Description of pathology
Muscular dystrophy (MD) is a heterogenous group of hereditary diseases that results in progressive muscular degeneration manifested as muscle weakness usually in a proximal-to-distal orientation. Patients have trouble performing normal muscular functions and in extreme cases may experience compromised cardiopulmonary function. MD is caused by mutations in dystrophin, a gene that encodes skeletal muscle proteins. The disorder can be detected by analyzing electromyograph readings in order to evaluate variations in normal electrical patterns, degree of denervation and myopathy. Alternatively, creatine kinase activity in the bloodstream is analyzed as the creatine kinase enzyme is released when degradation of muscle fibers occurs. Although there are many known variations of MD, the most common are Becker, Duchenne, limb-girdle, facioscapulohumeral, and myotonic dystrophy. Becker and Duchenne muscular dystrophies are specifically associated with mutation of the dystrophin gene and are sex-linked, thus mostly affect male patients. Duchenne MD is characterized by complete absence of dystrophin and occurs at early childhood between two and six years of age. On the other hand, dystrophin is produced in small amounts in Duchenne MD patients and generally occurs during adolescent years. Other variations of MD involve mutations in multiple genes and mainly affect extracellular molecules, with autosomal inheritance. Regardless, all cases of muscular dystrophy result in degeneration and attempted regeneration of muscle fibers, where regenerated fibers are abnormally large and ultimately transform into fats and scar tissue ( Girolami, 2006) .
Body systems involved
Muscular dystrophy affects the muscles, where symptoms vary with the type of disorder. In some cases, all body muscles are involved while in others, only specific muscles are affected. For instance, limb-girdle dystrophy is known to affect pelvic muscles while facioscapulohumeral dystrophy affects the face muscles and portions of the upper body. As for myotonic MD, which is the most prevalent type among adults, major body organs may be involved including the nervous system, heart, eyes as well as the endocrine and digestive system.
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Normal anatomy of major body system effected
Muscular dystrophies are known to target skeletal muscles. However, in severe cases of MD, other muscle types including cardiac muscles may be involved. The size of skeletal muscles ranges from very small strands such as the ear muscles to large ones such as the thigh muscles. They also vary in shape, either broad or narrow, and arrangement. Muscle masses are made up of numerous muscle fibers, where each fiber equates to one muscle cell. These muscle fibers are surrounded by a connective tissue (endomysium), to form bundles of fascicles. Fascicles are further bound by perimysium and also a strong outer casing known as epimysium. Skeletal muscle fibers have a plasma membrane known as sarcolemma. Mutations associated with MD compromise the plasma membrane, resulting in progressive muscle weakness and breakdown.
Normal physiology of body system effected
Skeletal muscles are responsible for body movements including ambulation. They have long fibers as compared to other muscle types and are usually stripped (striated). Skeletal muscles are the only voluntary muscles in the human body and thus contract under voluntary action. Muscle fibers comprise of long cylindrical masses called myofibrils and a plasma membrane called sarcolemma. Contraction of the sarcomere in myofibrils results in skeletal muscle contraction. The sliding filament theory suggests that energized heads of myosin attach to actin myofilaments, forming a cross bridge. The myosin head releases its energy and twists towards the sarcomere while pulling actin myofilaments along. When the myosin head is re-energized, the cross bride disengages and the cycle begins again, resulting in muscular contractions.
5. Mechanism of pathophysiology
Dystrophin is one of the proteins involved in ensuring sarcolemmal stability. It encodes 427-kd muscle proteins (Dp427) and can be found on the short arm of the X chromosome. Despite the small proportion of Dp427 proteins in stripped muscle (0.002%), dystrophin plays a major role in promoting the proper functioning of muscle membranes. It achieves this by aggregating as a homotetramer ad also associating with actin and dystrophin-associated glycoproteins to form stable complexes which interact with extracellular components such as laminin ( Emery, 2001) . Therefore, absence or low production of dystrophin results in cellular instability characterized by leaking of intracellular substances, resulting in muscular degradation. This leakage accounts for the high amounts of creatine phosphokinase in blood samples.
6. Prevention
Muscular dystrophy is a genetic disorder. Therefore, prevention after birth is impossible. To prevent incidence of MD, genetic screening should be performed especially on individuals with a family history of dystrophies to help identify carriers of the trait. Additionally, counselling should be offered to give insight on the possibility of future occurrences. These preventive practices ensure timely diagnosis of MD, which proves essential in early disease management.
7. Treatment
Currently, there is no known cure for the various types of muscular dystrophy. Therefore, timely diagnosis is important as it ensures early disorder management. This might be in the form of corrective surgeries aimed at removal of contractures. Although corrective surgeries are not recommended at early ages, they have proven effective in prolonging ambulation, reducing muscular pain and discomfort as well as preserving function of vital organs such as the lungs and heart ( Bushby, 2010) .
Alternatively, various medications including glucocorticoids and corticosteroids may be used to reduce progression of MD. Physical therapy, exercises and provision of mechanical supports may also prove invaluable in boosting muscular strength and effectiveness. In addition to these, all associated infections should be treated promptly and obesity controlled to reduce straining of muscles due to excessive body weight.
8. Clinical Relevance
Although MD does not have a cure, medical management has proved vital in improving strength of muscles and reducing rate of deterioration. The discomfort associated with dystrophies has been reduced and ambulation prolonged. Additionally, effects of severe cases including heart and lung failure have become more manageable, improving patient lifespan.
References
Girolami, P. (2006). Muscular dystrophy . Bethesda, MD: National Institute of Neurological Disorders and Stroke, National Institutes of Health.
Bushby, K. M. (2010). Muscular dystrophy: methods and protocols . Totowa, N.J: Humana Press.
Emery, A. E. (2001). The muscular dystrophies . Oxford: Oxford University Press.
