Clinical Criteria Identify Patients Able to Undergo LP Without CT Scan
Abstract & Commentary
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.
The emergency evaluation of a patient with suspected meningitis typically includes a lumbar puncture (LP) following a computed tomography (CT) scan. Imaging is intended to rule out an intracranial lesion with significant mass effect, since the pressure gradient produced by lumbar puncture may lead to the complication of downward herniation. Delay in obtaining a CT may result in a prolonged waiting time for initiation of antibiotic therapy or in the blind use of empiric antibiotics prior to lumbar puncture.
Hasbun and colleagues investigated whether a group of patients may be identified in whom lumbar puncture may be performed safely without prior CT. Hasbun et al prospectively evaluated 301 patients with suspected meningitis; 235 of these underwent CT. In 56 of the 235, the results of CT were abnormal. Mass effect was present in 11. Clinical features significantly associated with an abnormal CT were: age older than 60, immunocompromise, a history of CNS disease, history of seizure within 1 week of presentation, and neurologic abnormalities as assessed by a modified NIH Stroke Scale. These included an abnormal level of consciousness, inability to answer orientation questions, inability to follow 2 commands, gaze palsy, abnormal visual fields, facial palsy, arm drift, leg drift, or abnormal language function. There were none of these findings in 96 patients, and CT was normal in 93 of these patients. The negative predictive value of these criteria was thus 97%. Of the 3 misclassified patients, only 1 had mild mass effect on CT and all 3 underwent LP without complication. Of note, papilledema, the clinical finding commonly associated with increased intracranial pressure and a danger to LP, was present in only 1 patient. Three patients with severe mass effect on CT demonstrated multiple clinical "red flags," and were deemed too unsafe for LP. Two of these soon died of herniation.
The time delay for LP was 5.3 hours in patients who underwent CT and only 3 hours for those who did not have CT. There was a trend, but no statistical difference between the time from admission to the ER to the initiation of antibiotics in the 2 groups. The ultimate diagnosis was meningitis in a minority of patients, 28%. Only 6% had a pathogen identified by microbiology and most of these were nonbacterial. A large proportion of patients were immunocompromised—approximately 25%—increasing the likelihood of such diagnoses as cryptococcus, toxoplasmosis, and lymphoma. CTs were ordered by treating physicians surveyed to rule out a suspect mass lesion in 59%; as a matter of "standard of care" in 34%; and due to "fear of litigation" in 5%.
Commentary
This report confirms that in the vast majority of cases, LP may be performed safely in patients suspected to have meningitis. These data convincingly show that among patients at low risk by clinical criteria, the likelihood of significant abnormalities on CT is nearly zero. In fact, if the 2 patients with abnormal CTs but no mass effect are reclassified as "safe," then the negative predictive value of these clinical criteria is greater than 99%.
The presence of CT scan abnormalities, including mild mass effect, furthermore does not preclude a safe LP. Small-volume spinal taps using smaller 22-gauge needles are likely of minimal to no risk in patients with supratentorial lesions. Indeed, it is the presence of posterior fossa masses that poses the highest risk.
These analyses may be made irrelevant in emergency rooms with easy access to a rapid CT scan. If time is of the essence, the time necessary to obtain CT may be quite brief. For example, in acute stroke patients, the "door-to-needle" time including CT may be as short as 1 hour. These data are perhaps better applied to patients who are unable to rapidly get a CT due to critical illness or complicating medical issues. Unfortunately, these patients would not be the young, neurologically normal patients whom the clinical criteria of Hasbun et al best fit. Also, the presence of a normal CT does not preclude the presence of increased intracranial pressure and does not guarantee that a herniation pattern may evolve with or without LP. —Alan Z. Segal, MD. Dr. Segal, Assistant Professor, Department of Neurology, Weill-Cornell Medical College, Attending Neurologist, New York Presbyterian Hospital, is Assistant Editor of Neurology Alert.
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