Magnetic Resonance Imaging Studies in Duchenne Muscular Dystrophy: Linking Findings to the Physical Therapy Clinic

Abstract

Duchenne muscular dystrophy (DMD) is a muscle degenerative disorder that manifests in early childhood and results in progressive muscle weakness. Physical therapists have long been an important component of the multidisciplinary team caring for people with DMD, providing expertise in areas of disease assessment, contracture management, assistive device prescription, and exercise prescription. Over the last decade, magnetic resonance imaging of muscles in people with DMD has led to an improved understanding of the muscle pathology underlying the clinical manifestations of DMD. Findings from magnetic resonance imaging (MRI) studies in DMD, paired with the clinical expertise of physical therapists, can help guide research that leads to improved physical therapist care for this unique patient population. The 2 main goals of this perspective article are to (1) summarize muscle pathology and disease progression findings from qualitative and quantitative muscle MRI studies in DMD and (2) link MRI findings of muscle pathology to the clinical manifestations observed by physical therapists with discussion of any potential implications of MRI findings on physical therapy management.

Keywords: Muscle Weakness; Muscular Dystroophies; Neuromuscular Diseases.

© The Author(s) 2020. Published by Oxford University Press on behalf of the American Physical Therapy Association. All rights reserved. For permissions, please email: journals.permissions@oup.com.

Figures

Figure 1

Magnetic resonance imaging (MRI) of qualitative muscle. (A) T1-weighted images of the calf in an unaffected 8-year-old control and a 7-year-old with Duchenne muscular dystrophy (DMD). On T1-weighted images, healthy muscle is homogenous in intensity, and fat appears in bone marrow and the subcutaneous tissue as bright signal intensity. In DMD, T1-weighted contrast highlights the fatty infiltration of muscle. Here, fatty infiltration is visible in the lateral gastrocnemius and soleus. (B) In unaffected controls, T2-weighted images of muscle have similar contrast to T1-weighted images. In DMD, T2-weighted imaging reveals higher signal intensity in the muscle that can be reflective of either inflammation, fat, or both. (C) T2-weighted images with suppressed fat signal can highlight muscle edema well, particularly in this steroid-untreated individual. No increased muscle signal intensity is visible in the control calf, but increased signal intensity is visible in the individual with DMD in the soleus and medial gastrocnemius. The vertical lines in these images are small artifacts from blood vessels. Note: Each of the 3 images from the control and individual with DMD are taken at the same slice location, demonstrating the ability of different image contrasts to highlight different pathologies. Fib = fibularis longus and brevis; LG = lateral gastrocnemius; MG = medial gastrocnemius; Sol = soleus; TA = tibialis anterior; TP = tibialis posterior.

Figure 2

Magnetic resonance imaging (MRI) of quantitative muscle. Chemical shift-encoded imaging of the thigh muscles produces 2 separate images: (A) 1 that only contains signal from water, and (B) 1 that only contains signal from fat. Unaffected muscle, such as that seen in this 9-year-old control, should be comprised nearly entirely of water signal. Fat signal is present in the subcutaneous tissue and in dystrophic muscle, including in the muscles of this 10-year-old with Duchenne muscular dystrophy (DMD). (C) When the water image is digitally colored red and fat image digitally colored green, they can be overlaid (fused) to create a visually informative image. (D) MRI T2 maps display the T2 values of each individual pixel of the MRI. MRI T2 is a value that differs for each tissue, but unaffected muscle has a low T2 value (~30–35 ms) while fatty/edematous muscle has higher T2 values (~35–80 ms). MRI T2 values are well correlated with fat fraction values in DMD. Add = adductor group; BF = biceps femoris long head; Gra = gracilis; RF = rectus femoris; Sar = sartorius; SM = semimembranosus; ST = semitendinosus; VI = vastus intermedius; VL = vastus lateralis; VM = vastus medialis.

