The term myopathy refers to the muscle disease and miyoshi myopathy is the distal muscular dystrophy which refers to the muscle problems of the forearms, calves, hands or the feet. The initial symptoms occur around adulthood which can include muscle weakness and wasting (atrophy). This condition can deteriorate with time affecting the upper arm and the shoulder muscles. As the result, the affected individual can present difficulty with walking, climbing stairs or even standing for a long duration. Some of the people with miyoshi myopathy may require the use of wheelchair eventually.
Causes
Miyoshi myopathy can develop as the result of the mutation in the genes DYSF or ANO5 that produce the proteins present in the muscles used for movements. The protein produced by the DYSF is known as dysferlin which is believed to be involved in repairing the thin membrane that surrounds muscle fibers (sarcolemma) when it becomes damaged. The protein called anoctamin-5 is produced by the gene ANO5 which is present in the membrane of the cell’s endoplasmic reticulum that is involved in the protein production, processing and transport. It is also considered to act as a channel regulating the chloride flow within muscle cells that are essential for the controlling of muscle relaxation and tensing. When the corresponding protein is either eliminated or produced in decreased quantity, the damage occurs and leads to atrophy of the muscle fiber. This condition is inherited in an autosomal recessive pattern meaning both the copies of the gene in each cell from the parents are defective.
Symptoms
The young adults typically present the muscle weakness and atrophy in the distal parts of the legs particularly the calf and the muscles of Achilles tendon. Over a period of time, they can present the weakness that can spread to the thighs making it difficult to walk, climb stairs and stand. The forearms may also become mildly involved presenting a decrease in the gripping strength. Although the progression of the disease is slow, it can eventually include the shoulder girdle muscles. Rarely, they can also present abnormal heart rhythm (arrhythmias).
Diagnosis
The diagnosis is possible with the thorough clinical evaluation, detailed history of the affected individual, identification of features characteristic of this condition and with various tests. Electromyography is a test that assesses the health of the muscles by inserting a needle electrode through the affected muscle that helps determine if the muscle weakness is caused by the muscle or by the nerves controlling the muscles. Additional tests can include blood tests that present very high levels of creatine kinase, MRI of muscle tissue and the biopsy of the affected muscle tissue which will detect the characteristic changes associated with this condition.
Treatment
There is no cure for distal myopathies but the treatment is focused on the specific symptoms evident in each case. Treatment options can include physical or occupational therapy and to improve the muscle strength. In some cases, the use of wheelchairs or braces may be necessary to assist with walking. Genetic counseling may also be beneficial for some of the affected individuals and their family.
References
https://ghr.nlm.nih.gov/condition/miyoshi-myopathy#inheritance
https://rarediseases.info.nih.gov/diseases/9676/miyoshi-myopathy#ref_3235
https://rarediseases.org/rare-diseases/distal-myopathy/