Aseptic vs. Bacterial Meningitis: What’s the Diff’?

abstract & commentary

Source: Negrini B, et al. Cerebrospinal fluid findings in aseptic versus bacterial meningitis. Pediatrics 2000;105:316-319.

When a patient who is being evaluated for meningitis has a minimally elevated cerebrospinal fluid (CSF) count without a predominance of polymorphonuclear (PMN) cells, the diagnosis of asep tic meningitis is often considered. What of the same patient with predominantly PMN cells seen on the differential 24 hours after the onset of illness—could this still be aseptic meningitis? The answer, in contrast to typical teaching, may be yes. Standard textbooks describe the typical CSF findings in aseptic meningitis as a pleocytosis of between 20 and 1000 white blood cells (WBCs), composed mainly of lymphocytes.

Although some patients with aseptic meningitis do have PMN predominance in the CSF, most studies have explained this as an early phenomenon, which is followed by a shift to nonuclear/lymphocyte cells within 24 hours. This often leads to uncertainty in the diagnosis and treatment of meningitis and is a source of confusion and controversy. The authors performed a retrospective chart review of 158 cases of meningitis in children 30 days to 18 years of age hospitalized during the peak months for enteroviral meningitis (April to October) between 1992 and 1997. There were 138 cases of aseptic meningitis (defined as having at least 20 WBCs/mm3 and the absence of bacterial growth on culture). The remaining 20 cases were diagnosed with bacterial meningitis (positive CSF culture or the presence of a CSF pleocytosis with positive cultures of the blood). CSF variables, including WBC differential and time from the onset of symptoms to the performance of a lumbar puncture, were analyzed. PMNs were considered to be predominant when the percentage of neutrophils added to juvenile forms was greater than 50%. Patients were excluded if they had received antibiotic therapy within the previous five days.

The results demonstrated that the percentage of CSF PMNs in aseptic meningitis was not statistically different for patients who had a lumbar puncture performed either within or beyond 24 hours of the onset of symptoms. Fifty-one percent of the 53 patients with aseptic meningitis and duration of illness greater than 24 hours had a PMN predominance. The mean CSF WBC and PMN percentage for aseptic meningitis was 391 ± 568 and 52% ± 32%, respectively. For the bacterial meningitis patients, the mean CSF WBC was 3461 ± 5841 with 78% ± 18% neutrophils. The sensitivity of a PMN predominance for aseptic meningitis was 57%, whereas the specificity was 10%. The positive predictive value of a PMN predominance for aseptic disease was 81%, but the negative predictive value was 3%. Alternative definitions of PMN predominance from 60% to 90% were not useful as a clinical indicator of bacterial disease. In conclusion, the majority of children in this study with aseptic meningitis had a PMN predominance in the CSF. The PMN predominance was not limited to the first 24 hours of illness. A review of the literature by the authors identified other investigations that report similar findings.

Comment by Richard J. Hamilton, MD, FAAEM, ABMT

In the ED, the diagnosis of aseptic meningitis must be made carefully. For the conservative physician, the patient must be clinically stable and must have a normal mental status, a modest cell count (less than 50 cells/mm3), no PMNs, and a normal glucose, protein,

Gram stain, and opening pressure. The use of less conservative cutoffs increases the risk of misdiagnosis. To make the diagnosis even more risk-free, a test for bacterial antigens such as countercurrent immunoelectrophoresis or latex agglutination test should be negative. This paper is interesting because it demonstrates that during enteroviral season, most patients actually have aseptic meningitis, even though they may have more than 50% PMNs and pleocytosis on CSF fluid analysis. It also reminds ED physicians that we admit patients for meningitis not because the source is often bacterial—often it is not—but because it is a serious illness with the potential for deterioration, even if the source is not bacterial.

32. The CSF shows a predominance of polymorphonuclear cells after lumbar puncture is performed 36 hours after the onset of fever and severe headache in a 5-year-old. The diagnosis is:

a. bacterial meningitis.

b. aseptic meningitis.

c. bacterial or aseptic meningitis.

d. not consistent with meningitis.