Myotonic dystrophy (DM) is a muscle disease presenting skeletal muscle problem, abnormalities of the heart function, cataracts, speech difficulties, cognitive impairment and breathing difficulties. This disorder is considered to be one of the complicated disorders with the severity of symptoms and the age of onset varying greatly. There are two major forms of this disorder; type 1 (DM1) and type 2 (DM2) which are caused by different genetic mutations. The type 1 is further classified as mild DM1, classic DM1 and congenital DM1. DM type 2 is a less severe form of the disorder that presents symptoms similar to DM1.
Causes
Myotonic dystrophy type 1 develops because of mutation in the gene DMPK and type 2 develops from the mutations in the CNBP gene. The protein produced by the DMPK gene is essential for the normal function of the cells in the brain, heart and the skeletal muscles. The genetic mutation results in the segment of DNA that is abnormally repeated several times. The defective gene produces an abnormally long messenger RNA that can affect the production of various other proteins which prevents the normal functioning of the muscle cells. Both the types of myotonic dystrophy are inherited in an autosomal dominant pattern meaning a single copy of the defective gene in each cell can cause the disorder.
Symptoms
The symptoms of this condition can vary greatly among the affected cases. The common abnormalities are the muscle weakness, trouble relaxing the muscle and muscle wasting that can become worse over time. This disease can also affect other body functions such as the heart, gastrointestinal system and the lungs. The weakness of the muscles necessary for breathing can cause hypoventilation and increases the risk of respiratory failure later in life. They may also experience difficulty swallowing, infertility among the males and problems with sleep. The adult onset of DM1 is the most common mild form which is associated with cataract, mild myotonia and diabetes. The juvenile form of DM1 can develop before the age of 12 and include intellectual and behavioral issues, difficulty with speaking, hearing problems and heart issues. The congenital DM1 is the severe form with the onset of the symptoms evident at the birth of the infant. DM2 can develop typically in the third decade of the individuals’ life with the symptoms appearing to be highly variable and is characterized by the muscle weakness, pain, myotonia and cataracts.
Diagnosis
The diagnosis of myotonic dystrophy is based on the symptoms evident, the family history of the individual and with various specialized tests. An electromyography test records the electrical activity of skeletal muscles which can indicate the condition. The genetic testing can also help confirm the disease by detecting the mutation that causes the disease.
Treatment
There is no cure for the disorder and the treatment is focused on the symptoms evident in each case. Physical therapy can help the muscle weakness and the severe cases may require the use of a wheelchair, braces and walkers. Genetic counseling may be beneficial for some of the affected cases and their family members.
References
https://rarediseases.org/rare-diseases/dystrophy-myotonic/
https://ghr.nlm.nih.gov/condition/myotonic-dystrophy#inheritance
