Sporadic acute benign calf myositis: Systematic literature review

Acute benign calf myositis is a rare infection-associated syndrome presenting with calf pain that occurs in epidemics or sporadically. Epidemic cases are usually associated with influenza virus type B. Sporadic cases, however, might be associated with a large number of microorganisms. Furthermore, during an outbreak there is a great alertness that promotes earlier diagnosis. In contrast, there is likely a lower awareness regarding the sporadic form, compromising early and correct diagnosis. In order to characterize the sporadic form of acute calf myositis and increase the knowledge of this condition, we systematically reviewed the literature reporting sporadic cases. We identified 72 reports, including 451 patients, 325 males and 126 females. Sporadic acute benign calf myositis affected subjects ≤18 years of age (N = 450; 99%), who followed a prodromal flu-like illness (N = 411; 91%), presented with pain and tenderness affecting only the calves for ≤1½ weeks (N = 441; 99%) and was never complicated by kidney involvement. The creatine kinase ratio was ≥10 in 310 (70%) out of 444 cases. Microbiological studies identified an infectious trigger in 181 cases, mostly influenza virus (type B more frequently than type A), Dengue, Ebstein-Barr or Parainfluenza virus and Mycoplasma pneumoniae. Sporadic acute benign calf myositis is a self-limited condition that can usually be diagnosed on a clinical basis. Unlike the epidemic form, many cases are due to microorganisms other than influenza virus B or A.