From the Guidelines
For suspected subarachnoid hemorrhage (SAH), a lumbar puncture (LP) should be performed if the non-contrast CT scan of the head is negative but clinical suspicion remains high, particularly when the CT is performed more than 6 hours after symptom onset, as LP can detect breakdown products of blood even days after the initial bleed 1. When a patient presents with symptoms suggestive of SAH, the initial diagnostic step should be a non-contrast CT scan of the head. However, if the CT is negative or nondiagnostic, an LP should be considered to look for xanthochromia or red blood cells in the cerebrospinal fluid, which would indicate SAH. The rationale for prioritizing LP over CT angiography (CTA) of the head and neck in this scenario is based on the high sensitivity of LP for detecting SAH, even when CT sensitivity decreases over time, and the potential severe morbidity and mortality associated with a missed SAH 1. Some key points to consider in the diagnostic workup of SAH include:
- A high level of suspicion for SAH should exist in patients with acute onset of severe headache 1
- Non-contrast head CT should be the initial diagnostic test, followed by LP if the CT is nondiagnostic 1
- CTA may be considered when there is a confirmed SAH on CT or LP to identify the underlying vascular abnormality, typically an aneurysm 1
- LP can detect breakdown products of blood even days after the initial bleed, making it valuable when clinical suspicion remains high despite a negative CT 1.
From the Research
Diagnostic Approach for Subarachnoid Hemorrhage (SAH)
- The diagnosis of SAH typically involves a noncontrast computed tomography (NCCT) scan, followed by a lumbar puncture (LP) if the NCCT is negative or inconclusive 2, 3.
- However, recent studies suggest that NCCT followed by CT angiogram (CTA) may be a reasonable approach in the evaluation of patients with suspected SAH, potentially reducing the need for LP 2.
- The decision to perform an LP versus a CTA HEAD AND NECK depends on various factors, including the time from symptom onset, the patient's clinical presentation, and the results of initial imaging studies 3, 4.
Timing of Lumbar Puncture
- If the patient presents within 6 hours of symptom onset, a noncontrast brain CT is highly sensitive for detecting SAH, and an LP may not be necessary if the CT is negative 3.
- However, if the patient presents more than 6 hours after symptom onset, an LP may be considered to rule out SAH, even if the initial CT is negative 3, 4.
- The probability of SAH in patients with a negative head CT obtained more than 6 hours from symptom onset is estimated to be around 1.0%, which may influence the decision to perform an LP 4.
Diagnostic Utility of Lumbar Puncture
- A systematic review and meta-analysis found that LP has a high enough proportion of positive results to warrant its continued use in patients with suspected SAH and a negative CT head, despite the sensitivity of modern CT scanners 5.
- The study estimated a pooled proportion of 0.383 for LP+ results in patients with suspected SAH and a negative CT head, highlighting the importance of LP in the diagnostic workup of SAH 5.