Neuroimaging in Susac's syndrome: focus on DTI

Susac's syndrome is an underdiagnosed disease that is thought to occur mainly in young women. It is characterized by the triad of hearing loss, branch retinal artery occlusions, and encephalopathy with predominantly cognitive and psychiatric symptoms. Treatment consists of immunosuppressive therapy. Focal ischemic lesions in the central portion of the corpus callosum detectable by conventional MRI ("snowballs") are a typical feature of Susac's syndrome. The appearance of these lesions is not, however, correlated with the type and severity of the neuropsychological deficits.

Nine patients with Susac's syndrome, four men and five women, were investigated using Diffusion Tensor Imaging (DTI), a non-invasive technique for the detection of macro- and microstructural impairment of fiber integrity on the basis of normal values for the fractional anisotropy (FA). Patients were compared to a group of 83 healthy controls on a voxel-by-voxel basis. Several regions of interest were defined.

Impairment of fiber integrity was found in every patient. As compared to the controls, every patient showed disruption of fiber integrity in the genu of the corpus callosum. Reduction of FA was found particularly in the prefrontal white matter.

The type and severity of the encephalopathic symptoms in Susac's syndrome are much better represented by the prefrontal FA reductions detected by DTI than by the mostly sparse white matter abnormalities seen on conventional MRI. The fiber damage in the genu seems to be specific for patients with Susac's syndrome.