Mycoplasma Pneumoniae: The Most Common Cause of Pediatric Encephalitis?

Christie LJ, Honarmand S, Talkington DF, et al
Pediatrics. 2007;120:305-313

Summary

This report details data collected as part of the California Encephalitis Project, begun in 1998. The current study patients represent those reported to the project from 1996-2006. Among many pathogens identified, Mycoplasma pneumoniae was the most common cause of encephalitis.

The authors completed this report to detail the history, physical, and laboratory findings of the children with M pneumoniae encephalitis in order to better understand the epidemiology of the process.

Cases were defined as patients with encephalitis and evidence of acute infection with M pneumoniae: those with positive serum IgM or rising IgG antibodies to M pneumoniae, or subjects with a positive polymerase chain reaction to M pneumoniae from either the spinal fluid or respiratory sample.

There were 1988 subjects in the project sample, and 111 (5.6%) had evidence of acute M pneumoniae infection. The majority of the patients (n = 84 [76%]) with M pneumoniae were younger than 19 years of age, with a median age of 11 years for those with M pneumoniae encephalitis. There were no gender differences.

Many of the patients had gastrointestinal symptoms (45%), but an almost equal percentage had respiratory symptoms. Only 15% of the adults with M pneumoniae encephalitis had gastrointestinal symptoms. Of the pediatric patients with M pneumoniae encephalitis, most (n = 59 [70%]) were diagnosed by virtue of an elevated serum IgM to M pneumoniae. An additional 19 pediatric cases had IgM values that were positive along with another marker (such as respiratory polymerase chain reaction (PCR), serum IgG change, etc).

Only 1 subject with M pneumoniae encephalitis had a positive cerebrospinal fluid (CSF) PCR for M pneumoniae. Nine percent of child cases had respiratory PCR positive for M pneumoniae. The median CSF white blood cell count was 33/mm³ among the patients with M pneumoniae encephalitis.

Overall, however, the CSF values for patients with M pneumoniae encephalitis did not differ from those of other etiologies. Magnetic resonance imaging demonstrated abnormalities in 49% of the M pneumoniae cases.

Fourteen percent of the patients suffered coma, and 4% of the M pneumoniae child cases died. The authors note that M pneumoniae was the identified agent twice as often as enterovirus among children and 7 times as often as herpes simplex virus, type 1.

The authors conclude that M pneumoniae was the most common causative agent identified in this series of patients, but CSF evidence of the infection is not usually found.

Viewpoint

The lesson of this article is that serum samples, usually IgM and a combination of IgM and IgG (acute and convalescent titers), are the best methods to make the diagnosis of M pneumoniae encephalitis. A second message is that the symptoms of these patients are so similar to those seen in other etiologies that M pneumoniae might not immediately come to mind during consideration of the differential; M pneumoniae has to be on the list of "usual suspects," or clinicians will likely greatly underdiagnose it in encephalitis cases. Finally, the authors note that the role of antibiotics in M pneumoniae encephalitis is not clear, primarily because it is not clear whether the CNS manifestations in M pneumoniae patients are the result of direct invasion or an autoimmune process which might not be improved directly by antibiotics.