Body composition in patients with primary neuromuscular disease assessed by dual energy X-ray absorptiometry (DXA) and three different bioimpedance devices.

Background: Patients with primary neuromuscular disease have reduced muscle mass, and use of body mass index to assess nutritional status and body composition can therefore be questioned. Dual emission X-ray absorptiometry (DXA) can estimate muscle mass, but is not always readily available. Bioimpedance is a simple, portable and "easy to use" method for the assessment of body composition.

Objectives: To assess muscle mass by DXA in 143 patients with primary neuromuscular disease and validate three bioimpedance devices; Impedimed SFB7, (BIS), Xitron4200 (BIS) and Tanita MC180MA (MFBIA).

Methods: Body composition was assessed by DXA in 143, by BIS in 116, by MFBIA in 104 and by BIS in 35 patients.

Results: Muscle mass assessed by DXA, and phase angle (PhA) were below reference values in all female and 96% of male patients. BIS underestimated muscle mass by 6.5 ± 14.2 kg (p < 0.001), but this could be corrected after exclusion of resistance (Ri) values > 3500 Ohm (p = 0.84). MFBIA overestimated muscle mass by 30.8 ± 9.1 kg (p < 0.001) with systematic bias, whereas BIS was in agreement with DXA, and without systematic bias. Muscle mass was strongly correlated to PhA (r = 0.75, p < 0.01).

Conclusion: Patients with primary neuromuscular disease have proportionally more fat and less muscle mass than the population in general, despite normal BMI. Muscle mass can be assessed by bioimpedance in these patients, but performance and bias depends on device. Phase angle by bioimpedance correlates to muscle mass, and could therefore potentially be used a surrogate measure of muscle mass during follow up.