When LP Is Not Necessary to Detect Subarachnoid Bleed
CT performed within 6 hours of symptom onset in neurologically intact patients had 100% negative predictive value in this prospective multicenter study.
Standard teaching is that lumbar puncture (LP) is essential in patients with suspected subarachnoid hemorrhage (SAH) despite normal head computed tomography (CT) scans. Researchers prospectively enrolled 3132 consecutive neurologically intact patients older than 15 who underwent head CT with third-generation multislice scanners to evaluate nontraumatic acute headache or headache with syncope at 11 tertiary emergency departments in Canada from 2000 to 2009. LP was performed at the discretion of the treating physician. Experienced radiologists who were blinded to the study interpreted all CT scans. SAH was defined by subarachnoid blood on CT, aneurysm on cerebral angiography, or xanthochromia in cerebrospinal fluid.
Mean headache peak pain severity was 8.7 on a 0–10 scale. LP was performed in 49% of patients after negative CT scans. Overall, 240 patients (7.7%) were diagnosed with SAH. The sensitivity of head CT for SAH was 92.9%, and the negative predictive value (NPV) was 99.4%. Emergency physicians identified all but three cases of SAH; all three patients were scanned >6 hours after headache onset. Among 953 patients who were scanned within 6 hours of symptom onset, head CT had 100% sensitivity and 100% NPV. Follow-up at 1 and 6 months did not identify any cases of missed SAH.
Comment: Because subarachnoid blood diffuses and hemolyzes within hours, CT might not be able to distinguish cerebrospinal fluid from blood as time passes. Patients with histories that raise concern for SAH should be prioritized to undergo CT within 6 hours of symptom onset. If CT is performed with a modern scanner and is interpreted as negative for SAH by an experienced radiologist, LP is unnecessary, unless it is being performed to detect other causes of headache.
— Kristi L. Koenig, MD, FACEP
Published in Journal Watch Emergency Medicine August 5, 2011
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