22 June 2012
A Population-Based Study of Neurologic Manifestations of Severe Influenza A(H1N1)pdm09 in California
In April 2009, the California Department of Public Health (CDPH), initiated a population based study for cases of severe influenza A(H1N1)pdm09 with influenza-associated neurologic complications (INC), the results of which were recently published in the Clinical Infectious Diseases journal . Standardized case history forms were used by local health departments to report demographic, clinical, and laboratory data on fatal or severe laboratory-confirmed H1N1 influenza to CDPH for the period between 15 April and 31 December 2009. Specific clinical information reported included the presence or absence of altered mental status, encephalitis, encephalopathy, and seizures. The records were reviewed to identify those with primary neurological manifestations; those found to have secondary neurologic manifestations, such as hypoxia due to severe respiratory disease, were excluded. Of 2,069 reported severe or fatal influenza A(H1N1)pdm09 cases, 77 (3.7%) met the investigator’s definition of an INC. Reported neurological manifestations included encephalopty/encephalitis (29 cases), seizures (44 cases), meningitis (3 cases), and Guillain-Barré Syndrome (1 case). Encephalopathy/encephalitis cases reported were the most severe. INCs occurred most commonly in pediatric patients and Asian/Pacific Islanders and had an overall estimated incidence of 1.2/100,000 symptomatic H1N1 illnesses. Most patients with INCs had a relatively short length of stay (median 4 days), however, there were four fatalities.
Primary neurological manifestations with or without respiratory symptoms have increasingly been recognized as a complication of influenza infection in recent years, however this is the first report to estimate the frequency of occurrence of these complications. In most case reports, children appear to be more commonly affected than adults and may be the most prominent sign of infection at the time of presentation [2,3,4]. Commonly reported neurological complications include seizures, decreased alertness, encephalopathy, and meningitis. Although most patients with neurological complications of influenza recover uneventfully, influenza infection has also been associated with long term neurological complications such as narcolepsy and Parkinson’s disease [5,6]. Other than age and perhaps ethnicity as suggested by the current study, no specific risk factors have been identified which predispose individuals to the development of neurological complications of influenza. Clinicians should maintain a high index of suspicion when seeing patients with acute neurological syndromes during periods when influenza is widely circulating in the community, even in the absence of obvious respiratory illness.
1. Glaser C, Winter K, DuBray K, et al. A Population-Based Study of Neurologic Manifestations of Severe Influenza A(H1N1)pdm09 in California. Clin Infect Dis. 2012 Jun 4. [Epub ahead of print]
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4. Ak O, Biteker F, Cag Y, et al. Influenza B-associated encephalopathy in two adults. J Infect Chemother. 2012 Apr 14. [Epub ahead of print]
5. Han F, Lin L, Warby SC, et al. Narcolepsy Onset Is Seasonal and Increased following the 2009 H1N1 Pandemic in China. Ann Neurol. 2011 Sep;70(3):410-7.
6. Toovey S, Jick SS, Meier CR. Parkinson’s disease or Parkinson symptoms following seasonal influenza. Influenza Other Respi Viruses. 2011 Sep;5(5):328-33.