Body composition and clinical outcome measures in patients with myotonic dystrophy type 1

- Fat-free mass (FFM) and fat mass (FM) are decreased and increased respectively in myotonic dystrophy type 1 (DM1) compared to matched controls.
- Regional body composition correlates with primary clinical outcome measures.
- The most responsive measures that were assessed, in this DM1 cohort over an 18 month interval, were grip strength, total FFM and FM.
- DEXA appears to be a cost-effective useful secondary outcome measure of disease progression in clinical trials.

Myotonic Dystrophy type 1 (DM1) is the most common form of adult onset muscular dystrophy. In this study we compared body composition in DM1 and matched controls and evaluated the relationship between these parameters and clinical outcome measures in DM1 patients. In addition we established the sensitivity to change of these measures in a prospective 18 month longitudinal study of the DM1 patient cohort. Clinical data, manual muscle testing (MMT), quantitative muscle testing (QMT) of ankle dorsiflexion, bilateral grip dynamometry, 6 minute walk test and a DM1 functional rating scale (DM1-Activ) were collected at baseline (n = 38) and 18 month follow-up (n = 36). The case-control analysis was performed comparing baseline data with 31 anthropometrically matched controls. Dual-energy X-ray absorptiometry (DEXA) was used to obtain regional measurements of fat-free mass index (FFMI) and fat mass index (FMI) and demonstrated significant reduction of FFMI in the legs (left p = 0.004; right p = 0.017) and trunk (p < 0.0001) and increased FMI localised to the trunk (p < 0.0001) in DM1 patients compared to controls. Regional left and right arm FFMI and FMI significantly positively and negatively correlated with grip strength and both total FFMI (p = 0.0009) and FMI (p = 0.02) decreased and increased by 0.38 kg/m2 and 0.31 kg/m2 respectively after 18 month follow-up. DEXA is likely to provide a useful secondary outcome measurement of disease progression in addition to muscle strength and timed functional tasks in clinical trials.