Imaging Study Shows Upper Leg Involvement in SMA

Researchers are offering a look into documented upper leg involvement in spinal muscular atrophy (SMA) among a large group of patients across an array of ages and disease severity, adding to mounting experience with quantitative MRI (qMRI).

Using DIXON, T2 mapping, and diffusion tensor imaging, the researchers obtained qMRIs for 31 patients with SMA types 2 and 3, comparing measures with clinical scores of motor function and muscle strength. The researchers also collected imaging on 20 matched controls.

“Few studies have addressed anatomy and function of muscle over time. Magnetic resonance imaging is a powerful noninvasive tool to study anatomy and tissue characteristics of skeletal muscle in vivo,” explained the researchers. “Most imaging studies in SMA have focused on a qualitative appreciation of muscle, grading fat infiltration visually. Quantitative MRI can complement qualitative evaluation of muscles and has shown promising results in other neuromuscular disorders, such as Duchenne muscular dystrophy and limb girdle muscular dystrophies.”

The cohort of patients with SMA showed increased fat fraction, with mean fat fraction in all upper leg muscles landing at 47.6% compared with 7.6% among the controls. Between the 15 patients with SMA type 2 and the 16 patients with type 3, fat fraction was more pronounced in those with type 2 (51.5% vs 36.7%), and these patients showed higher averages of fat fraction across all 3 muscle groups: quadriceps, hamstrings, and adductor. However, fat fraction was significantly lower for patients with type 3 in only the hamstrings.

The researchers noted that despite severe involvement of the quadriceps, there was no impediment in the ability to walk among some patients with type 3.

The imaging also revealed that the group of patients with SMA had decreased mean diffusivity (MD) (1.13 vs 1.47) and T2 compared with the controls. In addition, compared with patients with type 3, patients with type 2 had significantly lower MD in the quadriceps and hamstrings.

The researchers also explored the relationship between these qMRI parameters and different clinical measures, finding a significant moderate correlation of disease duration with fat fraction and MD but not with fractional anisotropy (FA).

Motor function showed a negative moderate correlation with FA and fat fraction, and there was a positive moderate correlation of MD and Hammersmith Functional Motor Scale Expanded (HFMSE). There was no correlation with T2 and HFMSE. Additionally, there was a moderate significant correlation between muscle strength and FA, fat fraction, and MD.

“The correlation of qMRI and clinical parameters suggest potential as a biomarker for disease progression or treatment effect,” wrote the researchers.

Reference

Otto L, van der Pol W, Schlaffke L, et al. Quantitative MRI of skeletal muscle in a cross-sectional cohort of patients with spinal muscular atrophy types 2 and 3. NMR Biomed. Published online July 18, 2020. doi:10.1002/nbm.4357.