Introduction

Benign acute childhood myositis (BACM), sometimes referred to as viral myositis or influenza-associated myositis is a striking cause of childhood immobility, which is typically self-limiting and benign.

It often presents in school-age children, a few days after a viral infection (typically an URTI).

It is more common in boys.

Treatment is supportive and it generally resolved within a few days.

Epidemiology and Aetiology

  • Epidemiology is unknown
  • It is thought to be under-diagnosed
  • Typically occurs several days after a viral infection
    • Influenza is the most common causatory organism, particularly influenza B

Presentation

  • Calf pain
    • Pain usually almost always confined solely to the calves
  • Bilateral
  • Way result in a characteristic tip-toe walk, or refusal to walk
    • Children often crawl to get around
  • Extending the knees causes calf pain
  • Acute onset
    • E.g. patient awakes with symptoms, or onset over the course of an hour or two
  • Children otherwise systemically well
  • Symptoms often progressive, e.g.
    • Day one – complaining of calf pain
    • Day two and three – refusing to walk – crawling on all 4s
    • Day three – tip-toe walking
    • Day 4 – return to normal gait and mobility
  • Tender calves on examination
  • Rarely rhabdomyolysis can occur

Diagnosis

  • Diagnosis usually clinical
  • Often quite distressing for parents
  • Due to under-recognition of the condition, there may be over-investigation for serious causes
  • Bloods
    • May show raised creatine kinase (CK)
    • Not usually indicated
  • Nasopharyngeal swab for influenza PCR
    • May confirm the diagnosis of influenza
    • Again, not usually needed

Differentials

Management

  • Reassure parents
  • Typically time for resolution of symptoms is 3 days
  • Simple analgesia
    • e.g. paracetamol 15mg/Kg QID
    • +/- ibuprofen 10mg/Kg TDS
  • Safetynetting for worsening symptoms, fevers, systemically unwell, or dark urine (rhabdomyolysis)

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