White matter lesions and cognitive deficits: relevance of lesion pattern?

Magnetic resonance imaging (MRI) permits efficient visualization of white matter lesions (WML). A growing body of literature deals with the correlation of WML and cognitive dysfunction with conflicting results. We studied the influence of lesion pattern as well as size by analyzing MRI and psychometric test performance in 2 patient collectives with different WML patterns. 22 patients with myotonic dystrophy (MD) and mainly subcortical WML were compared with 39 patients with multiple sclerosis (MS) and mainly periventricular lesions. 73% of MD patients had WML, the extent of which correlated with cognitive deficits. Severely impaired patients had psychometric findings compatible with "subcortical" dementia. In MS the extent of WML alone did not correlate significantly with cognitive deficits. Significant cognitive dysfunction was observed with extension of WML to areas of white matter immediately underlying cortex, but not with exclusively periventricular lesions. Cerebral atrophy had less impact. Comparison of MD and MS indicates that WML immediately subjacent to cortex are likely to cause significant cognitive deficits, whereas extensive periventricular demyelination may cause no major dysfunction. This may relate to early disturbance of associative fibers by subcortical lesions. Our results emphasize the significance of pattern as well as total extent of WML. Myotonic dystrophy is a useful model to study the effect of subcortical lesions, due to a typical lesion pattern unusual in other conditions.