Selective, Reversible Thalamic Involvement With Influenza A Infection

Abstract

To the Editor. My colleagues and I have read with interest the report by Ruggieri et al1 regarding selective, reversible thalamic involvement with measles infection. Concerning the etiology of selective, reversible thalamic involvement, the authors concluded that deep cerebral vein thrombosis (DCVT) caused it. We could not deny their hypothesis. However, recently we observed two cases with similar magnetic resonance imaging (MRI) findings and clinical course attributable to influenza A infection. These cases were diagnosed as having the mild form of acute necrotizing encephalopathy (ANE).2 The authors never mentioned ANE as a possible etiology.Our cases were previously healthy Japanese boys, aged 2 and 5.3 Both had acute encephalopathy without pleocytosis in cerebrospinal fluid. Their computed tomography (CT) and MRI showed selective, reversible thalamic involvement and no evidence of DCVT. They recovered without sequelae and disturbance of sensation. Influenza A infection was confirmed serologically in both cases.It is obvious that influenza A infection also shows selective, reversible thalamic involvement on the MRI, as well as measles infection. Despite the different pathogenic virus, similar mechanisms causing these characteristic MRI findings may be present. The problem is the mechanism. ANE is a novel disease entity proposed by Mizuguchi et al.2 in 1995, which is prevalent in Japan and Far East countries. The onset of ANE is usually preceded by viral infection.2 Twenty-four percent of patients have evidence of influenza A infection2 and one case with measles infection (just as the cases the authors described) was also reported in ANE.2 In ANE, the CT and the MRI show multifocal, symmetric involvement of the thalamus, brainstem, and cerebellum, which is very similar to the authors' findings. ANE is considered to be different from other previously known encephalopathy and DCVT. Brain necropsy revealed no evidence of DCVT in ANE.2 Its etiology still remains unknown. Typical ANE usually shows irreversible multifocal involvement including thalamus and left severe sequelae. On the contrary, the mild form of ANE, like our two cases, shows selective, reversible involvement only in the thalamus on the MRI and it has a good prognosis. It is sometimes difficult to distinguish the mild form of ANE from other disorders because of the lack of the pathologic findings of the brain. We propose that the mild form of ANE may be present, and the cases the authors reported were also compatible with the mild form of ANE. We would appreciate any comment regarding the possibility of ANE in their cases.