Purpose: To assess the distribution of fat replacement along the proximodistal muscle axis in individual leg muscles of patients with Duchenne Muscular Dystrophy (DMD).
Introduction: In DMD, muscle tissue is progressively replaced by fat, resulting in increasing muscle weakness and functional limitations with age (1). However, the pathophysiology of the muscle degeneration is not fully understood (2). The progression of fat replacement between different muscles has been studied previously, but not within individual muscles. This information can be valuable both for the application of quantitative MRI as an outcome measure, as well as for understanding the pathophysiology of the muscle degeneration process in DMD.
Methods: 3-point gradient echo Dixon images (FOV 180x180 mm; resolution 1x1x10 mm3; 23 slices; slice gap 5 mm; TR 210 ms; TE 4.41 ms; ∆TE 0.76 ms; flip angle 8 ͦ) were acquired to assess fat infiltration in the right lower leg (knee to ankle coverage) of 22 DMD patients (mean age 9.3±3.1 years, range 5-16 years) using a 3T MR system (Ingenia, Philips Healthcare, Best, the Netherlands) with a 32 element receive coil.
Data-analysis: Fat and water images were reconstructed using a six-peak model coded in Matlab (3) without correction for T2*relaxation. Regions of interest (ROIs) were drawn manually using Medical Image Processing, Analysis and Visualization (MIPAV) software (http://mipav.cit.nih.gov) for all individual lower leg muscles: the lateral head of the gastrocnemius (GL), medial head of the gastrocnemius (GM), soleus (SOL), tibialis anterior (TA), peronei (PER), the tibialis posterior (TP), the extensor digitorum longus (EDL), extensor hallucis longus (EHL), flexor digitorum longus (FDL) and the flexor hallucis longus (FHL) muscles. Fat fractions are reported as a mean value of all pixels within a ROI per individual slice. Generalized estimating equations were used to evaluate the effect of location on the proximodistal axis and muscle volume (mm3) on fat percentage (%fat), assuming a parabolic relation with distance as a variable. The significance level was set at p<0.001.
Results: A higher %fat was observed in the more distal and proximal muscle segments compared to the muscle belly (Fig. 1). Group analysis showed that the location along the proximodistal muscle axis had a significant effect on %fat for the SOL, TA, TP, EDL and FDL muscles. In the GCL and PER muscle, muscle volume showed a significant correlation with %fat with higher fat fractions being found in a smaller muscle volume. Location along the proximodistal axis did not have an effect. For the GCM and FHL both the location along the proximodistal axis and the muscle volume had a significant effect on %fat.
Discussion: In DMD almost all muscles in the lower leg show non-uniform muscle degeneration decreasing outwards along the proximodistal muscle axis. Replacement of muscle tissue with fat is more pronounced near the tendons and lowest at the muscle belly. This could be due to the fact that in healthy skeletal muscle mechanical strain is not distributed uniformly along the proximodistal muscle axis; in the muscle end regions the intrafasciculair strain is higher and the Anatomical Gear Ratio is lower compared to the muscle belly (4). Moreover, preclinical work has shown that the musculotendinous junction is the weakest point in the healthy muscle tendon complex and most prone to stress-induced damage (5). In addition, the protein dystrophin, which is missing in DMD patients, has been found to be concentrated near the musculotendinous junction in healthy mice (6,7). Taking all of these observations into account, indicates that more advanced degeneration processes in DMD appear in high stress and muscle strain regions. This leads us to propose that stress-induced muscle degeneration starts at the level of the myotendinous junction both proximally and distally, whereas in later stages of the disease muscle degeneration becomes more homogeneous along the proximodistal muscle axis. In conclusion, we have shown the importance of considering heterogeneous fat infiltration in DMD within individual muscles. This can have an influence on quantitative MR measurements and biopsy outcomes potentially used as outcome measures, and presents a new insight into the disease progression of DMD.