Calf ultrasound shows promise for detecting muscle loss in PWS
Study: Method may offer alternative to radiation-based scans
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A simple ultrasound measurement of the calf muscle can help identify low muscle mass in people with Prader-Willi syndrome, offering a potential alternative to radiation-based scans, a study in Taiwan suggests.
Among 48 people with the genetic condition, all of whom had obesity, researchers found that more than half met criteria for sarcopenic obesity, a combination of obesity and sarcopenia (low muscle mass and strength).
“Ultrasound of the [calf muscle] is a valid, radiation-free predictor of skeletal muscle mass and serves as a practical diagnostic tool for sarcopenic obesity in PWS,” researchers wrote.
The study, “Diagnostic Utility of Muscle Ultrasound for Sarcopenia in Prader–Willi Syndrome: A Cross-Sectional Study,” was published in the Journal of Cachexia, Sarcopenia and Muscle.
PWS most common genetic cause of life-threatening obesity
PWS, a complex genetic disease, is “the most common genetic cause of life-threatening obesity,” the researchers wrote. However, the disease’s hallmark body composition alterations go beyond obesity, and also include reduced lean (muscle) body mass and muscle strength.
This profile meets the diagnosis of sarcopenic obesity, which is “associated with greater metabolic, functional and cardiovascular risks than either obesity or sarcopenia alone,” the researchers wrote.
Therefore, accurately measuring muscle mass is essential for managing PWS. The gold-standard method for measuring muscle mass is a full-body scan called dual-energy X-ray absorptiometry (DXA). However, it uses radiation, it’s costly, and people with severe obesity may not fit on the scanning table or may have difficulty cooperating with the procedure.
In this study, a team of scientists in Taiwan examined whether ultrasound of specific muscles could serve as a radiation-free alternative to DXA for detecting low muscle mass in people with PWS.
A total of 48 people with PWS, with a median age of 19.5 years, were recruited at a specialty clinic in Taiwan between October 2022 and June 2024. Just under half (47.9%) were girls/women, and all had obesity and a history of standard growth hormone treatment.
Using DXA and standard diagnostic criteria, the researchers found that 60.4% of participants had low muscle mass. About half (52.1%) met criteria for sarcopenia (reduced muscle mass and strength), and one-fifth (20.8%) had severe sarcopenia, meaning they also had reduced physical performance on tasks like walking speed or balance.
Ultrasound used to measure thickness of 2 thigh muscles, 1 calf muscle
The researchers used ultrasound to measure muscle thickness at several sites: the rectus femoris and vastus lateralis (two thigh muscles), and the gastrocnemius medialis (a calf muscle). They then compared these ultrasound measurements to DXA results.
The ultrasound-assessed calf muscle thickness showed the strongest association with DXA-measured muscle mass, followed by the cross-sectional area of the rectus femoris (thigh muscle). Statistical analyses accounting for body size showed that ultrasound-measured calf muscle thickness remained an independent predictor of DXA-measured muscle mass.
A separate analysis confirmed the utility of calf muscle thickness, as measured by ultrasound, for detecting sarcopenia in PWS, with an area under the curve (AUC) of 0.759. AUC is a measure of a test’s diagnostic accuracy with values ranging between zero and one and the latter reflecting perfect discrimination.
Using a cutoff calf muscle thickness value of 1.69 cm, the test showed a 86.2% sensitivity (correct identification of those with low muscle mass) and 63.2% specificity (correct exclusion of those without).
We recommend integrating gastrocnemius medialis ultrasound assessment, specifically of the calf muscle, alongside standard [body] and demographic indices, as a practical, radiation-free strategy to screen for sarcopenia and monitor muscle health in the PWS population.
Distal calf muscle thickness, or farthest from the body, was also strongly associated with handgrip strength, a measure of upper body strength, while no significant links were detected between thigh muscle thickness and strength.
Regarding muscle architecture, the angle between the calf muscle fibers and the central tendon showed a weak but significant correlation with DXA-measured muscle mass.
There was also a notable difference between upper- and lower-body function among patients. Grip strength was consistently poor across the group, but measures of lower body function, such as walking speed and overall physical performance scores, were relatively preserved.
The team proposed that the calf muscle may be a more accurate ultrasound site in this population for two reasons. First, they noted that the excessive fat tissue in the thigh of PWS patients can attenuate ultrasound beam penetration. Second, they suggested that because people with PWS generally retained their ability to walk, the calf muscle’s role in movement might better reflect functional muscle mass.
“We recommend integrating gastrocnemius medialis ultrasound assessment, specifically of the calf muscle, alongside standard [body] and demographic indices, as a practical, radiation-free strategy to screen for sarcopenia and monitor muscle health in the PWS population,” the researchers concluded.
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