When Is CT Really Necessary Before LP in Patients with Suspected Meningitis?
Source: Hasbun R, et al. Computed tomography of the head before lumbar puncture in adults with suspected meningitis. N Engl J Med 2001;345: 1727-1733.
This study was performed in an academic emergency department (ED). Patients who were being evaluated for suspected meningitis were enrolled prospectively prior to obtaining a computed tomography (CT) scan or performance of a lumbar puncture (LP). Baseline characteristics pertaining to subject demographics, symptoms, and physical findings were recorded. CT scans, when obtained, were examined by an independent radiologist and categorized as normal, focal abnormality with or without mass effect, or nonfocal abnormality with or without mass effect. The authors analyzed the baseline characteristics to determine which factors were associated with an abnormal CT.
Over four years, 301 subjects were enrolled. A CT of the head was obtained prior to LP in 235 subjects (78%). Of those undergoing CT, 76% had a normal study, 19% had abnormalities without mass effect, and 5% had evidence of a mass effect. Baseline characteristics associated with an abnormal CT were: age at least 60 years; immunocompromise; history of central nervous system (CNS) disease; seizure within previous week; abnormal level of consciousness; inability to follow commands or answer questions; gaze palsy; abnormal visual fields; facial palsy; leg or arm drift; or language abnormalities.
The features found to be associated with abnormal CT were analyzed to determine their sensitivity and predictive values in detecting CT abnormalities. Ninety-six subjects had none of the features associated with an abnormal CT; the CT was normal in 93 of the 96, yielding a negative predictive value of 97%. Of the three patients misclassified (i.e., the CT was abnormal although there were no abnormal baseline characteristics), only one had a mild mass effect on CT and all underwent an uneventful LP. The authors conclude that, in adults with suspected meningitis, specific clinical features can be used to identify patients unlikely to have an abnormal CT, and who may, therefore, safely undergo an LP without prior imaging studies.
Commentary by David J. Karras, MD, FAAEM, FACEP
This is the best study to date demonstrating the safety of performing an LP in patients with a normal sensorium and normal neurologic examination when meningitis is being excluded. The results can be taken at face value as a confirmatory study supporting what generally has been regarded as standard of care. Indeed, one of the study’s authors stated unequivocally in a 1997 New England Journal of Medicine review that CNS imaging was not required before LP in patients with suspected meningitis, unless coma, papilledema, or focal neurologic abnormality was noted.1 The present study adds age older than 60 years, immunocompromise, a history of CNS disease or recent seizures, and any impairment in consciousness to this list of indications for obtaining a CT.
I find the authors’ portrayal of the conventional wisdom in the ED evaluation of meningitis to be interesting and revealing. None of the authors was an emergency physician. Seventy-eight percent of the subjects underwent CT before their LP; 41% of these subjects had no indication for CNS imaging. When the physicians ordering the scans were interviewed, 34% stated they ordered scans because they believed it was the standard of care, and 5% cited fear of litigation as the primary reason for ordering the study. This is not the most favorable portrait of emergency physicians at the study site.
Of greater concern is the delay in treatment documented in this study. Of patients undergoing CT, the mean time from ED arrival to LP was 5.3 hours, and 3.8 hours elapsed until administration of antibiotic. Among subjects not undergoing CT, the mean time to LP was 3.0 hours, and time to antibiotic was 2.9 hours. While it may be argued that a CT was ordered in some cases because the diagnosis of meningitis was less apparent, these figures underscore the delays in life-saving treatment that occur when therapy is delayed for the performance of an unnecessary test. The delay in antibiotic administration noted in this study is exceptionally long; in contrast, another recent, retrospective study found a mean time to antibiotics of 1.1 hours in ED patients with meningitis.2 While the issue certainly is open to debate, it generally is accepted that a delay in antibiotic administration is highly undesirable in managing patients with suspected meningitis.1 The 3-4 hour delay encountered by the patients in this study is therefore troublesome.
References
1. Quagliarello VJ, et al. Treatment of bacterial meningitis. N Engl J Med 1997;336:708-716.
2. Miner JR, et al. Presentation, time to antibiotics, and mortality of patients with bacterial meningitis at an urban county medical center. J Emerg Med 2001;21 :387-392.
Dr. Karras, Associate Professor of Emergency Medicine, Department of Emergency Medicine Temple University School of Medicine, Director of Emergency Medicine Research, Temple University Hospital, Philadelphia, PA, is on the Editorial Board of Emergency Medicine Alert.
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