Objective: To examine masseter muscle size and its association with brain structure and motor endpoints in participants with myotonic dystrophy type 2 (DM2).
Methodology: Brain MRIs were acquired on a 3T Siemens Skyra MRI scanner in 20 adults with DM2 and 14 controls. Average cross-sectional areas (CSA) of masseter muscle were assessed using T1 MP-RAGE images. Diffusion tensor imaging (DTI) parameters, including fractional anisotropy (FA), radial diffusivity (RD), and axial diffusivity (AD), were analyzed to evaluate cerebral white matter (WM) integrity. FA is an index of non-uniform movement of water molecules ranging from 0 (CSF) to 1 (healthy WM), while RD and AD represent rates of water diffusion (high values indicate high water dispersion; low values represent well-organized WM fibers). A comprehensive battery of motor performance was measured and correlated with the average CSA of the masseter muscle (Spearman’s correlation [ρ] with Bonferroni correction, significance level at 0.008).
Results: Compared to controls (n=14, age 59±11years, 57% female), the DM2 group (n=20, 61±9years and 70% female) had smaller masseter CSA (mean difference, -113mm2 [-178, -47]; p-value=0.001). On the subset of participants with measures of motor endpoints and WM integrity (11 DM2, 10 controls), masseter CSA is significantly correlated with RD (ρ=-0.61, p-value=0.004), and with AD (ρ=-0.69, p-value <0.001) in the DM2 group. Reduced masseter CSA was also strongly associated with lower 6-minute walk distance (ρ=0.70, p-value<0.001), lower short physical performance battery (SPPB) score (ρ=0.60, p-value 0.005), and weaker grip strength (ρ=0.75, p-value<0.001). Conclusions: Our pilot data demonstrates masseter muscle size is reduced in patients with DM2, and its size correlates to impaired WM integrity, as well as lower motor performance. If confirmed in a larger study, this measure could be used to obtain meaningful data on cerebral WM health and, opportunistically, motor performance in DM2.