Muscular dystrophies in children are a group of uncommon, genetically determined primary myopathies characterized by progressive muscle wasting. The natural history is variable.
Duchenne Muscular Dystrophy
This dystrophy is the most common lethal genetic disorder of childhood. It is caused by the absence of or impaired dystrophin. The dystrophin gene is located on the X chomosome. Dystrophin stabilizes the cell membrane and protein complexes within the muscle cell. A loss of this function causes the disease. In addition, to progressive musclar degeneration, intellectual impairment is common, with an average of about a 20-point drop in intelligence quotient (IQ).
Clinical features include males with an onset in early childhood, a wide base gait, impaired running, positive Gowers sign, delayed motor development, intellectual slowness, calf pseudohypertrophy, and often a positive family history. About one-third are new mutants.
Laboratory findings Serum CK is elevated 200–300 times normal values. DNA obtained from blood shows a large deletion in 60% of patients. Now the gene can be fully sequenced and in most of those not showing deletions will show a mutation that prevents dystrophin synthesis. Muscle biopsy is currently rarely necessary. Specimens show variation in fiber size, loss of fibers, increased fat, and sometimes centralization of nuclei. EMG shows myopathic changes.
Natural history Most boys show progressive deterioration with flexion–abduction contractures of the hips, and flexion contractures of the knees and ankle with loss of walking at about 10 years of age. Once walking ability is lost, scoliosis develops progressively. Cardiomyopathy and pulmonary compromise progress, and death usually occurs in late teens.
Medical management Treatment with deflazacort or prednisone reduces the inflammatory reaction that follows the breakdown of muscles. This preserves muscle function and dramatically slows the progression of disability. Steroids, vitamin D, and calcium are administered starting in midchildhood. This results in a prolongation of motor function with about three-quarters retaining their walking ability into the midteen years. Assisted ventilation adds to life expectancy. Dystrophin replacement may be feasible in the future.
Mobility With new treatment, walking can be preserved until the midteen years. Provide effective mobility. An electric wheelchair and a van for the family are useful. Make a home wheelchair accessible. Foot deformities should be corrected before the child becomes wheelchair dependent.
Family support This disease causes guilt in the mother, stress in the family, and depression in the child. Provide a support system and make counseling available as necessary.
Long-term ventilatory support Prolongation of survival is technically possible, but creates many ethical issues.
Spine management Prepare the family in advance of the onset of scoliosis. Progressive scoliosis develops once the child becomes nonambulatory. Avoid bracing. The use of steroids has dramatically reduced the risk of scoliosis and the need for operative correction. In boys treated with corticosteroids, curve progression is unclear. Curves under 30°–40° may be stable. In boys not on corticosteroids, surgery may be necessary if the curve approaches 20°, and before the vital capacity falls below 30%, perform a segmental instrumentation and fusion from T2 to L5 or the sacru.
Becker Muscular Dystrophy
Becker muscular dystrophy is an uncommon, mild muscular dystrophy due to dystrophin abnormality with clinical findings similar to DMD but much less severe. Early features of BMD include leg cramps and gait problems during childhood. Scoliosis is rare. Survival into midadult life is usual. In some cases, the diagnosis is not established until the teens. Management problems are similar to DMD but are required at an older age. Provide physical therapy, release of contractures, and effective mobility.
Emery-Dreifuss Muscular Dystrophy
This ia a rare sex-linked recessive muscular dystrophy with elbow, triceps, and posterocervical muscle contractures, slowly progressive muscle wasting, and cardiomyopathy. Weakness becomes apparent during the first decade and more pronounced during adolescence. Differentiate this from BMD and DMD by only mildly elevated CK levels and by dystrophin testing. DNA testing is available for these patients. Patients may require contracture releases, spinal stabilization, and referral to a cardiologist for insertion of a cardiac pacemaker.
Autosomal Dominant Muscular Dystrophies
Myotonic dystrophies include a heterogeneous group of diseases characterized by myotonia. Myotonia is the inability of the muscle to relax after contracting. Severe forms are seen.
Myotonia congenita has an onset in infancy but becomes more evident during adolescence. Clinical features include generalized muscle hypertrophy, absence of skeletal deformities, and minimal long-term disability.
Congenital myotonic dystrophy The severity is proportional to the number of repeats. Repeats of less than 20 are within normal limits. Repeats of 20–200 cause mild forms and >500 severe involvement.
Children with the severe form are weak at birth and require assisted ventilation for months. They become progressivcly stronger, and most will walk. The myotonia does not manifest until the second decade. Cataracts may develop in adolescence. Orthopedic problems include clubfeet, hip dislocations, and lower limb contractures.