Diagnosing Subarachnoid Hemorrhage: Key Methods and Clinical Insights

Subarachnoid hemorrhage (SAH) is a critical neurological condition that requires prompt and accurate diagnosis to improve patient outcomes. Early detection plays a vital role in preventing complications such as cerebral vasospasm, hydrocephalus, or even death. The diagnosis of SAH relies on a combination of clinical presentation, imaging studies, and cerebrospinal fluid analysis. Recognizing the hallmark symptoms and utilizing the appropriate diagnostic tools are essential for timely intervention.

Recognizing the Clinical Signs of SAH

One of the most characteristic features of subarachnoid hemorrhage is a sudden, severe headache—often described by patients as "the worst headache of my life." This abrupt onset headache is frequently accompanied by nausea, vomiting, photophobia, and altered mental status. In many cases, neck stiffness (nuchal rigidity) develops due to irritation of the meninges. These symptoms, when occurring together, should raise immediate suspicion of SAH, especially in individuals with risk factors such as hypertension, smoking, or a family history of aneurysms.

The Role of Non-Contrast Head CT in Diagnosis

Non-contrast computed tomography (CT) of the brain is the first-line imaging modality for detecting subarachnoid hemorrhage. It is fast, widely available, and highly sensitive, with a detection rate exceeding 95% when performed within the first 24 hours after symptom onset. The presence of hyperdense (bright white) areas in the subarachnoid spaces—such as the interpeduncular cistern, sylvian fissure, or perimesencephalic cistern—strongly suggests bleeding.

For example, blood located in the interpeduncular cistern may indicate an aneurysm in the anterior communicating artery or anterior cerebral artery. Hemorrhage in the sylvian fissure often points to a middle cerebral artery aneurysm. In contrast, perimesencephalic bleeding patterns typically carry a better prognosis and are less commonly associated with aneurysms.

Time Sensitivity of CT Scans

The sensitivity of head CT decreases over time. While it remains above 95% within the first day, it drops to approximately 80% by day three and may fall to around 50% after one week as the blood is gradually reabsorbed. Therefore, early imaging is crucial. Delays in scanning can lead to false-negative results, making it more challenging to confirm the diagnosis based on CT alone.

When CT Is Inconclusive: The Use of Lumbar Puncture

If a non-contrast CT scan does not show clear evidence of bleeding but clinical suspicion remains high, a lumbar puncture (LP) becomes a valuable diagnostic tool. However, this procedure is typically not performed within the first 12 hours after symptom onset. This precaution is due to the risk of precipitating brain herniation in patients with elevated intracranial pressure caused by acute hemorrhage.

After 12 hours, if an LP is carried out and reveals uniformly bloody cerebrospinal fluid (CSF) without clearing across tubes, it supports the diagnosis of SAH. Additional CSF analysis—such as xanthochromia (a yellowish tint caused by bilirubin from lysed red blood cells)—can further confirm the presence of prior bleeding, especially in cases where the CT was negative but symptoms persist.

Advanced Imaging: MRI and Its Advantages

In cases where both CT and lumbar puncture yield inconclusive results, magnetic resonance imaging (MRI) offers a powerful alternative. MRI demonstrates high sensitivity for detecting subarachnoid blood even one to two weeks after the initial bleed, maintaining a positive detection rate of over 90% during this period. Sequences such as gradient echo (GRE) or susceptibility-weighted imaging (SWI) are particularly effective at identifying small amounts of blood products that might be missed on standard CT scans.

This makes MRI especially useful in delayed presentations or when evaluating patients with recurrent headaches following a suspected but unconfirmed hemorrhagic event.

Confirming the Source: Cerebral Angiography

Once SAH is confirmed, identifying the underlying cause—most commonly a ruptured cerebral aneurysm—is critical. Computed tomography angiography (CTA) is now routinely used as a non-invasive method to detect aneurysms with high accuracy. In some cases, digital subtraction angiography (DSA) may still be required for a more detailed vascular assessment, particularly when planning endovascular treatment.

Early identification of the bleeding source allows for timely interventions such as surgical clipping or endovascular coiling, which can prevent rebleeding—a major cause of mortality in SAH patients.

Conclusion: A Multimodal Diagnostic Approach

Diagnosing subarachnoid hemorrhage involves a stepwise approach combining clinical evaluation, neuroimaging, and laboratory testing. While non-contrast head CT remains the gold standard in the acute phase, lumbar puncture and MRI serve as essential adjuncts when initial imaging is negative. Timely and accurate diagnosis significantly improves prognosis and guides life-saving treatments. Healthcare providers must maintain a high index of suspicion, especially in patients presenting with thunderclap headaches, to ensure no case goes undetected.