Acute benign myositis common in children with flu infection

Children with influenza A virus infection are susceptible to acute benign myositis (ABM), with most patients showing clinical characteristics consistent with postfebrile myalgia, reports a study.

“[A]ccording to the corresponding characteristics of elevated muscle enzymes, a clinical diagnosis can be made quickly and early,” the researchers said. “Moreover, ABM in children is self-limiting, the prognosis is good, and these reports aim to reduce unnecessary examinations and treatments.”

This retrospective analysis utilized clinical data of 118 children with ABM associated with influenza A virus infection. These patients were admitted to the Children’s Hospital of Chongqing Medical University in China during the epidemic of influenza A from February 2023 to May 2023.

Most of the paediatric patients included in this study were preschool- or school-age. Sudden lower limb pain after fever was the most common symptom, and many children also experienced bilateral calf myalgia. [Pediatr Infec Dis J 2024;43:626-629]

The muscle enzyme spectrum had a range of 187.21‒32,191.00 μg/L (median 3,053 μg/L), while that for creatinine kinase isoenzymes was 5.15‒749.7 μg/L (median 41.82 μg/L). Additionally, myoglobin ranged from 102.79 to 1,200 μg/L, with a median of 674.9 μg/L.

Children with ABM underwent examinations via electromyography, muscle color Doppler ultrasound, radiograph, and cardiac color Doppler ultrasound, which were rarely positive.

All patients had a good prognosis with no serious consequence. Apart from bed rest and a reduction of physical activity, children received oseltamivir treatment, medications to improve energy metabolism, fluid replacement, or alkalinization of symptomatic treatment.

“All patients were followed up within 1–2 months,” the researchers said. “There was no recurrence of clinical symptoms or sequelae, and their routine blood and muscle enzyme spectra returned to normal.”

Clinical characteristics

Childhood ABM is characterized by early fever and sudden lower limb pain after reaching a normal body temperature. In the current study, 63.56 percent of children showed a consistent disease course, while 60.17 percent had myalgia within 3 days following defervescence.

“Therefore, we should suspect [childhood ABM] in patients with the above characteristics,” the researchers said.

The mechanism behind this remains unclear. Several studies suggested potential causes of virus-induced muscle damage, including direct invasion of the muscle by viral particles and virus-triggered immune-mediated muscle damage. [Eur J Pediatr 2021;180:577-584; Neuromuscul Disord 2014;24:342-346; Kurume Med J 1984;31:209-216; JAMA 1980;243:461-462; Muscle Nerve 2019;60:586-590]

“Our study suggested that the course of early fever and sudden lower limb pain after defervescence may be related to immune-mediated muscle damage caused by the virus. When fever occurred simultaneously with myalgia, this may have been related to the direct invasion of the muscle by viral particles. The above mechanisms need to be further confirmed by basic research,” the researchers said.

“It is necessary to closely follow up on the duration of myalgia, the characteristics of elevated muscle enzymes, and the recovery of muscle enzymes to exclude other neuromuscular diseases and metabolic myopathies,” they noted.

Paediatric ABM is a transient, self-limiting inflammatory disease of the skeletal muscle. It normally occurs following a viral infection and has a good prognosis.