The aim of this study was to determine structural brain abnormalities in patients with adult-onset myotonic dystrophy type 1 (DM1) and correlate them to clinical and neuropsychological measures.

DM1 is a dominantly inherited repeat expansion disorder with variable clinical phenotype and central nervous system involvement. Prior neuroimaging studies in DM1 have identified widespread morphological abnormalities including global atrophy, ventriculomegaly and white matter hyperintensities.

Structural T1 weighted images were collected in 36 DM1 patients and 36 age and gender matched controls. Image preprocessing and voxel-based morphometry analysis were conducted using the Computational Anatomy Toolbox (CAT12). Global and regional gray (GM) and white matter (WM) volume differences were assessed through group comparisons and linear regression analysis with clinical and neuropsychological measures.

Compared to healthy controls, DM1 patients had widespread global and regional GM and WM volume reductions that were correlated with disease severity. An enlargement of mesial temporal structures including parahippocampal gyrus, fusiform gyrus, uncus, entorhinal cortex, hippocampus and amygdala was noted in DM1. GM volume loss in right inferior parietal lobule/angular gyrus and WM volume loss in left middle temporal gyrus were correlated with excessive daytime sleepiness in DM1. Volume loss in lentiform nucleus was inversely correlated with motor speed in DM1. 

In this study, we describe an abnormal enlargement of mesial temporal structures in the setting of widespread GM and WM volume loss in DM1 patients. GM and WM volume loss were associated with disease severity in DM1. Specific regional patterns of volume loss were associated with excessive daytime sleepiness and motor speed abnormalities in DM1. The mesial temporal enlargement could be compensatory or pathological and may underlie apathy and impaired social and emotional cognition and functioning noted in DM1. It is also possible that these changes represent chronic effects of sleep disordered breathing that are prevalent in DM1.