Contents
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
- Trauma or non-accidental injury
- Guillain-Barré syndrome
- Non-specific myositis
- Infection
- DVT
- Malignancy
- Juvenile arthritis
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)