Skeletal muscle magnetic resonance biomarkers correlate with function and sentinel events in Duchenne muscular dystrophy

Abstract

Objective: To provide evidence for quantitative magnetic resonance (qMR) biomarkers in Duchenne muscular dystrophy by investigating the relationship between qMR measures of lower extremity muscle pathology and functional endpoints in a large ambulatory cohort using a multicenter study design.

Methods: MR spectroscopy and quantitative imaging were implemented to measure intramuscular fat fraction and the transverse magnetization relaxation time constant (T2) in lower extremity muscles of 136 participants with Duchenne muscular dystrophy. Measures were collected at 554 visits over 48 months at one of three imaging sites. Fat fraction was measured in the soleus and vastus lateralis using MR spectroscopy, while T2 was assessed using MRI in eight lower extremity muscles. Ambulatory function was measured using the 10m walk/run, climb four stairs, supine to stand, and six minute walk tests.

Results: Significant correlations were found between all qMR and functional measures. Vastus lateralis qMR measures correlated most strongly to functional endpoints (|ρ| = 0.68-0.78), although measures in other rapidly progressing muscles including the biceps femoris (|ρ| = 0.63-0.73) and peroneals (|ρ| = 0.59-0.72) also showed strong correlations. Quantitative MR biomarkers were excellent indicators of loss of functional ability and correlated with qualitative measures of function. A VL FF of 0.40 was an approximate lower threshold of muscle pathology associated with loss of ambulation.

Discussion: Lower extremity qMR biomarkers have a robust relationship to clinically meaningful measures of ambulatory function in Duchenne muscular dystrophy. These results provide strong supporting evidence for qMR biomarkers and set the stage for their potential use as surrogate outcomes in clinical trials.

Fig 1. Representative T₂ maps, ¹H MRS voxel placement, and ¹H MR spectra.

A) T₂ maps were created from the 40, 60, 80, and 100ms TE spin echo images of the lower leg and thigh. Utilizing the popliteus and biceps femoris short head as standardized anatomical landmarks to guide lower leg and thigh slice selection, the TA, PER, TP, SOL, MG, VL, BFLH, and GRA muscles were traced on three contiguous slices to determine mean T₂ for each muscle. B) T₁-weighted images demonstrate the location of voxel placement for MR spectroscopy used to quantify FF. Voxels were sized as large as possible while staying completely within the SOL and VL muscles, avoiding the fascia. The voxel position from prior years was referenced to position voxels at an anatomically similar location at follow-up visits. C-F) T₁-weighted images of the calf and thigh with corresponding SOL and VL spectra and FFs from a representative participant in each of four functional groups (based on the 10m walk/run test). Participants have increasing levels of SOL and VL fat infiltration (appreciated visually on the T₁-weighted images and quantified using spectroscopy) with decreasing ambulatory ability. 10m walk/run times were 3.56s, 7.16s, and 11.32s for the example participants in panels C, D, and E, respectively. (TA = tibialis anterior, PER = peroneus longus and brevis, TP = tibialis posterior SOL = soleus, MG = medial gastrocnemius, VL = vastus lateralis, BFLH = biceps femoris long head, GRA = gracilis, FF = fat fraction).

Fig 2. Scatterplots showing the correlations between VL MRI T₂ and FF and functional endpoints.

The qMR biomarkers most strongly correlated to function were VL MRI T₂ and VL FF. A) VL MRI T₂ versus 10m walk/run, B) VL FF versus 10m walk/run, C) VL MRI T₂ versus 6MWD, D) VL FF vs 6MWD. Participants who lost the ability to perform the timed tests were given the maximum time of 45 seconds (open circles) for the first visit in which they were unable to complete the test. Participants who lost the ability to perform the 6 MWT have distances of 0 m (open circles). As indicated in Table 2, all correlations were significant with the 95% bootstrap confidence intervals excluding zero. (VL = vastus lateralis, 6MWD = six minute walk distance, 6MWT = six minute walk test).

Fig 3. FF and quantitative and qualitative measures of walking/running ability.

A and B) FF was assessed based on 10m walk/run performance. Data points from participants who could complete the test were divided into bins of <5 sec (n = 152), 5–8 sec (n = 184), and >8 sec (n = 127). The LOA group represents data points from subjects that could no longer complete the 10m walk/run at the time of the visit (n = 35). The amount of SOL and VL fatty infiltration was significantly different between functional performance groups. C) Groups were created based on the qualitative rating of participants’ 10m walk/run test at each time point. Mean VL FF was significantly different between each qualitative grade group. * = significantly different; bootstrap confidence intervals excluded zero. (SOL = soleus, VL = vastus lateralis).

Fig 4. Loss of function in participants with different VL FFs.

Data points were binned by VL FF and further divided by the number of functional skills the participants could complete at the time of the visit. Black = ambulatory, able to climb 4 stairs, able to move supine to stand. Dark gray = ambulatory, able to climb 4 stairs, unable to rise from supine. Light gray = ambulatory, unable to perform stairs, unable to rise from supine. White = nonambulatory. As FF increased, there was a progressive decrease in the percent of participants who could perform the functional tasks. At VL FFs less than 0.19, 99% of participants could perform all functional tests. Apart from one outlier, participants began losing ambulation at VL FFs ≥ 0.40. (VL = vastus lateralis, STS = supine to stand).

Fig 5. qMR biomarkers of muscle pathology at the first visit following loss of ambulation.

A) MRI T₂ and B) MRS FF are plotted for each muscle at the first time point at which participants were nonambulatory (n = 35). Whiskers represent the 5^th-95^th percentiles with outliers represented by the filled dots. MRI T₂ was highest in the BFLH and VL and lowest in the GRA, TA, and TP. The MRI T₂ and FF of the VL were higher than that of the SOL as expected based on the proximal to distal pattern of involvement in DMD.