Duchenne muscular dystrophy
Duchenne muscular dystrophy (DMD) is a dystrophinopathy and the most common muscular dystrophy.
DMD has an incidence of 1 in 3500 to 5000 males . The condition is extremely rare in females due to its inheritance pattern, as discussed below .
The characteristic feature is progressive muscle weakness with fatty replacement of muscle that begins in early childhood . Typically, proximal lower limb and truncal muscles are affected first, but eventually, there is progression to upper limb muscles and extremities .
In addition to gross motor weakness and delay, patients also exhibit muscle pseudohypertrophy especially of the calves, a waddling gait, scoliosis, joint contractures, and the classic yet non-specific Gower’s sign of proximal muscle weakness . Generally, children become wheelchair-bound by approximately 12 years of age .
Furthermore, patients have many other non-musculoskeletal features:
- dilated cardiomyopathy leading to congestive heart failure; often remains subclinical due to physical inactivity
- cardiac conduction anomalies , characteristic electrocardiographic anomalies include:
- interventricular conduction delay with widening of the QRS complex (duration >0.12 seconds)
- narrow (< 0.04s), deep Q waves in the lateral praecordial (V5, V6) and high lateral (I, aVL) leads
- may be differentiated from the Q waves present after a lateral myocardial infarction by the duration/width of the deflection (infarction Q waves are characteristically wider, with a duration >0.04 seconds)
- prominent R waves in V1 (R/S >1)
- the terminal portion of the QRS in lead V1 is often distorted, with R’ and S’ deflections common
- restrictive lung disease
- intellectual disability
DMD is inherited in an X-linked recessive pattern, and thus nearly exclusively occurs in males . It is due to a mutation in the DMD gene that normally encodes for dystrophin, a protein involved in strengthening skeletal and cardiac muscle fibers by acting as a mechanical link between the cytoskeleton and the extracellular glycoprotein matrix of cells in these tissues .
Unlike in Becker muscular dystrophy where a mutation in the DMD gene results in a partially functioning dystrophin protein, in Duchenne muscular dystrophy dystrophin is non-functioning, which results in a more severe phenotype .
Musculoskeletal MR features, especially those from the lower limb, are most commonly described, however other imaging modalities may be useful as well.
CT is not commonly performed due to risks of ionizing radiation in this young patient demographic . However, CT confirms plain radiograph findings and furthermore, in the affected skeletal musculature (see below), typically shows low attenuating fatty infiltration and resultant pseudohypertrophy .
MRI is the imaging modality of choice in DMD, with T1-weighted sequences traditionally being the most useful . Due to its superior soft tissue contrast, T1-weighted MRI shows characteristic patterns of muscular fatty infiltration . Affected muscles demonstrate high T1-weighted signal that is initially streaky, but becomes more confluent in nature as the condition progresses .
There tends to be a typical pattern of muscle involvement:
- lower limb and pelvic musculature:
- early involvement of the gastrocnemii muscles
- eventual involvement of the gluteus maximus, gluteus medius and adductor magnus muscles, followed by involvement of the psoas, iliacus, quadriceps, rectus femoris, biceps femoris, peroneus longus, and soleus muscles
- there is characteristic relative sparing of the sartorius, gracilis, semitendinosus, semimembranosus, and tibialis posterior muscles, even in advanced stages of the condition
- upper limb musculature:
Furthermore, recent research suggests T2 mapping techniques may also be useful applications of MR, especially in relation to clinicoradiological severity correlations, but this is an ongoing field of research .
Bone mineral density can be reduced leading to osteoporosis due to derangement of calcium metabolism +/- effects of corticosteroid therapy .
Cardiac MRI reveals ventricular dilation consistent with a dilated cardiomyopathy . Late gadolinium-enhancement images show subepicardial delayed myocardial enhancement characteristically affecting the basal inferolateral wall, reflecting subepicardial fibrosis of the basal inferolateral wall . This pattern of enhancement can mimic viral myocarditis , although the clinical presentation differs significantly.
Treatment and prognosis
Corticosteroids, such as prednisolone and deflazacort, are effective in improving muscle strength and respiratory function , and are the mainstay of treatment. However, novel therapies such as eteplirsen may have potential for far greater benefit in certain genetic cohorts .
Otherwise, the management of DMD is multidisciplinary and involves rehabilitation and surveillance of respiratory, cardiac, and orthopedic complications . Many patients require heart transplants to prolong survival .
The prognosis remains relatively poor, with most patients not surviving past the third decade of life .
History and etymology
This disorder is named after Guillaume-Benjamin-Amand Duchenne de Boulogne (1806-1875), a French neurologist, who first described the condition in his 1861 book .