Coexistence of Alzheimer disease neuropathology with herpes simplex encephalitis

Several unusual features were observed during routine histopathological confirmation of a clinical diagnosis of Alzheimer's disease (AD) in an 85-year-old, right-handed, married male. The patient presented with a 12-year history of slowly progressive cognitive impairment, which increased in severity just prior to death. Detailed postmortem examination of the frontal lobes revealed a significant number of neuritic plaques and neurofibrillary tangles. Multifocal spongiform encephalopathic changes, mononuclear perivascular infiltrates, subcortical demyelination and gliosis were also found. Of particular interest were well-defined neuronal and astrocytic intranuclear inclusion bodies (Cowdry type I and I), suggestive of viral disease. Electron microscopy, immunohistochemical and immunohistofluorescent studies confirmed a Herpes simplex type I encephalitis (HSV-I). These histological results and the clinical history of progression suggest that reactivation of a latent viral infection may have contributed to the rapid progression of dementia prior to death. The present analysis underscores the fact that multiple etiologic factors may act simultaneously to produce dementia. While one such process may be identified or diagnosed (in the present case AD), it is necessary to be open to the possibility that another mechanism may come into play during the time course of that illness. A differential diagnosis may be difficult when the symptoms of the two disease processes are very similar. Such may be the case if there is reactivation of a previously undiagnosed herpes virus infection. With the development of PCR and in situ hybridization diagnosis will be simplified and more definitive.