Clinical prediction tool useful for identifying patients at low risk for Lyme meningitis

Cohn KA. Pediatrics. 2012;149:46-53.

Patients who are classified as low risk for Lyme meningitis using the “Rule of 7’s” clinical prediction tool may be managed as outpatients, but patients should still be closely monitored until serology data become available, according to a study published online.

Keri A. Cohn, MD, of Children’s Hospital Boston, and colleagues said the Rule of 7’s is a useful predictive tool in screening for whether to hospitalize children while awaiting serology results.

The Rule of 7’s classifies children at low risk for Lyme meningitis when each of the following three criteria are met: less than 7 days of headache; less than 70% cerebrospinal fluid (CSF) mononuclear cells; and absence of seventh or other cranial nerve palsy.

In the retrospective study, Cohn and colleagues tested the efficacy of the Rule of 7’s on a multicenter cohort of 423 children presenting with symptoms of CSF pleocytosis at children’s hospitals based in Boston, Wilmington, Del., and Philadelphia.

The researchers said among 423 children, 117 had Lyme meningitis, 306 had aseptic meningitis and none had bacterial meningitis. Only five of 130 children who were classified as having a low risk under the Rule of 7’s had Lyme meningitis.

“The Rule of 7’s performed better and was considerably easier to apply than the Avery predictive model, which required a complicated mathematical calculation to estimate the probability of Lyme meningitis,” the researchers wrote. “The Rule of 7’s also performed well in the subgroup of patients with occult Lyme disease (ie, no physician-documented [erythema migrans] rash).”

The researchers said there were some study limitations; notably, the study’s retrospective design, which may have limited access to medical records or clinical predictors. Regardless, treating those patients classified as low risk may reduce unnecessary antibiotic use and hospitalizations.

Disclosure: The researchers report no relevant financial disclosures.

The goal of this study is admirable: to assess the validity of the “Rule of 7’s” to identify which children in areas endemic for Lyme disease who present with what appears to be aseptic meningitis are at low risk of actually having Lyme meningitis.

However, the investigators used an unreliable definition for which patients had Lyme meningitis. Patients with pleocytosis and positive serology in the peripheral blood for either immunoglobulin M or IgG antibodies to Borrelia burgdorferi were classified as having Lyme meningitis. It is well recognized that the criteria for positive IgM serology are not sufficiently stringent and that false-positive results are common, yet nearly half of the patients with Lyme meningitis had a positive IgM alone. Moreover, most experts would not accept positive serology in the blood alone as evidence of Lyme meningitis (antibody concentration in CSF is more of a gold standard). Consequently, it is likely that the number of patients with Lyme meningitis in this sample is substantially overestimated, which makes the conclusions of the investigators subject to uncertainty.

While it is likely that patients in the low-risk category are indeed at low risk of Lyme meningitis, the implication that the larger number of subjects not in the low-risk category should be hospitalized and treated for possible Lyme meningitis is disturbing. Not only is it likely that a majority of these patients do not have Lyme meningitis, but also there is now good evidence from clinical trials in Europe that Lyme meningitis can be treated effectively with doxycycline administered orally.

- Eugene Shapiro, MD
Infectious Disease News Editorial Board member

Disclosure: Dr. Shapiro reports no relevant financial disclosures.

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