Neuroimaging in rabies.

Rabies remains a virtually incurable disease once symptoms develop. Neuroimaging studies demonstrate lesions in the different parts of the neuroaxis, even before brain symptoms are evident. These abnormalities have been detailed in both rabies virus-infected humans and dogs with magnetic resonance imaging (MRI). MRI disturbances were similar in both forms (furious or paralytic) in human rabies; however, they were more pronounced in paralytic than in furious rabies virus-infected dogs in which examination was done early in the disease course. Abnormalities were not confined only to neuronal structures of hippocampus, hypothalamus, basal ganglia, and brain stem but also extended to white matter. The blood-brain barrier (BBB) has been clearly shown to be intact during the time rabies virus-infected patients and dogs remained conscious, whereas leakage was demonstrated as soon as they became comatose. Although the location of MRI abnormalities can help diagnosing rabies, the intensities of signals are usually not very distinct and sometimes not recognizable. Newer techniques and protocols have been developed and utilized, such as diffusion-weighted imaging and diffusion tensor imaging, and the latter provides both qualitative and quantitative data. These techniques have been applied to normal and rabies virus-infected dogs to construct fractional anisotropy and mean diffusivity maps. Results showed clear-cut evidence of BBB intactness with absence of vasogenic brain edema and preservation of most neuronal structures and tracts except at the level of brainstem in paralytic rabies-infected dogs. Neuroimaging is one of the most useful tools for the in vivo study of central nervous system infections.