How to Diagnose Subarachnoid Hemorrhage: Key Medical Procedures and Imaging Techniques

Understanding Subarachnoid Hemorrhage

Subarachnoid hemorrhage (SAH) refers to bleeding that occurs in the subarachnoid space—the area between the arachnoid membrane and the pia mater, which are two of the protective layers surrounding the brain. This space normally contains cerebrospinal fluid (CSF), which cushions the brain and spinal cord. When blood leaks into this region, it can lead to sudden and severe neurological symptoms, including an intense headache often described as "the worst headache of my life."

Primary Diagnostic Tool: Non-Contrast CT Scan

The first and most critical step in diagnosing SAH is a non-contrast computed tomography (CT) scan of the head. Modern CT imaging is highly sensitive, especially within the first 6 to 12 hours after symptom onset, detecting over 95% of cases. On a CT scan, fresh blood appears as a hyperdense (bright white) area in the subarachnoid space, typically around the base of the brain, in the Sylvian fissures, or within the cerebral sulci. This immediate visualization allows for rapid diagnosis and prompt intervention.

When CT Results Are Inconclusive

If the CT scan does not show clear evidence of bleeding but clinical suspicion remains high—especially if the patient presents with a thunderclap headache—further diagnostic testing is essential. In such cases, a lumbar puncture (LP) is performed to analyze the cerebrospinal fluid.

During a lumbar puncture, a small amount of CSF is extracted from the lower back. In patients with SAH, the CSF will appear xanthochromic (yellowish) due to the breakdown of red blood cells, even if initial visual inspection shows bloody fluid. Spectrophotometry may also be used to detect bilirubin, confirming the presence of degraded blood and ruling out traumatic tap as a cause of red CSF.

Supportive Role of MRI and Advanced Imaging

While CT remains the frontline tool, magnetic resonance imaging (MRI), particularly sequences like gradient-echo (GRE) or susceptibility-weighted imaging (SWI), can also detect blood products in the subarachnoid space. These are especially useful in delayed presentations or when CT findings are subtle. However, MRI is less practical in emergency settings due to longer scan times and limited availability.

Identifying the Underlying Cause

Once SAH is confirmed, identifying the source of bleeding is crucial. The majority of spontaneous SAH cases are caused by the rupture of an intracranial aneurysm. To locate such vascular abnormalities, clinicians use catheter cerebral angiography or CT angiography (CTA). These imaging modalities provide detailed views of the brain's blood vessels and help guide treatment decisions, including surgical clipping or endovascular coiling.

Conclusion: A Multistep Diagnostic Approach

Diagnosing subarachnoid hemorrhage requires a systematic approach combining clinical assessment with advanced neuroimaging. Immediate non-contrast CT scanning offers the fastest and most reliable detection method. When CT is negative but suspicion persists, lumbar puncture becomes a vital diagnostic tool. Together with MRI and angiographic studies, these techniques ensure accurate diagnosis and effective management of this life-threatening condition.