Are Neuroimaging Findings in Novel Influenza A(H1N1) Infection Really Novel?
============================================================================

* R.K. Gupta

Novel H1N1 (referred to as “swine flu” earlier) is a new influenza virus causing illness in humans. This virus was first detected in people in Mexico and the United States in April 2009. It is spreading worldwide from person-to-person, probably in much the same way as the spread of regular seasonal influenza. The latter can be associated with neurologic complications,1,2 but the frequency with which these occur with the novel influenza A(H1N1) virus infection is unknown. Neurologic sequelae such as seizures, encephalopathy, or encephalitis within 5 days of the initial illness were reported in 4 children with H1N1 infection for the first time in Dallas, Texas.3 Brain imaging findings were normal in these children.

The first case of neuroimaging abnormalities in H1N1 infection was reported in a child from Texas presenting with imaging features of acute necrotizing encephalitis.4 Subsequently, 2 more cases of encephalitis associated with H1N1 infection have been reported by Haktanir from Turkey5 and Ormitti et al from Italy.6 Neuroimaging findings in influenza-associated encephalopathy might be normal, but in severe cases, abnormalities can include diffuse cerebral edema and bilateral thalamic lesions.2 Lack of evidence of H1N1 viral infection in the CSF suggests that neurologic manifestations might be an indirect effect of respiratory tract infection, similar to the ones observed in influenza A and B viral infections.1,2

The imaging findings may resemble those of acute necrotizing encephalitis or may present as encephalitis with hemorrhage and typically involve the bilateral thalami as seen in all 3 case studies.4(#ref-5)(#ref-6)(#ref-7)–8 These imaging features have also been described in Arbovirus encephalitis and may overlap these conditions.9 These case studies suggest that imaging may be abnormal in H1N1-associated encephalitis with normal CSF; and in the presence of flu-like symptoms in the endemic zones, H1N1-associated encephalitis should be considered as an important differential diagnosis. Because these patients are known to recover completely with treatment, early recognition of H1N1-associated encephalitis will result in early institution of therapy specific to H1N1 and will possibly help in reducing the associated morbidity and mortality.

## References

1.  1. Morishima T, Togashi T, Yokota S, et al. Encephalitis and encephalopathy associated with an influenza epidemic in Japan. Clin Infect Dis 2002;35:512–17 
    
    [Abstract/FREE Full Text](http://www.ajnr.org/lookup/ijlink/YTozOntzOjQ6InBhdGgiO3M6MTQ6Ii9sb29rdXAvaWpsaW5rIjtzOjU6InF1ZXJ5IjthOjQ6e3M6ODoibGlua1R5cGUiO3M6NDoiQUJTVCI7czoxMToiam91cm5hbENvZGUiO3M6MzoiY2lkIjtzOjU6InJlc2lkIjtzOjg6IjM1LzUvNTEyIjtzOjQ6ImF0b20iO3M6MTk6Ii9ham5yLzMxLzMvMzkzLmF0b20iO31zOjg6ImZyYWdtZW50IjtzOjA6IiI7fQ==) 

2.  2. Maricich SM, Neul JL, Lotze TE, et al. Neurologic complications associated with influenza A in children during the 2003–2004 influenza season in Houston, Texas. Pediatrics 2004;114:e626–33 
    
    [Abstract/FREE Full Text](http://www.ajnr.org/lookup/ijlink/YTozOntzOjQ6InBhdGgiO3M6MTQ6Ii9sb29rdXAvaWpsaW5rIjtzOjU6InF1ZXJ5IjthOjQ6e3M6ODoibGlua1R5cGUiO3M6NDoiQUJTVCI7czoxMToiam91cm5hbENvZGUiO3M6MTA6InBlZGlhdHJpY3MiO3M6NToicmVzaWQiO3M6MTA6IjExNC81L2U2MjYiO3M6NDoiYXRvbSI7czoxOToiL2FqbnIvMzEvMy8zOTMuYXRvbSI7fXM6ODoiZnJhZ21lbnQiO3M6MDoiIjt9) 

3.  3. Centers for Disease Control and Prevention (CDC). Neurologic complications associated with novel influenza A (H1N1) virus infection in children: Dallas, Texas, May 2009. MMWR Morb Mortal Wkly Rep 2009;58:773–78 
    
    [PubMed](http://www.ajnr.org/lookup/external-ref?access_num=19629027&link_type=MED&atom=%2Fajnr%2F31%2F3%2F393.atom) 

4.  4. Lyon JB, Remigio C, Milligan T, et al. Acute necrotizing encephalopathy in a child with a H1N1 influenza infection. Pediatr Radiol 2010;40:200–05. Epub 2009 Dec 18 
    
    [CrossRef](http://www.ajnr.org/lookup/external-ref?access_num=10.1007/s00247-009-1487-z&link_type=DOI) 
    
