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The Muscular Dystrophy Association lists more than forty diseases under the umbrella term of Muscular Dystrophy (http://www.mdausa.org/disease/). Notably for this series, they include Myasthenia Gravis - MG, as discussed in the previous blog, and Amyotrphic Lateral Sclerosis - ALS, which we will cover in the next installment of The Lab Rats' Guide to the Brain. Notably absent, despite the fact that it is a neuromuscular disease, is Multiple Sclerosis - MS, perhaps because it has its own foundation, fundraising and research network.
The catch-all term Muscular Dystrophy, or MD, refers primarily to diseases of the muscles, typically not exclusively autoimmune diseases like MS and MG, but frequently genetic diseases that arise from faulty or missing genes. MD specifically refers to muscle weakness, and can significantly include wasting or atrophy of muscles. Also, unlike MS and MG, which are generalized diseases that affect all muscle and neuron sites in the body, the various subtypes of MD can include only specific muscle groups, with the consequence that weakened muscles on one side of a bone or joint cannot counter the contraction of normal muscles on the other side, resulting in deformities around the spine and joints.
The most notable of the dystrophies "Duchenne's Muscular Dystrophy" and the less severe variant "Becker's Muscular Dystrophy" result from the genetic lack of "dystrophin," a structural protein in muscle cells. With low levels of protein, or abnormal protein, the muscles cannot grow normally, accompanied by weakness of the muscles of voluntary movement, such as walking, particularly since the muscles of the legs and hips often show the first signs of the disease. Duchenne's appears in early childhood, and eventually affects heart and breathing muscles, such that survival past 30 years of age is rare. Becker's typically appears in late adolescence and early adulthood, and a near-normal lifespan is possible. Given that both diseases affect the same protein, it is not surprising that they have a common cause - a faulty gene on the X chromosome. The disease is "sex-linked recessive" most common in males, who receive their faulty X chromosome from a typically non-diseased mother. The disease is rare in females, and would likely co-occur with other genetic disorders, since the likelihood of a female with two faulty X chromosomes being able to have children is highly unlikely.
Other dystrophies such as Facioscapulohumeral Muscular Dystrophy (FMD) affect men and women alike. FMD is caused by a missing gene on the fourth chromosome, and affects mostly upper body muscles, rather than lower body muscles such as Duchenne's and Becker's dystrophies. FMD is quite often accompanied by loss of control of facial and neck muscles, preventing facial expression and causing sloped shoulders, drooping eyelids, impaired speech, and inability to whistle or "purse" the lips.
There are also metabolic causes of of MD. Myotonia Congenita is a another genetic disease (present from birth, hence the name from the language root "congenital") of the ion channels in muscle. Abnormal chloride channels in muscle prevent chloride from quickly entering muscle cells after contraction. During the process of contraction, positively charged sodium and calcium ions enter the muscle cell, much the same as during an action potential in a neuron. Potassium typically flows out and chloride into the cells to "repolarize" or returning to the normal negative resting voltage. With defective chloride channels, the muscles can relax only very slowly, resulting in rigidity, slow movement, difficulty in swallowing. Fortunately, continued movement relaxes muscles, and movement becomes easier. The disease is well treated with anesthetic-like muscle relaxants, and symptoms can improve as the patient ages.
Again, with over forty diseases and disorders in this category, it is difficult to do more than sample the types of muscular dystrophies. For more information, I highly suggest the MD page at PubMed Health (http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002172/) as a starting point, and the excellent listing of diseases at the MDA site (http://www.mdausa.org/disease/) for a clickable list of diseases, symptoms, causes and treatments. For writers, the illustrations and personal stories can assist with character development if you choose to use one of these diseases in your writing. For patients and families, there are useful links to medical information, support groups and physician referrals.
After a short break for the Stellarcon science fiction convention, we will conclude the series on neuromuscular diseases with ALS - the disease that cut down "The Pride of the Yankees", baseball player Lou Gehrig, in his prime - but has failed to stop the intellect of physicist Stephen Hawking after nearly 50 years.