Distal myopathy or distal muscular dystrophy (DD) is a general term for a group of rare progressive genetic disorders characterized by wasting (atrophy) and weakness of the voluntary distal muscles in the extremities, particularly in the hands, feet, lower legs, and lower arms. Although the age of onset can occur anytime from infancy to adulthood, most forms develop later in life between ages 40 and 60 and are slowly progressive. DD affects both men and women.
The causes of Distal muscular dystrophy are difficult to determine.This is because it is a genetic disorder that can be a result of a mutation in any one of eight different genes. The disease can be inherited either from a single parent or from both the parents.In this condition, it can be dominant or recessive. The latter means that the person will not exhibit any symptoms. It can be a result of a dominant or a recessive gene present in one or both the parents.
There are different types of distal muscular dystrophy which include:
- Distal myopathy with vocal cord and pharyngeal weakness – linked to chromosome 5 in the same region as the gene that’s defective in limb-girdle MD type 1A. Symptoms first appear between about 35 and 60 years of age and include weakness of the hands, legs or voice. Difficulty in swallowing may be a feature.
- Finnish (tibial) distal myopathy – Finnish muscular dystrophy (also called tibial MD) can be severe or benign, and typically affects only people of Finnish descent. Weakness starts after age 40 in the lower extremities and progressing slowly to the upper extremities and trunk muscles. Cardiac problems can be a feature. Those with only one defective gene experience mild weakness of the tibial leg muscles (front of the calf) sometime after age 40. Those with two defective genes have progressive weakness starting in childhood and may lose the ability to walk by age 30.
- Gowers-Laing distal myopathy – Onset is from childhood to 25 years of age. Weakness is first seen in the leg and neck muscles and progresses slowly to include upper leg muscles, hands, and more neck muscles. This disorder results from mutations in the MYH7 gene, which instructs for myosin heavy chain 7, a protein that participates in muscle contraction.
- Hereditary inclusion-body myositis type 1 – Symptoms appear between the ages of 25 and 40, first affecting the muscles that lift the front of the foot and the thigh muscles. Other muscles can be affected later. The cause is unknown.
- Miyoshi distal myopathy – Miyoshi distal myopathy causes weakness that begins in the calf muscles. It shows up between ages 15 and 30. Initially, affected individuals may be unable to stand on their toes. Eventually, muscle weakness spreads to affect the proximal muscles of the upper legs often causing difficulties climbing stairs, standing or walking. Muscles in the hands, forearms, and shoulder area may also become affected. As the disease progresses, affected individuals may eventually have significant difficulty walking and require a wheelchair. The genetic defects that cause Miyoshi myopathy are in the gene for the dysferlin protein.
- Nonaka distal myopathy – It is characterized by weakness of the anterior compartment of the lower limbs with onset in early adulthood between 20 and 40 years of age, and sparing of the quadriceps muscles. As the inclusion body myopathy, NM is histologically characterized by the presence of numerous rimmed vacuoles without inflammatory changes in muscle specimens. NM, also known as inclusion body myopathy is related to inclusion body myositis and gene-related myopathy and has symptoms including gait disturbance, distal muscle weakness, and elevated serum creatine phosphokinase. An important gene associated with Nonaka Myopathy is GNE (Glucosamine (UDP-N-Acetyl)-2-Epimerase/N-Acetylmannosamine Kinase).
- Welander’s distal myopathy – Occurs in individuals more than 40 years of age in most cases. Certain muscles of the hands and feet (intrinsic muscles and long extensors) and certain muscles of the fingers and toes (extensors) are predominantly affected. The progression of muscle weakness is slow. Muscles in the hands are affected first with the muscles in the legs becoming involved later on or not at all. This type has been identified with greater frequency in Sweden and Finland.
Since no distal myopathy has been linked to the X-chromosome, distal myopathies affect males and females in equal numbers. The exact incidence of the distal myopathies is unknown. Some forms have been identified with greater frequency in certain populations. Udd distal myopathy occurs with greater frequency in Finland where the prevalence is estimated to be 7 in 100,000 individuals. Welander distal myopathy occurs with greater frequency in Sweden where the prevalence is estimated to be 1 in 1,000 individuals. Approximately 220 cases of IBM2 have been identified in the medical literature. The muscular dystrophies as a whole are estimated to affect 250,000 individuals in the United States. The estimated prevalence of distal myopathy in the northern region of England is 1/300,000.
To diagnose distal muscular dystrophy, physicians carry out extensive tests. The first step is a thorough family history and physical examination. Blood serum analysis, CT scans, and MRIs may be advised. Genetic studies in the form of gene mapping may be ordered to understand the level of mutation better. Once the level of mutation and its severity has been identified, the patient can then be classified as one of the many types of Distal Muscular dystrophies. A serum protein assay could show elevated serum Creatine Kinase (often referred as CK). Sometimes a muscle biopsy is recommended.
There is no cure for DD. But supportive care can help keep the patient’s strength and flexibility. Physical therapy is important to keep the range of motion. Occupational therapy can help with ways to adapt to activities such as eating, walking, or computer use. Respiratory and cardiac care us recommended. Certain aids may improve the quality of life including:
- Lower-leg braces to treat foot drop
- Other lightweight leg braces for support
- Handwriting aids
- Mouse and keyboard alternatives
- Cuffs, braces, openers, and other adaptive devices
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