Neurologic and muscular complications of the 2009 influenza A (H1N1) pandemic.

In the spring of 2009 a new triple-reassortant of influenza A (H1N1) virus appeared in Mexico and rapidly spread around the world, becoming a pandemic that primarily infected children and uncommonly older adults. Accompanying the pandemic were associated neurologic and muscular syndromes that affected primarily children and included febrile seizures, encephalopathy/encephalitis with or without seizures, delirium, focal neurologic syndromes, Guillain-Barré syndrome, myositis, and myocarditis. Neither the frequency nor the severity of these syndromes appears different from those recognized during periods of infections of previous influenza A viruses. I review the clinical, laboratory, neuroimaging, and pathologic characteristics of the associated syndromes appearing in the first wave of the pandemic, compare them to similar cases occurring in previous years, and explore several theories of pathogenesis.