A large outbreak of influenza B-associated benign acute childhood myositis in Germany, 2007/2008.

BACKGROUND

Benign acute childhood myositis (BACM) is a rare syndrome associated with various viral infections. Bilateral calve pain may lead to inability to walk. During winter 2007/2008, we investigated a nationwide outbreak of influenza-associated BACM (IA-BACM) to identify etiologic (sub)type, describe the course of disease, and explore how well the syndrome is known among physicians.

METHODS

We performed retrospective and prospective case finding in all German federal states. Physicians returned patient-based questionnaires containing information about sex, age, disease progression, patient-management, and number of BACM cases treated previously. We compared IA-BACM cases with influenza cases from the German virologic sentinel surveillance system for influenza.

RESULTS

We investigated 219 children with IA-BACM. They coincided with the curve of influenza B of the German virologic sentinel surveillance system for influenza. Median age was 7 years, 74% (160/216) of cases were male, median time between the onset of fever and onset of BACM-symptoms was 3 days lasting for a median of 4 days. Almost half of the affected children had presented at hospitals. One case with beginning renal impairment occurred, but the patient recovered completely. Most reporting physicians had not seen BACM-patients previously. Multivariable analysis showed IA-BACM's strong association with influenza B, male sex, and age between 6 and 9 years.

CONCLUSIONS

Influenza B caused a large BACM outbreak in Germany. Onset of BACM symptoms followed shortly after the onset of influenza symptoms. The course of this disease was almost exclusively mild and self-limiting. Diagnosis of this rare but distinct clinical entity by the alert physician can spare the patient potentially unneeded invasive testing and hospital admission.