood and **T**ypical **A**nterior circulation distribution.