Muscular dystrophies are the inherited disorders characterized by the progressive degeneration of the muscular tissue.
Muscular dystrophy (MD) is a wide group of hereditary noninflammatory disorders causing progressive muscular tissue loss and subsequent decreasing muscle strength despite the fact that both central and peripheral innervation is preserved. As the disease progresses affected individuals become severely disabled and eventually wheelchair-bound. Muscle weakness results in poor posturing, scoliosis that may impair lung functioning and cause respiratory issues. Furthermore, another significant problem is the development of osteopenia and osteoporosis that in turn lead to pathologic fractures. There are four main types of MD distinguished, they include Duchenne muscular dystrophy, limb-girdle muscular dystrophy, facioscapulohumeral dystrophy, and myotonic muscular dystrophy. Each group has its own characteristic features with a specific distribution of muscle weakness, progression rate and inheritance pattern. Muscular deficiency prevalence varies greatly depending on the type with Duchenne MD considered the most common.
These disorders develop due to genetic mutations that disrupt the formation of the muscular tissue and cause its progressive degeneration which leads to muscular weakness.
Duchenne and Becker muscular dystrophy (also referred to as dystrophinopathy)
The Duchenne and Becker muscular dystrophies develop due to mutations in the dystrophin gene located on X-chromosome. The structural elements of the muscle cells appear to be disrupted, especially the cellular membranes for which dystrophin and the dystrophin-associated glycoproteins (DAGs) are essential. These proteins are important for the maintenance of stability and integrity of the muscle cell membranes.
In muscular dystrophies there are various gene mutations leading to the impairment of dystrophin (dystrophin gene is located on the X chromosome) or DAGs (the involved genes are located on different autosomal chromosomes) synthesis; dystrophin deficiency results in cellular instability and leakage of the intracellular components.
In Duchenne muscular dystrophy, the disease usually manifests at the age of 2-3 years, though some evidence suggesting of myopathy may be observed even from birth. The child cannot run, jump or climb on the stairs. Gradually the disease progresses until a kid becomes wheelchair-bound. Dilated cardiomyopathy and conduction abnormalities are also typical. Furthermore, cognitive retardation and developmental delay are observed in individuals with Duchenne myopathy. The prognosis is poor and most of those who were diagnosed with Duchenne’s do not reach their 30ies.
In Becker’s muscular dystrophy the disease progression is somewhat slower than in the case of Duchenne MD. The first symptoms usually occur in kids at the age of 5-15 years, though it can manifest as late as in the person’s 30ies or 40ies. Becker’s MD is associated with decreased life expectancy, however, most of the affected individuals survive even until 50-60 years. Unlike in Duchenne MD, mental retardation is not common for Becker’s.
Emery-Dreifuss muscular dystrophy
Emery-Dreifuss muscular dystrophy (humeroperoneal muscular dystrophy) may be inherited as an X-linked recessive, autosomal dominant, or autosomal recessive pattern. The underlying mutations affect nuclear membranes proteins (emerin, lamin A and C, nesprin 1 and 2) synthesis.
Autosomal recessive and autosomal dominant MD
Limb-Girdle muscular dystrophy
Limb-girdle muscular dystrophy includes several disorders characterized by the progressive weakness of the muscles of pelvic (pelvifemoral type) and shoulder (scapulohumeral type) girdle. About 10 types of Limb-girdle MDs are inherited as an autosomal recessive pattern, 5 – as an autosomal-dominant pattern.
Facioscapulohumeral muscular dystrophy
Facioscapulohumeral MD manifests with progressive weakness of the muscles of the face, upper arm, lower leg, and hips, though the muscles are affected asymmetrically. This disease is caused by the mutation causing underexpression of the DUX4 gene that is inherited as an autosomal dominant pattern. The disorder onset may take place at any point from infancy up to middle age. As in this case muscular dystrophy progresses relatively slowly prognosis is favorable.
Oculopharyngeal dystrophy is a rare autosomal-dominant disorder characterized by impaired articulation (dysarthria) and swallowing (dysphagia) associated with the dropping of the upper eyelid (known as ptosis) and occasionally limb muscles weakness.
Myotonic dystrophy includes two autosomal dominant disorders – DM1 known as Steinert disease and DM2. The onset of muscular dystrophy usually takes place at adulthood. Except for muscular weakness and hypotonia affected individuals develop cataracts, insulin resistance (diabetes mellitus) and heart disorders.
- Elevated levels of creatine phosphokinase (CPK) is suggestive of muscle disease and is considered specific for muscular dystrophy, especially if it’s levels are significantly increased. Later when there is extensive muscle atrophy the CPK levels are not so high anymore, though still elevated;
- Transaminase, lactate dehydrogenase and aldolase levels may also be elevated;
- Electromyography is conducted to verify the muscle weakness reason – this procedure helps to distinguish whether there is the primary lesion of the muscle itself (indicative of muscular dystrophy) or the muscles sustain atrophy due to impaired innervation and there is a lesion either in the central or peripheral nervous system;
Unfortunately, to date, there is no curative treatment for muscular dystrophy.
- Steroids (prednisone or deflazacort) may slow the progression of the Duchenne’s muscular dystrophy;
- Cardiac abnormalities may be managed by administration of angiotensin-converting enzyme (ACE) inhibitor and/or a beta-blocker, diuretics, and digoxin for symptomatic relief;
- It is essential to make sure that a person with muscular dystrophy receives vaccinations against seasonal influenza and pneumococcus;
- Physical therapy is necessary to prevent the development of contractures and maintaining muscle function as long as possible.