The muscular dystrophies are a group of genetic diseases that affect muscle and cause myofibril degeneration and regeneration and are characterized by progressive muscle wasting and weakness. They are subdivided into several groups, including congenital forms (congenital muscular dystrophy [CMD]); Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD); limb-girdle muscular dystrophy (LGMD); Emery-Dreifuss muscular dystrophy (EDMD); oculopharyngeal; distal; and facioscapulohumeral. Here we are focusing on the muscular dystrophies primarily or commonly affecting children: DMD, BMD, LGMD, and EDMD. CMD is discussed in Chapter 43.
DUCHENNE AND BECKER MUSCULAR DYSTROPHIES
ESSENTIALS OF DIAGNOSIS AND TYPICAL FEATURES
X-linked recessive muscle dystrophies primarily affecting boys
Duchenne mutations are typically out of frame, whereas Becker mutations are in frame in DMD gene
Characteristic dystrophic findings with myofiber degeneration and regeneration on muscle biopsy, with primarily absent dystrophin protein expression in DMD and decreased dystrophin protein expression in BMD
Clinically present with motor difficulty, calf pseudohypertrophy, and pelvic girdle muscle weakness; onset of symptoms for DMD is before age 5, whereas that for BMD is typically around age 12 or later
Multisystem involvement, including progressive weakening of respiratory and cardiac muscles, leads to respiratory insufficiency and dilated cardiomyopathy
DMD and BMD are X-linked recessive degenerative muscle diseases caused by genetic mutations in the DMD gene that primarily affect boys. In addition, DMD is the most common childhood-onset muscle disease. Besides the clinical features, genetic analyses and/or muscle biopsy are essential to establish the diagnosis. Management is provided by a multidisciplinary neuromuscular team. Glucocorticoid steroids are standard treatment for DMD in addition to supportive care, and various genetic modification therapies are now available for DMD patients with specific genetic mutations (eg, exon-skipping medications). Cardiac medications are routinely prescribed to slow down the progression of cardiomyopathy and to optimize cardiac function.
Symptom onset varies from early childhood before age 5 for DMD to around age 12 or later for BMD. Delayed motor development milestones are common in DMD. Patients present with symptoms of motor difficulty, calf pseudohypertrophy, and progressive pelvic girdle muscle weakness. Muscle weakness is more proximal and affects the lower extremities initially. Patients have difficulty with running and getting up from the floor with a positive Gowers sign due to weak knee and hip extensors. Subsequently, weakness worsens and spreads to the upper extremities as well. Deep tendon reflexes are usually depressed or absent. Patients have increased difficulty with step climbing, walking, and ambulation, eventually becomes impaired with ambulation. DMD patients lose independent ambulation around age 12 in most cases, whereas BMD patients lose independent ambulation in adolescence or later in adult life. In addition to skeletal muscle involvement, cardiac and respiratory muscles are affected. Patients have progressively decline in pulmonary function and cardiac systolic function. Chewing and swallowing functions are affected later. Cognitive function ...