A new study published in Ultrasound in Medicine & Biology has revealed that muscle ultrasound changes significantly correlate with functional status and is beneficial for quantifying muscular changes in spinal muscular atrophy (SMA). The study highlighted that the diaphragm thickening ratio could appear normal even with a severe compromise of the respiratory muscles in quantitative analysis.
The researchers investigated muscles (the biceps brachii, rectus femoris, intercostals, diaphragm, and thoracic multifidus) of 41 patients experiencing type 1 to 4 SMA and 46 sex-matched and healthy age control individuals through various techniques, including b-mode Ultrasound for gray-scale analysis (GSA), the cross-sectional images (biceps brachii and rectus femoris) i-e area analysis and the diaphragm thickening ratio.
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The research team applied the functional scales to the recruited patients and examined the ultrasound abnormalities in select clinical subtypes. They also assessed related data regarding the functional status.
Study results revealed that the patients with SMA, compared with the control individuals, were found to have reduced muscle area, and significantly increased mean GSA for all the examined muscles (P< 0.001).
This decrease in muscle area was demonstrated through the established correlation between the increase in GSA and the SMA severity for brachii, biceps, rectus femoris, and the intercostals when using the Hammersmith Functional Motor Scale Expanded (HFSME). Additionally, the diaphragm thickening ratio was found normal in most patients, while the intercostal muscles were reported to have higher GSA than the diaphragm in relation to the controls.
“We expected these findings considering SMA’s natural history, and it is conceivable that it could be different in the future for patients who start treatment earlier in life,” the authors wrote. The results mentioned above signify the use of ultrasound in quantifying muscular changes in SMA and its correlation with the functional status.
“Future longitudinal studies with a larger group of patients with SMA type 1 and comparing data at multiple stages during treatment can validate our findings and the usefulness of Ultrasound to monitor treatment response,” the authors highlighted.
SMA is a neurodegenerative disorder that affects lower motor neurons. Genetic variations in the SMN1 gene cause the most prevalent form of this disorder. SMA is categorized into 3 subtypes based on the age of onset and motor milestones. Type 1 is the most severe and affects infants up to 6 months old, while type 2 affects children between 6 and 18 months old, and type 3 affects children over 2 years old. A gradual increase in muscle weakness characterizes a mild adult form (type 4).
Recent advances in disease-modifying therapies have improved motor milestones for treated children with SMA, the researchers noted. While clinical trials have primarily relied on functional tests, ultrasound can be a useful and less expensive alternative, the study team said.
It is easy to perform, is reproducible, and can quickly assess multiple muscles without requiring patient cooperation, they added. Quantitative muscle ultrasound can effectively assess muscle size and echogenicity, the team noted.
Children with SMA typically have decreased muscle thickness and hyperechogenic muscles due to fibrosis and fat substitution of the myofibrils, the investigators continued.
Moreira AL, Mendonça RH, Polido GJ, et al. Muscle ultrasound changes correlate with functional impairment in spinal muscular atrophy. Ultrasound Med Biol. Published online April 8, 2023. doi:10.1016/j.ultrasmedbio.2023.02.021