    [PubMed](http://www.ajnr.org/lookup/external-ref?access_num=20020117&link_type=MED&atom=%2Fajnr%2F31%2F3%2F393.atom) 
    
    [Web of Science](http://www.ajnr.org/lookup/external-ref?access_num=000273470200009&link_type=ISI) 

5.  5. Haktanir A. MR imaging in novel Influenza A(H1N1)-associated meningoencephalitis. AJNR Am J Neuroradiol 2010;31:394–95 
    
    [Abstract/FREE Full Text](http://www.ajnr.org/lookup/ijlink/YTozOntzOjQ6InBhdGgiO3M6MTQ6Ii9sb29rdXAvaWpsaW5rIjtzOjU6InF1ZXJ5IjthOjQ6e3M6ODoibGlua1R5cGUiO3M6NDoiQUJTVCI7czoxMToiam91cm5hbENvZGUiO3M6NDoiYWpuciI7czo1OiJyZXNpZCI7czo4OiIzMS8zLzM5NCI7czo0OiJhdG9tIjtzOjE5OiIvYWpuci8zMS8zLzM5My5hdG9tIjt9czo4OiJmcmFnbWVudCI7czowOiIiO30=) 

6.  6. Ormitti F, Ventura E, Summa A, et al. Acute necrotizing encephalopathy in a child during the 2009 influenza A(H1N1) pandemia: MR imaging in diagnosis and follow-up. AJNR Am J Neuroradiol 2010;31:396–400 
    
    [Abstract/FREE Full Text](http://www.ajnr.org/lookup/ijlink/YTozOntzOjQ6InBhdGgiO3M6MTQ6Ii9sb29rdXAvaWpsaW5rIjtzOjU6InF1ZXJ5IjthOjQ6e3M6ODoibGlua1R5cGUiO3M6NDoiQUJTVCI7czoxMToiam91cm5hbENvZGUiO3M6NDoiYWpuciI7czo1OiJyZXNpZCI7czo4OiIzMS8zLzM5NiI7czo0OiJhdG9tIjtzOjE5OiIvYWpuci8zMS8zLzM5My5hdG9tIjt9czo4OiJmcmFnbWVudCI7czowOiIiO30=) 

7.  7. Mizuguchi M, Abe J, Mikkaichi K, et al. Acute necrotizing encephalopathy of childhood: a new syndrome presenting with multifocal, symmetric brain lesions. J Neurol Neurosurg Psychiatry 1995;58:555–61 
    
    [Abstract/FREE Full Text](http://www.ajnr.org/lookup/ijlink/YTozOntzOjQ6InBhdGgiO3M6MTQ6Ii9sb29rdXAvaWpsaW5rIjtzOjU6InF1ZXJ5IjthOjQ6e3M6ODoibGlua1R5cGUiO3M6NDoiQUJTVCI7czoxMToiam91cm5hbENvZGUiO3M6NDoiam5ucCI7czo1OiJyZXNpZCI7czo4OiI1OC81LzU1NSI7czo0OiJhdG9tIjtzOjE5OiIvYWpuci8zMS8zLzM5My5hdG9tIjt9czo4OiJmcmFnbWVudCI7czowOiIiO30=) 

8.  8. Yoshikawa H, Yamazaki S, Watanabe T, et al. Study of influenza-associated encephalitis/encephalopathy in children during the 1997 to 2001 influenza seasons. J Child Neurol 2001;16:885–90 
    
    [Abstract/FREE Full Text](http://www.ajnr.org/lookup/ijlink/YTozOntzOjQ6InBhdGgiO3M6MTQ6Ii9sb29rdXAvaWpsaW5rIjtzOjU6InF1ZXJ5IjthOjQ6e3M6ODoibGlua1R5cGUiO3M6NDoiQUJTVCI7czoxMToiam91cm5hbENvZGUiO3M6NToic3BqY24iO3M6NToicmVzaWQiO3M6OToiMTYvMTIvODg1IjtzOjQ6ImF0b20iO3M6MTk6Ii9ham5yLzMxLzMvMzkzLmF0b20iO31zOjg6ImZyYWdtZW50IjtzOjA6IiI7fQ==) 

9.  9. Gupta RK, Jain KK, Kumar S. Imaging of nonspecific (nonherpetic) acute viral infections. Neuroimaging Clin N Am 2008;18:41–52, vii 
    
    [CrossRef](http://www.ajnr.org/lookup/external-ref?access_num=10.1016/j.nic.2007.12.004&link_type=DOI) 
    
    [PubMed](http://www.ajnr.org/lookup/external-ref?access_num=18319154&link_type=MED&atom=%2Fajnr%2F31%2F3%2F393.atom) 

*   Copyright © American Society of Neuroradiology