Motor unit magnetic resonance imaging (MUMRI) as a novel biomarker of muscle activity in spinal muscular atrophy

Motor unit MRI (MUMRI) non-invasively detects fasciculation, a common symptom of SMA and potential biomarker for clinical trials. We applied MUMRI in ten SMA III patients and ten controls comparing fasciculation rates. Images of the tongue, upper arm, paraspinal and thighs & lower legs were acquired using MUMRI and 3-point Dixon (fat fraction) sequences. Fasciculation rate (cm^-3min^-1) was significantly higher in SMA than controls for: paraspinal 0.15 ± 0.20 vs. 0.003 ± 0.006, p = 0.001, thighs 1.28 ± 1.76 vs. 0.008 ± 0.005, p = 0.002 and lower legs 0.53 ± 0.85 vs. 0.02 ± 0.02, p = 0.001, but not for the tongue 0.20 ± 0.20 vs. 0.06 ± 0.09, p = 0.082 or upper arm 0.45 ± 0.95 vs. 0.002 ± 0.004, p = 0.014. Fat fraction %, was significantly higher in SMA than controls for: upper arm 35.0 ± 25.4 vs. 4.2 ± 1.1, p<<0.001, paraspinal 41.4 ± 31.0 vs. 7.4 ± 4.5, p = 0.002, thighs 54.8 ± 23.8 vs. 5.7 ± 1.0, p<<0.001 and lower legs 29.6 ± 23.5 vs. 4.4 ± 0.9, p = 0.0003, but not for the tongue 13.9 ± 3.2 vs. 13.0 ± 3.3, p = 0.393. MUMRI is an attractive non-invasive biomarker, which could be used to monitor progression & response in SMA clinical trials.