In a 5-year-old girl who had fever and cough‑cold a week ago and now presents with severe bilateral lower‑limb myalgia, what is the appropriate evaluation and management?

Benign Acute Childhood Myositis (Post-Viral Myositis)

This 5-year-old girl most likely has benign acute childhood myositis (BACM), a self-limited post-viral syndrome that typically follows influenza or other respiratory viral infections by 1-7 days and presents with bilateral lower limb pain, particularly in the calves.

Clinical Recognition

The presentation is classic for BACM 1:

• Timing: Symptoms appearing 7 days after a febrile respiratory illness
• Age: Peak incidence in school-aged children (mean 6.8 years), more common in boys but occurs in girls 1
• Location: Bilateral leg pain (92% of cases), predominantly affecting the calves 1
• Associated symptoms: Recent fever (80% of cases) and difficulty walking/inability to ambulate (56% of cases) 1

The viral prodrome of fever, cough, and cold followed by severe bilateral lower limb myalgia is pathognomonic for this condition 2, 3.

Immediate Evaluation

Check creatine kinase (CK) levels immediately - this is the single most important diagnostic test:

• CK levels are markedly elevated, typically ranging from 1,000-4,000 U/L, though can reach >8,000 U/L 3, 1, 4
• CK-MB and myoglobin are also elevated 4
• C-reactive protein is only mildly elevated or normal 1
• Complete blood count may show mild leukopenia or thrombocytopenia 2
• Renal function and urinalysis must be checked to exclude myoglobinuria and rhabdomyolysis 2

Viral testing should include:

• Nasopharyngeal swab for influenza A and B (most common causes) 3, 1, 4
• Consider testing for RSV, adenovirus, enterovirus, and parainfluenza if influenza is negative 1

When to Admit vs. Outpatient Management

Admit if any of the following are present 5:

• Inability to walk or severe functional impairment
• Signs of dehydration
• CK levels >10,000 U/L (risk of rhabdomyolysis)
• Abnormal renal function or myoglobinuria
• Altered mental status or drowsiness
• Respiratory distress

Outpatient management is appropriate if:

• Child can ambulate with assistance
• Normal renal function and no myoglobinuria
• Adequate oral intake maintained
• Reliable follow-up within 24-48 hours

Management Protocol

Supportive Care (Primary Treatment)

• Rest and activity restriction - avoid physical exertion until symptoms resolve 4
• Hydration: Encourage oral fluids; IV fluids if unable to maintain intake or if CK >5,000 U/L to prevent renal complications 2, 4
• Analgesia: Paracetamol or ibuprofen for pain (avoid aspirin in children) 5

Antiviral Therapy

Consider oseltamivir if:

• Influenza confirmed and symptoms <48 hours from viral illness onset
• Severe symptoms requiring hospitalization 4
• Note: Most cases present after the window for effective antiviral therapy

Monitoring

If admitted 2:

• Daily CK levels until declining
• Daily renal function monitoring
• Urine output and color (watch for dark urine indicating myoglobinuria)
• Repeat assessment of ability to ambulate

If managed outpatient:

• Follow-up within 24-48 hours
• Repeat CK if symptoms worsen
• Return immediately if develops dark urine, decreased urine output, or inability to walk

Expected Course and Prognosis

Complete recovery occurs within 3-10 days in all cases 2, 3, 1:

• CK levels normalize within 24-72 hours 2
• Pain resolves within 2-5 days
• Full ambulation returns within 3-7 days
• Mean hospital stay when admitted: 3.6 days 1
• No long-term sequelae or complications 4

Critical Pitfalls to Avoid

Do not pursue extensive workup if presentation is typical:

• Electromyography is rarely positive and unnecessary 4
• Muscle ultrasound may show inflammation but doesn't change management 3, 4
• Muscle biopsy is not indicated for typical presentations 4
• Cardiac evaluation only if CK-MB disproportionately elevated or cardiac symptoms present 4

Red flags requiring alternative diagnosis consideration:

• Asymmetric or upper extremity involvement
• Persistent fever beyond 4-5 days
• Progressive weakness rather than pain-limited movement
• Altered mental status or meningismus (consider MIS-C if recent COVID exposure) 6, 7, 6
• Rash, conjunctivitis, or mucosal changes (consider MIS-C or Kawasaki disease) 6
• Recurrent episodes (consider familial Mediterranean fever or metabolic myopathy) 8

This child requires CK measurement, viral testing, renal function assessment, and supportive care with close monitoring for rhabdomyolysis - the condition is self-limited with excellent prognosis.