What is the most likely cause of a patient with a two‑week worsening upper‑respiratory infection who now presents with 12 hours of severe, intractable leg spasms?

Influenza-Associated Myositis with Guillain-Barré Syndrome as a Critical Differential

The most likely cause is influenza-associated myositis presenting with severe leg spasms, but you must urgently rule out Guillain-Barré syndrome, which can develop after respiratory infections and presents with ascending weakness that may initially manifest as leg spasms or difficulty walking. 1, 2

Immediate Diagnostic Approach

Primary Consideration: Influenza-Associated Myositis

• Influenza-associated myositis characteristically develops 2-4 days after respiratory symptoms begin to subside, presenting with extreme exhaustion, severe muscle pain (particularly in the calves), and marked functional impairment. 2

• The timing in this case—12 hours of intractable leg spasms after two weeks of worsening URI—fits the classic pattern where acute respiratory symptoms improve before myositis emerges. 2

• Check creatine phosphokinase (CPK) immediately; levels typically exceed 1,000 U/L in influenza-associated myositis and can reach >5-10 times normal, indicating risk for rhabdomyolysis. 2, 3

• Examine for focal calf muscle tenderness on palpation, assess whether the patient can walk or refuses to walk due to pain, and look for bilateral involvement. 2

Critical Differential: Guillain-Barré Syndrome

• Guillain-Barré syndrome typically shows albuminocytologic dissociation (elevated CSF protein with normal cell count), whereas influenza-associated myositis has a normal cerebrospinal fluid profile—this distinction is essential when evaluating post-influenza gait disturbance. 2

• Upper respiratory tract infection is the most common antecedent event reported by 22-53% of patients with Guillain-Barré syndrome in multiple regions, with symptoms developing within 4 weeks of the infection. 1

• If the patient demonstrates ascending weakness, areflexia, or any signs of respiratory muscle involvement, perform lumbar puncture immediately to check for albuminocytologic dissociation. 1, 2

Laboratory Evaluation Priority

• Obtain CPK, urinalysis for myoglobin, blood urea nitrogen, creatinine, and electrolytes immediately to identify rhabdomyolysis and assess renal function. 2, 3

• If CPK is markedly elevated (≥5-10 times upper normal limit), the patient requires aggressive intravenous hydration to prevent acute kidney injury from myoglobin precipitation. 2, 3

• Monitor renal function daily with BUN, creatinine, and electrolytes until CPK levels trend downward. 2

Management Based on Diagnosis

If Influenza-Associated Myositis Confirmed:

• Initiate aggressive intravenous hydration immediately if CPK is extremely elevated to prevent myoglobin-induced acute kidney injury. 2, 3

• Use acetaminophen as first-line treatment for pain; avoid NSAIDs if renal function is compromised, and never use aspirin in children under 16 years due to Reye's syndrome risk. 2, 3

• Consider oseltamivir if within 48 hours of initial symptom onset (75 mg twice daily for adults, weight-based for children), though the patient is likely beyond this window given the two-week duration. 2

• Most cases are self-limiting with complete recovery within 3 days of supportive care initiation. 2, 3

If Guillain-Barré Syndrome Suspected:

• Admit immediately for monitoring of respiratory function and consideration of intravenous immunoglobulin or plasmapheresis, as Guillain-Barré can rapidly progress to respiratory failure. 1

• The risk of invasive meningococcal disease rises after influenza infection, so maintain vigilance for signs of meningococcal disease even without classic meningitis signs. 2

Common Pitfalls to Avoid

• Do not assume cardiac disease based on CPK-MB alone; troponin is the definitive test for myocardial injury if cardiac involvement is suspected. 2

• Do not miss rhabdomyolysis—ensure urine is examined for myoglobin in any patient with severe post-influenza muscle pain. 2, 3

• Do not attribute bilateral calf tenderness to septic arthritis, which presents with single joint effusion and localized swelling, not bilateral muscle pain. 2

• If no improvement occurs within 48 hours of supportive care, or if the patient develops altered mental status (indicating severe metabolic derangement), re-evaluate immediately. 2

• Persistent fever over 4-5 days may indicate secondary bacterial infection requiring antibiotics. 2