Last updated Jan. 26, 2022, by Marisa Wexler, MS
Fact-checked by José Lopes, PhD
Myotonic dystrophy is the most common form of muscular dystrophy in which the symptoms begin during adulthood. It affects about 10 out of every 100,000 adults of European ancestry, though it is rarer in nonwhite populations.
The disease gets its name from the characteristic symptom of myotonia, where muscles are unable to relax after use. Myotonic dystrophy is often abbreviated “DM,” which refers to the Greek name for the condition, dystrophia myotonica.
Types of DM
There are two types of myotonic dystrophy, referred to as type 1 DM (DM1) and type 2 DM (DM2). DM1 is caused by mutations in the DMPK gene, and DM2 is caused by mutations in a gene called CNBP — also sometimes called ZNF9. The symptoms of the two types overlap, though DM2 is generally milder.
DM1 is also called Steinert disease, after the German doctor who published the first formal descriptions of the condition in 1909. DM1 can be further divided into two forms, the mild type and the congenital type. In the mild type, symptoms usually don’t develop until adulthood, whereas in congenital DM1, the symptoms may be apparent from birth.
Causes of DM
Both types of DM are caused by mutations in which a segment of DNA in the particular gene — DMPK or CNBP — is abnormally repeated many times.
When genes are “read” to produce a protein, the cell makes an intermediary molecule called messenger RNA (mRNA) to carry the genetic code from the nucleus, where DNA is stored, out to the cell’s protein-making machinery. DM-causing mutations result in the production of an abnormally long mRNA that forms clumps in cells, interfering with normal cellular processes and ultimately causing the disease’s symptoms.
Inheritance of DM
For both the DMPK and CNBP genes, everyone inherits two copies, one from each biological parent. Both DM1 and DM2 are inherited in an autosomal dominant fashion, which means that just one mutated copy of the gene is enough for the disease to develop. A person with DM who has a biological child has a 50% chance of passing along the disease-causing mutation.
Symptoms of DM
Symptoms of DM usually first appear when a person is in their 20s or 30s, though there is substantial variation person-to-person and among DM types; it is possible for symptoms to appear at any point in life, from childhood to late adulthood. In general, an earlier age at symptom onset is associated with more severe symptoms.
The namesake symptom of DM is myotonia — when muscles are unable to relax after a contraction. For example, patients may have difficulty letting go of someone’s hand after a handshake, or releasing their grip on a doorknob or handle. Myotonia also can contribute to slurred speech or locking of the jaw.
Like other forms of muscular dystrophy, myotonic dystrophy is characterized by muscle weakness and wasting that gets progressively worse as time goes on. In people with DM1, it’s typical for muscle weakness to mainly affect the distal muscles, or the muscles further from the trunk, like those in the lower legs, hands, neck, and face. By contrast, in DM2, it’s more common for proximal muscles — those close to the trunk, such as ones in the shoulders, elbows, and hips — to be affected.
In addition to affecting muscles needed for movement, DM also can cause problems with muscles that are involved in involuntary bodily processes — including the muscles of the heart, those needed for breathing, and muscles that move food through the digestive tract. As a result, heart rhythm issues, as well as breathing problems during sleep, can develop as the disease progresses, and patients often have digestive symptoms such as trouble swallowing, constipation, and gallstones. Weakness in muscles around the uterus can cause complications in pregnancy and labor.
Beyond muscles, DM also can affect other parts of the body. It is common for patients to develop cataracts (opaque or cloudy spots in the lenses of the eyes) as well as diabetes. While many people with DM experience normal neurological development, the disease can cause cognitive disabilities and developmental delays in some patients, particularly those who start to experience symptoms during childhood.
Management and prognosis of DM
There currently is no cure for DM, and medical management mainly involves working to ease or control symptoms in order to maximize patients’ quality of life. For example, many patients may benefit from support like physical or occupational therapy to help preserve muscle function and find strategies for navigating life while living with DM.
Long-term prognosis and life expectancy for DM depend on the severity of the disease and the age at onset. Available data suggest that people with DM1 tend to live for a shorter length of time than the general population; heart and breathing problems are a leading contributor to death in this disease type. Life expectancy is not substantially affected in most people with DM2, available evidence indicates.
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