Severe myopathy caused by the new pandemic influenza A (H1N1) in a child

Introduction

Myopathy is a rare complication of influenza infections. ¹ In March and April 2009, a novel influenza A H1N1 virus led to human infection in Mexico and spread rapidly to all regions of the world. ² Due to the rare association of myopathy with the new pandemic influenza A H1N1, we report on an eight-year-old girl with severe myopathy caused by this pandemic virus.

Case history

A previously healthy girl aged eight years presented to the hospital having had a generalized tonic-clonic seizure. She also had cough and mild fever for two days. There was no history of previous convulsion. On physical examination, she was confused, and could only respond to painful stimuli. Body temperature was 38.2°C, pulse 164/min, blood pressure 110/60 mmHg and respiratory rate 28/min. Deep tendon reflexes were normal. Laboratory investigations revealed: whole blood count of 21200/mm³, Hb 12 g/dL, platelet 326,000/mm³, aspartate aminotransferase (AST) 124 and alanine aminotransferase (ALT) 31 U/L, creatine phosphokinase (CPK) 11803 U/L (35–195), cardiac fraction of creatine kinase (MB) 203 ng/mL (0–3.23) and C-troponin I 12 ng/mL (0–1). Computed cranial tomography (CT) and cerebrospinal fluid (CSF) sample showed normal findings. Investigations for viral hepatitis A, B, C, D, E viruses and Epstein-Barr virus were negative. Urine analysis was normal. Her mental status recovered completely in almost 6 h without any focal deficit, but she was extremely exhausted and hardly able to move her extremities when she woke. With a diagnosis of myopathy, possibly due to pandemic influenza A (H1N1) virus, oseltamivir treatment was begun. Echocardiographic examination revealed mild left ventricular enlargement, decreased left ventricular functions (ejection fraction: 51.6% [N: 74–95%], fractional shortening: 25.6% [N: 28–44%]), global left ventricular wall hypokinesis and mild mitral valve regurgitation which confirmed myocarditis. No arrhythmia developed and pulse rate returned to normal levels on the second day. Electroencephalography (EEG) revealed no pathology. Her respiratory sample was found positive for pandemic influenza A (H1N1) virus, tested by real-time polymerase chain reaction. On the fourth day, CPK levels increased up to 108085 U/L, and AST and ALT levels to 2369 and 1253 U/L, respectively. She was quite well, could easily move her extremities and was able to walk on the seventh day. She recovered fully within 10 days and was discharged with almost normal laboratory values.

Discussion

Influenza-associated myopathy (IAM) is characterized by the sudden onset of calf pain, muscle tenderness and difficulty in walking and it is usually recognized in school-aged children during influenza outbreaks and epidemics. Serum CPK levels are almost always elevated. ¹ Although IAM has been reported with seasonal influenza viruses previously, the first IAM case with pandemic influenza A (H1N1) virus was reported by Koliou et al. ³ recently. In our case, seizure was the prominent finding at the beginning with mild flu symptoms and later, severe myopathy. With the help of very high levels of CPK and the H1N1 pandemic, the child was evaluated as having influenza A (H1N1). Subsequently, a virologic study confirmed the diagnosis. As she had seizures, severe myopathy and mild myocarditis, oseltamivir treatment was begun for the patient and she recovered rapidly.

The mechanisms by which the virus leads to muscle involvement are not clear. The two most commonly proposed mechanisms are direct viral infection and immune-mediated muscle damage triggered by the virus. ¹ Seizure is the most common neurological complication experienced by children hospitalized with influenza. We diagnosed our patient as having seizure (not febrile seizure because of her age) in the absence of an underlying seizure disorder or brain pathology with normal CSF, EEG, cranial CT findings and altered mental status lasting <24 h. ⁴

Myocarditis and hepatitis are rare complications of influenza A and B infections. ⁵ Elevated levels of transaminases in our cases might have originated from the muscles, but we did not rule out mild hepatitis. Although we could not do a virological study of CSF or tissue specimens of the brain, myocardium or liver, the clinical picture of our case suggested a systemic viral dissemination, as myopathy was predominant. Consequently, even if rare, atypical clinical pictures of influenza, such as myopathy, should be borne in mind in order to avoid unnecessary investigations, especially during outbreaks of influenza.