Diagnosing Subarachnoid Hemorrhage: Key Tests and Medical Evaluations
Subarachnoid hemorrhage (SAH) is a life-threatening condition that requires prompt diagnosis and treatment. Early detection through accurate diagnostic methods can significantly improve patient outcomes. When SAH is suspected, healthcare providers rely on a combination of clinical assessments and advanced medical tests to confirm the presence of bleeding in the subarachnoid space—the area between the brain and the thin tissues that cover it.
Physical Examination: Identifying Neurological Signs
A thorough neurological assessment is often the first step in evaluating a potential subarachnoid hemorrhage. Physicians look for classic signs of meningeal irritation, which commonly occur when blood irritates the protective layers surrounding the brain. One of the most telling symptoms is nuchal rigidity—stiffness in the neck that prevents the chin from touching the chest. Doctors measure this by checking how many fingers fit between the chin and sternum.
Additional diagnostic maneuvers include testing for Kernig's sign and Brudzinski's sign. A positive Kernig's sign is indicated by pain and resistance when the leg is extended at the knee with the hip flexed. A positive Brudzinski's sign occurs when neck flexion causes involuntary bending of the hips and knees. These physical findings strongly suggest meningeal inflammation, frequently associated with SAH.
Imaging Studies: CT Scans as the First-Line Tool
Non-contrast head CT scanning is the primary imaging method used to detect subarachnoid hemorrhage. It is fast, widely available, and highly sensitive—especially within the first 6 to 12 hours after symptom onset. On a CT scan, fresh blood in the subarachnoid space appears as bright white (hyperdense) areas along the brain's surface, in the fissures, or around the base of the brain.
While MRI can also be used, it is generally less effective than CT in the acute phase due to longer scan times and lower sensitivity for detecting early bleeding. However, if the CT scan is inconclusive and clinical suspicion remains high, an MRI with specific sequences like FLAIR (Fluid-Attenuated Inversion Recovery) may help identify subtle signs of blood products in the cerebrospinal fluid spaces.
When CT Is Negative but Symptoms Persist
In cases where the initial CT scan does not show clear evidence of bleeding—particularly if the patient presents several hours or days after the event—further investigation becomes essential. Delayed imaging may miss SAH because the blood has started to disperse and mix with cerebrospinal fluid (CSF), reducing its visibility on scans.
This is where lumbar puncture plays a critical role in confirming the diagnosis.
Lumbar Puncture: The Gold Standard for Confirmation
Lumbar puncture (LP) remains the definitive test for diagnosing subarachnoid hemorrhage when imaging results are normal but clinical signs point toward SAH. During this procedure, a small amount of cerebrospinal fluid is collected from the lower back using a thin needle. In a healthy individual, CSF should be crystal clear and free of red blood cells (RBCs).
If RBCs are found uniformly across multiple CSF tubes and do not decrease in number from the first to the last tube—a test known as the "three-tube technique"—it suggests true bleeding rather than a traumatic tap caused by the needle puncturing a small vessel during insertion. Furthermore, xanthochromia—yellowish discoloration of the CSF caused by the breakdown of hemoglobin—can be detected under spectrophotometry, providing strong evidence of prior hemorrhage even when RBCs have lysed.
Follow-Up Testing After Diagnosis
Once SAH is confirmed, additional imaging such as CT angiography (CTA), MR angiography (MRA), or digital subtraction angiography (DSA) is typically performed to locate the source of bleeding—most commonly a ruptured cerebral aneurysm. These vascular studies help guide treatment decisions, including surgical clipping or endovascular coiling.
Early and accurate diagnosis of subarachnoid hemorrhage is crucial for preventing complications such as rebleeding, vasospasm, and hydrocephalus. By combining physical examination findings with advanced neuroimaging and lumbar puncture analysis, clinicians can swiftly confirm SAH and initiate life-saving interventions.