Amnesia following herpes simplex encephalitis: diffusion-tensor imaging uncovers reduced integrity of normal-appearing white matter.

PURPOSE: To explore white matter (WM) and gray matter tissue integrity in the apparently unaffected hemisphere of patients with herpes simplex encephalitis (HSE) who have gross unilateral medial temporal lobe (MTL) lesions and both verbal and visuospatial memory deficits.
MATERIALS AND METHODS: This study had institutional ethics committee approval and included written informed consent. Magnetic resonance (MR) imaging was performed in five patients who had recovered from HSE (one woman, four men; median age, 32 years; interquartile range, 28.5-37 years) and 51 age-matched controls (30 women, 21 men; median age, 32 years; interquartile range, 28-37 years). As markers of microstructural WM integrity, fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity, and radial diffusivity (RD) derived from diffusion-tensor (DT) imaging were used. An automated regional brain segmentation approach yielded estimates of subcortical volumes, including hippocampus, and cortical thickness. Group differences were evaluated by using permutation tests.
RESULTS: Examination of standard MR images found unilateral lesions in all patients. The patients with HSE showed reduced FA and increased MD and RD in several WM tracts contralateral to lesions (P < .05, corrected for multiple comparisons). Affected tracts included connections between the MTLs and other parts of the brain. No significant group differences were observed in subcortical volume or cortical thickness.
CONCLUSION: These results suggest that patients with HSE have reduced microstructural integrity of normal-appearing WM contralateral to grossly visible lesions. These subtle lesions, detectable at DT imaging, probably contribute to the memory impairment manifested by these patients. SUPPLEMENTAL MATERIAL: http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.10100179/-/DC1. © RSNA, 2010