Muscle ultrasound may help to monitor SMA progression: Study

Patients show atrophy, signs of fat, connective tissue where muscle should be

Lindsey Shapiro, PhD avatar

by Lindsey Shapiro, PhD |

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An illustration shows the bands of a muscle.

Muscle abnormalities observed with ultrasound imaging correlated with motor function in people with spinal muscular atrophy (SMA) in a recent study.

While the findings varied somewhat by muscle group and SMA type, ultrasound data generally indicated SMA patients exhibited muscle atrophy and signs of fat and connective tissue where muscle would normally be. A greater degree of these changes was associated with more significant functional impairments in SMA types 2-4.

A muscle ultrasound may thus be, “useful for quantifying muscular changes in SMA,” the researchers said in “Muscle Ultrasound Changes Correlate With Functional Impairment in Spinal Muscular Atrophy,” which was published in Ultrasound in Medicine & Biology.

SMA is marked by the progressive loss of the specialized nerve cells involved in voluntary movement (motor neurons), leading to muscle weakness and wasting. To monitor its progression and responses to treatment in the clinic and in clinical trials, functional tests are used to evaluate patients’ ability to perform certain activities and meet age-appropriate motor milestones.

Ultrasound is a widely used type of imaging test that uses sound waves to generate a picture of tissues and structures in the body. Certain types of ultrasound can look at the size and health of muscles.

The tests are reliable and easy to perform, and allow for a quick assessment of multiple muscles. As such, they could be a good way to monitor muscle status and disease progression in SMA, possibly bypassing the need for functional tests that are more time-consuming, or for supplementing them.

Some data indicates children with SMA have reduced muscle thickness, reflecting muscle atrophy, and their muscles appear hyperechogenic on an ultrasound, which suggests tissue abnormalities or damage relative to healthy tissue. Hyperechogenicity refers to an elevated sound wave density relative to the surrounding structures. It’s often seen in bone and fat calcifications.

Still, most studies that evaluated the utility of muscle ultrasound for SMA included few patients and didn’t examine if ultrasound findings correlated well with patients’ functional status.

To learn more, the scientists performed muscle ultrasounds on 41 patients with SMA types 1-4 who were seen at the Neuromuscular Clinic of Hospital das Clinicas, Brazil and compared them with ultrasounds from 46 healthy age-and sex-matched people.

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Disease progression in muscle groups according to SMA type

Of the patients, ages 4 months to 43 years, nine had SMA type 1, 16 had SMA type 2, 12 had type 3, and four had type 4.

All had ultrasound testing at different muscles — biceps brachii (upper arm), rectus femoris (thigh), diaphragm (a breathing muscle), intercostal muscles (ribcage; involved in breathing), and the thoracic multifidus of the spine.

For all the examined muscle groups, SMA patients in general showed signs of hyperechogenicity relative to healthy people, results showed. This hyperechogenicity arises from the substitution of normal muscle with connective tissue and fat that change the image, the scientists said.

Still, the findings for specific muscle groups in each subtype varied. Type 4 patients showed the least difference in ultrasound values compared with the healthy people, who served as controls, with no significant differences observed.

The average bicep muscle was significantly reduced in type 1 patients relative to healthy people. Moreover, the size of the rectus femoris was significantly reduced among all types relative to the control group.

The functional status of type 1 patients was assessed by a physical therapist using the Children’s Hospital of Philadelphia Infant Test of Neuromuscular Disorders (CHOP INTEND), whereas all other patients were evaluated with the Hammersmith Functional Motor Scale Expanded (HFMSE).

HFMSE scores were significantly correlated with bicep muscle size, as well as the degree of hyperechogenicity in the biceps, rectus femoris, and intercostal muscles, reflecting a relationship between the ultrasound findings and patients’ functional status. CHOP-INTEND scores didn’t correlate with ultrasound findings with type 1.

“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 researchers wrote, noting more analyses to compare the findings in these muscles with lung function tests “could validate the usefulness of our findings as a predictor of neuromuscular respiratory compromise,” given that limited changes were seen in the diaphragm, an important muscle for breathing.