Benign Acute Childhood Myositis in a Pediatric Patient Post Influenza B Infection

Benign acute childhood myositis (BACM) is a mild and self-limited sudden onset of lower extremity pain during or following recovery from a viral illness. It is characterized by difficulty walking due to severe bilateral calf pain, which usually resolves in three days. It is typically appreciated during times of large influenza outbreaks and epidemics. The most severe complication can be rhabdomyolysis without proper treatment and can lead to renal damage and potential renal failure. There are limited reported cases of BACM and therefore no clear guidelines in the treatment or management of the condition. This case is unique in the sense that the patient had leg pain the entire month prior to presentation in the absence of trauma or injury, and it is believed that the pre-existing myalgia may have been exacerbated by an upper respiratory infection (URI) that started a few weeks after the leg pain onset. In addition, this patient's creatine kinase peaked at over 13,000 U/L, which is three to five times higher than the average of other reported children with this condition. The patient is a five-year-old male who presented to the emergency department with bilateral leg pain and difficulty ambulating. His guardian reported that the leg pain began one month prior to presentation and worsened to the point where he could no longer ambulate, following a fever and cough that began one week prior to presentation. A respiratory viral panel was positive for influenza B, and initial creatine kinase (CK) levels were greater than 10,000. A diagnosis of BACM was made, and supportive care was initiated. BACM is an infrequent complication following a viral infection that is typically treatable with hydration management and routine CK monitoring. Symptoms of BACM are usually limited to muscle pain and weakness, but it can progress to rhabdomyolysis and renal failure if not managed properly. It is therefore crucial that physicians monitor CK values daily until a downtrend is noticed and symptoms begin to resolve.