Edie, an eight-year-old girl, is seen via telehealth consultation with 24 hours of bilateral calf pain and disturbed gait, worsening despite regular paracetamol administration.
She developed URTI symptoms seven days ago, as well as a low-grade fever (37.9°C), rhinorrhoea, dry cough and sore throat, which have now almost completely resolved. Her four-year-old brother had parainfluenza virus the preceding week.
Edie is usually very well, with normal growth and development. There is no family history of note.
Via video examination, Edie is comfortable, happy and chatty when seated; she has good colour and no respiratory distress.
Edie’s parents demonstrate that her calves are very tender on palpation, with no other leg muscle or joint tenderness.
Passive hip and knee movements are normal, but attempted passive ankle movement causes nine out of 10 pain. Edie refuses to demonstrate walking.
What is the most likely diagnosis?
Correct!
In this case, the history of recent viral infection and localised calf muscle findings favour benign acute childhood myositis (BACM).
That said, the differential diagnosis for the condition includes a number of serious pathologies — including Guillain–Barré syndrome, juvenile idiopathic arthritis, malignancy, dermatomyositis, polymyositis, muscular dystrophy and trauma — so careful diagnostic and management reviews are warranted in suspected BACM.
The condition is characterised by sudden-onset muscle pain, primarily affecting the calves, during or following recovery from a viral illness. It is predominantly observed in school-aged children. BACM is most commonly associated with influenza A or B, with cases also reported following parainfluenza, coxsackie, HSV, Epstein–Barr and adenovirus infections.
The onset of myositis typically occurs with resolution of the acute viral symptoms — usually 1-2 days after fever, cough and coryza subside.
The child may refuse to walk, or ambulate with difficulty, as a result of pain or true muscle weakness. They will resist passive ankle dorsiflexion because of pain, maintaining their ankles in a plantar-flexed position at rest.
The pathogenesis is thought to be viral invasion of myocytes causing muscular necrosis. Muscle enzymes, such as CK, may be elevated up to 30 times the normal limit, but rhabdomyolysis is a rare sequela.
Management is supportive, including rest and analgesia, with full clinical recovery typically seen in 3-10 days and normalisation of muscle enzymes within three weeks. Depending on the degree of clinical concern and/or uncertainty, patients may be managed with close observation at home. Those with significantly disabling symptoms or any diagnostic concern may warrant admission.
In this instance, Edie is referred to ED and admitted under paediatrics. Investigations confirm parainfluenza virus, with mild leucopenia, normal renal function and a CK of 5518 U/L (normal: 55-200). Edie is admitted for IV fluids, pain management (with regular ibuprofen and paracetamol) and renal function monitoring.
Within 24 hours, her pain drops to a three out of 10 and her CK to below 3000 U/L. Forty-eight hours later, she is discharged home, walking normally and comfortably, with a CK of 1300 U/L. At one-week review, she is back to her usual active self.