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.