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Background-We recently showed that the severity of MRI signal abnormalities increases with age in CADASIL, an arteriopathy due to mutations of notch 3 gene on chromosome 19. Previous results also suggest that the various hemispheric subcortical areas have a different vulnerability to ischemia in this disease. The distribution of the lesions at the brain stem level has not yet been reported. Case Descriptions-We reviewed the MRIs of 68 affected patients having signal abnormalities in the hemispheric white matter to assess the distribution and clinical consequences of brain stem signal abnormalities in CADASIL. We found hypersignals on T2-weighted images in the brain stem in 45% of the subjects. The pons was more frequently involved (100%) than the mesencephalon (69%) and the medulla (35%). Hyposignals on T1-weighted images, at the brain stem level, were observed only in two thirds of these subjects. The lack of signal abnormalities reaching the brain stem surface and the absence of cerebellar lesions were noteworthy. It is responsible for recurrent subcortical strokes in midadulthood, and it leads progressively to dementia 10 to 30 years after onset. 3 A major feature of this disease is the presence of increased signal intensities (hypersignals) on T2-weighted images in the hemispheric white matter in all clinically affected subjects and in asymptomatic carriers of the mutated gene. 4,5 In a series of 75 patients, we previously demonstrated 6 that the severity of hemispheric white matter hypersignals dramatically increases with age in CADASIL. We also showed that whereas hypersignals were constant in the periventricular white matter, they were less common at a distance from these areas, suggesting a variable topographical susceptibility to ischemia among the different regions irrigated by the perforating arteries. The aim of the present study was to investigate the distribution of signal abnormalities within the brain stem. Conclusions-Brain Subjects and MethodsWe studied the frequency, location and clinical significance of brain stem MRI signal abnormalities in 68 CADASIL patients having hemispheric hypersignals on T2 weighted images (28% with hypersignals located in basal ganglia). All of them carried the affected haplotype and belonged to families with at least 1 member having a proven deleterious mutation in notch 3 gene (error risk of Ͻ1/10000 compared with direct screening of mutation in each subject). 2,7 MRI examinations performed in different neuroradiological centers between 1993 and 1997 included coronal, axial, or sagittal T1-weighted and coronal or axial T2-weighted images of thicknesses varying between 5 and 10 mm. The following parameters were evaluated on MRI: (1) frequency of brain stem lesions according to age; (2) frequency and location of lesions at the 3 brain stem levels; and (3) maximal surface of hypersignals at the midpons (calculated with tracing paper) and correlation with the severity of hemispheric hypersignals calculated according to the Scheltens method (sum of the scores of hypersi...