Diagnostic dilemmas and subarachnoid subtleties: What to do when the evidence gives you a headache

Abstract

Mann’s article 1 in this issue of CJEM (see page 102) illustrates some of the problems encountered in the emergency department (ED) assessment of suspected subarachnoid hemorrhage and addresses the role of lumbar puncture (LP) and imaging modalities in rural Canada. Mann describes two cases. In the first, a 68-year-old woman with minimal clinical findings undergoes LP shortly after the abrupt onset of her worst-ever headache. The LP results are negative, but 3 hours later she deteriorates, requiring emergent intubation and transfer. An intracerebral hematoma is subsequently evacuated and the patient recovers with permanent neurological deficits. In the second case, a 19-year-old woman presents with a vague history of gradual-onset headache and constitutional symptoms. Her exam is normal apart from “slight” meningismus. Despite observation and rehydration, her headache and meningismus persist. An LP is done, and a diagnosis of subarachnoid hemorrhage (SAH) is made based on red blood cells and xanthochromia in her cerebrospinal fluid (CSF). She is transferred, treated appropriately and recovers uneventfully. As an urban emergency physician, what are most striking about these cases are not the differences, but the similarities between small town and tertiary care practice. Mann’s discussion highlights dilemmas facing all practitioners fearful of missing a subtle SAH and helps underscore the paucity of evidence surrounding SAH diagnosis. Indeed, there are several fundamental questions that are answered poorly by current literature. What is the sensitivity of computed tomography for diagnosing SAH? Mann suggests that computed tomography (CT) sensitivity peaks at 24 hours; however, most references suggest that sensitivity is highest immediately after the bleed (i.e., less than 12 hours), then falls rapidly as blood in the CSF is broken down. 2,3 To provide a more useful answer to the sensitivity question, several factors should be considered. First, how much blood is in the CSF? Not surprisingly, patients with major neurological findings tend to have larger bleeds than those with headache alone. CT is, therefore, very sensitive in clinically obvious cases and less sensitive in subtle cases — the very ones where we need the most help with diagnosis. Many studies have combined patients with different grades of SAH, and the relatively high sensitivities reported in these studies cannot necessarily be generalized to patients with subtle clinical findings. The second factor involves the timing of the bleed. Blood is most radio-dense immediately after it enters the CSF. The longer the CT is delayed, the more red blood cells (RBCs) break down and the less likely they are to be seen on CT. The third critical factor is who interprets the CT. Studies suggest that neuroradiologists are more accurate than other radiologists,