How should I evaluate and manage a child with recent leg pain that started during a viral illness and who is otherwise well?

Evaluation and Management of Pediatric Leg Pain Following Viral Illness

In an otherwise well child with recent leg pain during a viral illness, the most likely diagnosis is benign acute childhood myositis (BACM), which requires assessment of creatine kinase levels and monitoring for rhabdomyolysis, while ruling out serious bacterial complications like meningococcemia or septic arthritis.

Initial Clinical Assessment

Red Flag Symptoms Requiring Immediate Action

Immediately assess for meningococcemia if the child has:

• Leg pain combined with fever, lethargy, or abnormal skin color 1, 2
• Petechial or purpuric rash anywhere on the body 1, 2
• Cold extremities, prolonged capillary refill >2 seconds, or signs of shock 1, 3
• Altered mental status, irritability, or poor feeding 3

If meningococcemia is suspected, administer parenteral antibiotics immediately after obtaining blood cultures, without waiting for confirmatory tests 2. This is critical because most deaths occur within the first 24 hours 2.

Distinguishing Benign Acute Childhood Myositis

BACM typically presents with:

• Bilateral calf pain (92% of cases) with symmetric involvement 4
• Difficulty walking or refusal to walk (56% of cases), often with tiptoe gait 5, 4
• Recent or concurrent viral illness, particularly influenza B, followed by influenza A 6, 4
• School-aged children (mean age 6.8 years), more common in boys 4
• Fever present in 80% of cases 4

The pain typically develops during or shortly after recovery from a viral upper respiratory infection 6, 5. Symptoms usually resolve within 3 days 5.

Diagnostic Workup

Essential Laboratory Testing

For suspected BACM, obtain:

• Creatine kinase (CK) level - expect elevation between 100-4000 U/L in typical cases, though levels can exceed 10,000-13,000 U/L in severe cases 6, 4
• Urine myoglobin to screen for rhabdomyolysis 5
• Basic metabolic panel to assess renal function if CK is significantly elevated 6

C-reactive protein is typically only slightly elevated in BACM 4, which helps distinguish it from bacterial infections.

When to Expand the Workup

Consider additional testing if:

• The child appears toxic, has unilateral symptoms, or has focal joint swelling - obtain blood cultures, complete blood count, and consider imaging for septic arthritis or osteomyelitis 7
• Symptoms persist beyond 3-5 days or worsen despite supportive care 5
• CK levels are markedly elevated (>10,000 U/L) - monitor daily until downtrending 6

Post-varicella patients with limb or joint pain require particular vigilance for musculoskeletal complications including septic arthritis, osteomyelitis, or pyomyositis, which may necessitate MRI imaging 7.

Management Strategy

Outpatient Management for Typical BACM

For well-appearing children with bilateral leg pain and stable vital signs:

• Provide supportive care with rest and analgesia 5
• Ensure adequate hydration to prevent rhabdomyolysis 6
• Arrange close follow-up within 24-48 hours 5
• Educate families about warning signs requiring immediate return: dark urine, decreased urine output, worsening pain, or inability to maintain hydration 6

Indications for Hospital Admission

Admit children with:

• CK levels >10,000 U/L or myoglobinuria 6, 5
• Signs of dehydration or inability to maintain oral intake 6
• Any concern for meningococcemia or other serious bacterial infection 1, 2
• Severe pain limiting mobility despite analgesia 6

Hospitalized patients require daily CK monitoring until levels are downtrending and symptoms begin to resolve 6. The mean hospital stay for BACM is 3.6 days 4.

Critical Pitfalls to Avoid

Do not:

• Dismiss leg pain in a febrile child without assessing for meningococcemia - half of children with meningococcal disease are initially sent home 2
• Delay antibiotics if meningococcemia is suspected while waiting for diagnostic tests 2
• Assume BACM is only a childhood condition - similar presentations can occur in adults 8
• Perform muscle biopsy in typical BACM cases, as the condition is self-limited and biopsy is unnecessary 8
• Overlook post-varicella musculoskeletal complications, which can be life- and limb-threatening despite occurring in otherwise healthy children 7

Viral Etiology Considerations

Beyond influenza A and B, multiple other viruses have been associated with BACM including herpes simplex, coxsackie, enteroviruses, adenovirus, respiratory syncytial virus, and parainfluenza viruses 4. However, specific viral testing is generally not necessary for management unless it changes clinical decision-making 5, 4.

The prognosis for BACM is excellent with appropriate supportive care and monitoring 5.