Influenzavirus B-associated acute benign myalgia cruris: an outbreak report and review of the literature.

Acute benign myalgia cruris is characterized by transient bilateral calf pain that leads to difficulty walking. A regional outbreak of influenza virus B-associated myalgia cruris was observed during the seasonal influenza outbreak observed in Switzerland from week 1 to 13 of 2013. We performed a prospective case finding among the Swiss-Italian pediatric emergency units and pediatricians. A review of the literature was also performed. The diagnosis of myalgia cruris was made in 49 Swiss-Italian children aged 3.0-14 years (♂:♀=1.7). Flu-like symptoms were resolving when bilateral calf pain began that remitted over ⩽ 7days. The creatine kinase-level, assessed in 28 patients, was elevated in 25. Nose swabs were positive for influenza virus B in 13 out of 14 cases. The blood cell count, measured in 41 cases, disclosed leucopenia in 12 and thrombocytopenia in 3. The review of the literature found 10 outbreaks of ⩾ 10 cases of influenza virus B-associated myalgia cruris, which included a total of 203 patients with a mean age of 7.3 years and a ♂:♀ ratio of 2.0. In conclusion influenza virus B caused a large Swiss-Italian outbreak of myalgia cruris. Our outbreak and the literature indicate that influenza virus B-associated myalgia cruris affects preschool- and school-aged children, primarily boys.