Introduction: An epidemic of acute myositis affecting children in Eastern Nebraska occurred in association with influenza infections in 2007. The myositis was quite distinct from the generalized myalgias that typically precede or accompany influenza infection. Myositis followed the initial influenza prodrome by a few days and almost exclusively involved the gastrocnemius and soleus muscles. Our objective was to evaluate influenza-associated acute myositis during 2 consecutive influenza seasons.
Methods: After the institutional review board approval, we conducted a retrospective chart review to identify children younger than 18 years diagnosed with influenza at the Children's Hospital in Omaha, Neb, between October 2005 and April 2007. Myositis was defined by elevated creatine kinase (CK) levels and clinical presentation.
Results: Eighty-nine children tested positive for influenza at the Children's Hospital in the 2005-2006 winter season, and 184 tested positive in the 2006-2007 season. In the first season, 90% of cases were influenza A and 10% influenza B, whereas in the second season, 57% were influenza A and 43% influenza B; the increase of influenza B in the community is statistically significant with a 2-sided P < 0.0001. There were 15 cases of acute myositis in association with influenza in 2007 and none in the 2006 season; 73% of the cases were secondary to influenza B and 27% to influenza A. This difference is significant with a 2-sided P = 0.0264. The age range of children with acute myositis was 3 to 14 years, with a mean of 7.93 years and a median of 7 years. Most cases presented with a few days of fever, cough, and rhinorrhea followed by acute calf pain, abnormal gait, and inability to ambulate. Thirteen of the 15 patients were hospitalized. Most cases were uncomplicated with full recovery within a week. The peak CK levels ranged from 264 to 26,582 U/L. The children with the 2 highest CK levels died from complications of their illnesses.
Conclusions: Acute myositis after influenza infection occurs more commonly with influenza B, although the patients with influenza A had more complicated courses. Myositis should be suspected in patients with acute change in gait, refusal to walk, and intense leg pain a few days after a viral prodrome.