Figure 3

Magnetic resonance imaging (MRI) of abdominal and chest muscle. (A) Axial fat/water fusion images of the trunk at the T5, T9, and L4 vertebral levels reveal little to no fatty infiltration in a control (age = 17 y). (B) Involvement of several muscles is visible in a representative early ambulatory individual with DMD (age = 11 y). (C) Once individuals become non-ambulatory, the majority of the trunk musculature becomes replaced by fatty tissue. This is a 12-year-old, steroid-untreated individual who has been non-ambulatory for approximately 2 y. EO = external oblique; Inf = infraspinatus; IO = internal oblique; Lat = latissimus dorsi; PMa = pectoralis major; PMi = pectoralis minor; PS = paraspinal muscle group; psoas = psoas major; QL = quadratus lumborum; RA = rectus abdominis; SA = serratus anterior; Sub = subscapularis; TM = teres major.

Figure 4

Magnetic resonance imaging (MRI) of pelvic girdle muscle. Fat (green) and water (red) fusion images of axial MRIs of the pelvis. (A) Unaffected controls have little to no visible pelvis muscle fatty infiltration. The individual shown here is 13 y old. (B) Gluteal muscle involvement is present in DMD beginning at a young age and even in individuals with preserved functional abilities. Shown here is an 11-year-old who could rise from the floor in 3.2 seconds (average control time = 1.5 s) and climb stairs in 1.9 seconds without rail support (average control time = 1.2 s). (C) By the late ambulatory phase, many individuals have complete replacement of the gluteal muscles by fat. Representative image from a 16-year-old who lost ambulation 9 months after image acquisition. GMax = gluteus maximus; GMed = gluteus medius; GMin = gluteus minimus; RA = rectus abdominis.

Figure 5

Magnetic resonance imaging (MRI) of upper and lower leg muscle. (A–D) T1-weighted axial MRIs of the upper leg and (E–H) lower leg in an unaffected control and 3 individuals with Duchenne muscular dystrophy (DMD). (A,E) In the unaffected control (age = 12 y), the muscles have a smooth-appearing texture and moderate signal intensity without evidence of fatty infiltration. (B,F) This individual with DMD (age = 11 y, same individual as in Fig. 4B) has visible fatty infiltration of the quadriceps, adductor magnus, and biceps femoris muscles. There is minimal fatty infiltration visible in the gastrocnemius muscles. (C,G) In this 16-year-old who requires 8.2 seconds to traverse 10 m (average control time = 2.7 s), the vastus lateralis (VL) has approximately 50% fat, the adductors (Add) are approaching complete fatty replacement, and the fibularis group and calf muscles have an estimated 40% fat. (D,H) This steroid-untreated individual (age = 12 y, nonambulatory for ~2 y) has nearly complete fatty replacement of muscle with some sparing of the gracilis (Gra) and tibialis posterior (TP). (I–K) Water, fat, and fat/water fusion images highlight patterns of muscle involvement and sparing as well as loss of contractile tissue in the upper and lower legs. (I) Despite plantar-flexion contractures, the tibialis anterior (TA) typically remains less involved than the calf muscles. (J) These images demonstrate dramatic involvement of the fibularis group in stark contrast to sparing of the TP that contributes to equinovarus contractures. (K) Contractures are ubiquitous in the nonambulatory stage, and these images reveal the lack of remaining contractile tissue in this stage. Note the complete replacement of the biceps femoris by fat. Add = adductor; BF = biceps femoris long head; Early amb = early ambulation; Fib = fibularis longus and brevis; late amb = late ambulation; LG = lateral gastrocnemius; RF = rectus femoris; Sar = sartorius; Sol = soleus; ST = semitendinosus; VI = vastus intermedius.

Figure 6

Magnetic resonance imaging (MRI) of upper extremity muscle. T1-weighted images of the shoulder, arm, and forearm. (A) The unaffected control has little to no visible fat in the upper extremity muscles. (B) In this early–ambulatory stage individual (age = 11 y) who can abduct the arms overhead without compensation, the scapular muscles and deltoid already show fatty infiltration. (C) In a late ambulatory individual who can abduct his arms overhead only by using compensatory movements (age = 16 y), progression of shoulder complex muscles is visible, and signs of upper arm muscle involvement are present. (D) Progressive fatty infiltration of the shoulder, arm, and forearm muscles is visible in this steroid-untreated 12-year-old who cannot abduct his arms overhead but can bring his hand to his mouth. Ant = anterior forearm muscle group; Bi = biceps brachii; Br = brachialis; Del = deltoid; Inf = infraspinatus; PMa = pectoralis major; Post = posterior forearm muscle group; Sub = subscapularis; Tri = triceps brachii.