A Population-Based Study of Neurologic Manifestations of Severe Influenza A(H1N1)pdm09 in California

Background. Reported influenza-associated neurologic complications are generally limited to case series or case reports. We conducted a population-based study of neurologic manifestations associated with severe and fatal influenza A(H1N1)pdm09 (2009 H1N1) cases. Methods. Medical records of patients with fatal or severe (hospitalized in intensive care unit) laboratory-confirmed 2009 H1N1 reported to the California Department of Public Health from 15 April 2009 through 31 December 2009 were reviewed to identify those with primary neurological manifestations. Cases with secondary neurologic manifestations (eg, hypoxia) were excluded. Primary influenza-associated neurologic complications (INCs) were classified into 4 groups: encephalopathy/encephalitis, seizures, meningitis, and other. Severe 2009 H1N1–associated neurologic incidence was calculated by using estimates of 2009 H1N1 illnesses in California. Results. Of 2069 reported severe or fatal 2009 H1N1 cases, 419 (20%) had neurologic manifestations. Of these, 77 (18%) met our definition of INCs: encephalopathy/encephalitis (n = 29), seizures (n = 44), meningitis (n = 3), and other (Guillain–Barré Syndrome) (n = 1). The median age was 9 years (range, 4 months–92 years); the highest rate of disease was among pediatric Asian/Pacific Islanders (12.79 per 1 000 000) compared with pedi-atric white, non-Hispanics (3.09 per 1 000 000), Hispanics (4.58 per 1 000 000), and blacks (6.57 per 1 000 000). The median length of stay (LOS) was 4 days (range, 1–142), and there were 4 fatalities. The estimated incidence o