Quantitative muscle ultrasound detects disease progression in Duchenne muscular dystrophy

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Clinical trials in Duchenne muscular dystrophy (DMD) generally rely on measures of function and strength, such as the 6‐minute walk test (6MWT), as their primary outcome.1 Imaging modalities, however, may also be valuable in demonstrating progressive alterations in muscle over time and the effect of therapy. For example, recent work has shown that magnetic resonance imaging (MRI) of muscles is very sensitive to disease progression in DMD.4 Another imaging modality, quantitative ultrasound, has also been shown preliminarily to identify disease status and progression in DMD. Muscle echo intensity increases with greater muscle fat and fibrosis,6 worsening strength and function,8 and when measured by an experienced examiner, can detect disease progression over time in both very young (age < 3 years)9 and older (age = 3–15 years) boys with DMD.10 Quantitative ultrasound also offers some practical advantages over MRI, as it can be performed at the bedside, is relatively inexpensive, and is technically easier for evaluating both upper and lower extremities and subjects who cannot lie still or flat.
In this 2‐year, nonblinded longitudinal study, we evaluated alterations in quantitative muscle ultrasound in boys with DMD and compared them to healthy controls, along with electrical impedance myography (EIM) measurements and functional assessments, as described in our companion article.11 Our goal was to determine whether quantitative muscle ultrasound, when performed by trained evaluators in a clinical trial setting, could provide effective biomarkers of disease progression in DMD. We measured muscle echo intensity in 2 ways. We utilized a direct analysis of the amplitudes of the reflected ultrasound echoes, measured in decibels, termed quantitative backscatter analysis (QBA), in addition to the more standard analysis of the gray scale pixel level (GSL) in a region of interest (ROI) within the fully processed image. The reason for evaluating QBA specifically is that the relatively large amounts of data contained in the raw backscattered energy may be lost or skewed when proprietary algorithms are used to compress the values into the 256 GSLs for image display. It is unknown whether QBA provides similar, better, or worse outcomes than using GSL for measuring disease progression in DMD.

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