Background In myotonic dystrophy type 1 (DM1), sleep disorders are common, with excessive daytime sleepiness (EDS) as a predominant feature. In myotonic dystrophy type 2 (DM2), the presence of sleep disturbances is unknown.

Objective To investigate the frequency of EDS, poor sleep quality and fatigue in DM2.

Methods 29 genetically proven DM2 patients were surveyed using the Epworth Sleepiness Scale, Pittsburgh Sleep Quality Index (PSQI) and Checklist Individual Strength. The results were compared with 29 adult onset DM1 patients and 65 population controls, both matched for age and sex.

Results Only 6.9% of DM2 patients had EDS compared with 44.8% of DM1 patients and 6.2% of population controls (DM2–DM1: p=0.001; DM2–controls: p=0.51). Sleep quality was poor (PSQI >5) in both DM2 and DM1 groups, and differed significantly from population controls (DM2 6.5±3.0; DM1 6.2±3.7; controls 4.3±3.0; DM2–controls: p=0.002). Poor sleep quality was not explained by depression or other comorbidity but was mainly due to sleep disturbances as a result of nocturnal pain. Comparable with the DM1 group, DM2 patients experienced severe fatigue (DM2 38.7±13.1; DM1 42.9±8.5; controls 21.1±11.1; DM2–controls: p<0.001). Results were not confounded by abnormal thyroid function or medication use.

Conclusion These results provide new insight into the phenotype of DM2 and have consequences for clinical treatment. In addition, the absence of EDS in DM2 is a new discriminative feature with adult onset DM1.