COMPUTED TOMOGRAPHY SCANNING The ED patient whose headache requires emergent investigation usually begins with a noncontrast computed tomography (CT) scan.4 The use of contrast material increases the time, expense, and risk of adverse effects (minor 10 percent, severe 0.1 percent), 5 and the noncontrast CT scan usually adequately excludes critical lesions or mass effects requiring emergent interventions. In particular, the noncontrast CT scan is the best neuro imaging test for diagnosing an acute SAH, but CT scan cannot rule it out. When there is strong suspicion of small lesions likely to be missed without contrast (e.g., in an AIDS patient suspected of cerebral toxoplasmosis or suspected small brain mass), then a CT with contrast material may be needed. 5
LUMBAR PUNCTURE Lumbar puncture (LP) is required in cases such as suspected meningitis, or suspected SAH with a normal CT scan. Contraindications to LP include the suspicion of raised ICP, which can be excluded by the absence of papilledema, normal level of consciousness, and normal findings on neurologic examination. Venous pulsations seen at the disc margins on fundoscopic examination with the patient upright effectively rule out ICP. Venous pulsations may be terminated temporarily with pressure on the globe. If these conditions are met, then a CT scan need not necessarily be carried out prior to LP, especially if the CT is likely to be delayed. In cases of suspected meningitis without evidence of raised ICP, LP may be done without prior CT in order to avoid any delay in starting antibiotics.4
MAGNETIC RESONANCE IMAGING The cost and restricted availability of magnetic resonance imaging (MRI) limit its utility in the emergency investigation of headache. MRI is more sensitive than CT in evaluating brain injuries, such as diffuse axonal injuries, small parenchymal contusions, or isodense subdural hemorrhages, and most tumors.5 In acute SAH, however, MRI is no more sensitive than CT in the first few days following a bleed.13 CT scanning and LP are adequate for the large majority of ED headache patients requiring emergent investigation.
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