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|Classification and external resources|
|Classification and external resources|
Myotonic dystrophy type 1 (DM1), also called Steinert disease, has a severe congenital form and an adult-onset form. Myotonic dystrophy type 2 (DM2), also called proximal myotonic myopathy (PROMM) is rarer than DM1 and generally manifests with milder signs and symptoms. Myotonic dystrophy can occur in patients of any age. Both forms of the disease display an autosomal dominant pattern of inheritance. Both "DM1" and "DM2" have Adult-Onset forms.
Two types of myotonic dystrophy exist. Type 1 (DM1), also known as Steinert disease, has a severe congenital form and a milder childhood-onset form as well as an adult-onset form [VALID CITATION NEEDED - Only Congenital and Adult-Onset are verifiable] . This disease in more often in the facial muscles, levator palpebrae superficialis, temporalis, sternocleidomastoids, distal muscles of the forearm, hand intrinsic muscles, and ankle dorsiflexors. Type 2 (DM2), also known as proximal myotonic myopathy (PROMM), is rarer and generally manifests with milder signs and symptoms than DM1.
Other forms of myotonic dystrophy not associated with DM1 or DM2 genetic mutations have been described. One case which was proposed as a candidate for the "DM3" label, was later characterized as an unusual form of inclusion body myopathy associated with Paget's disease and frontotemporal dementia.
Myotonic dystrophy (DM) is an inherited disease, affecting males and females approximately equally. About 30,000 people in the United States are affected. Symptoms may appear at any time from infancy to adulthood. DM causes general weakness, usually beginning in the muscles of the hands, feet, neck, or face. It slowly progresses to involve other muscle groups, including the heart. DM affects a wide variety of other organ systems as well. A severe form of DM, congenital myotonic dystrophy, may appear in newborns of mothers who have DM. Congenital means that the condition is present from birth. The incidence of congenital myotonic dystrophy is thought to be about 1:20,000. DM occurs in about 1 per 7,000–8,000 people and has been described in people from all over the world.
Presentation of symptoms and signs varies considerably by form (DM1/DM2), severity and even unusual DM2 phenotypes. DM1 symptoms for DM2 include problems with executive function (e.g., organization, concentration, word-finding) and hypersomnia. Conduction abnormalities are more common in DM1 than DM2, but all patients are advised to have an annual ECG. Both types are also associated with insulin resistance. Myotonic dystrophy patients may have a cortical cataract with a blue dot appearance, or a posterior subcapsular cataract.
DM2 is generally milder than DM1, with generally fewer DM2 patients requiring assistive devices than DM1 patients. In addition, the severe congenital form that affects babies in DM1 has not been found in DM2 and the early onset of symptoms is rarely noted to appear in younger patients in the medical literature.
In DM1, the affected gene is called DMPK, which codes for myotonic dystrophy protein kinase, a protein expressed predominantly in skeletal muscle. The gene is located on the long arm of chromosome 19.
In DM1, there is an expansion of the cytosine-thymine-guanine (CTG) triplet repeat in the DMPK gene. Between 5 and 37 repeats is considered normal, while individuals with between 38 and 49 repeats are considered to have a pre-mutation and are at risk of having children with further expanded repeats and, therefore, symptomatic disease. Individuals with greater than 50 repeats are almost invariably symptomatic, with some noted exceptions.[ref] Longer repeats are usually associated with earlier onset and more severe disease.
DMPK alleles with greater than 37 repeats are unstable and additional trinucleotide repeats may be inserted during cell division in mitosis and meiosis. Consequently, the children of individuals with premutations or mutations inherit DMPK alleles which are longer than their parents and therefore are more likely to be affected or display an earlier onset and greater severity of the condition, a phenomenon known as anticipation. Interestingly, paternal transmission of the condition is very uncommon, possibly due to selection pressures against sperm with expanded repeats, but anticipation tends to be less severe than in cases of maternal inheritance.
DM2 is caused by a defect of the ZNF9 gene on chromosome 3. The specific defect is a repeat of the cytosine-cytosine-thymine-guanosine (CCTG) tetranucleotide in the ZNF9 gene. As it involves the repeat of four nucleotides, it is not a trinucleotide repeat disorder, but rather a tetranucleotide repeat disorder.
The repeat expansion for DM2 is much larger than for DM1, ranging from 75 to over 11,000 repeats. Unlike in DM1, the size of the repeated DNA expansion in DM2 does not appear to make a difference in the age of onset or disease severity. Anticipation appears to be less significant in DM2 and most current reviews only report mild anticipation as a feature of DM2.
The diagnosis of DM1 and DM2 can be difficult due to the large number of neuromuscular disorders, most of which are very rare. More than 40 neuromuscular disorders exist with close to 100 variants.
As a result, patients with multiple symptoms that may be explained by a complex disorder such as DM1 or DM2 will generally be referred by their primary care physician to a neurologist for diagnosis. Depending on the presentation of symptoms, patients may be referred to a number of medical specialists including cardiologists, ophthalmologists, endocrinologists, and rheumatologists. In addition, the clinical presentation is obscured by the degree of severity or the presence of unusual phenotypes.
It is common that the clinical presentation for both DM1 and DM2 patients does not conform to the perceptions of these diseases held by many neurologists. Clinicians who are less familiar with the myotonic dystrophies in their day to day practice may expect patients with both forms to present with the more severe classic symptoms of DM1. As a result, patients may remain undiagnosed or be misdiagnosed. A useful clinical clue for diagnosis is the failure of spontaneous letting go of the hands following strong handshakes due to myotonia (delayed relaxation of muscles after contraction) which accompanies muscle weakness.
Even though there is presently no cure for DM and management is currently symptom based, a precise diagnosis is still necessary because of multiple other problems that may develop over time (e.g. cataracts). An accurate diagnosis is important to assist with appropriate medical monitoring and medical management of symptoms. In addition, genetic counseling should be made available to all patients because of the high risk of transmission. Potentially serious anesthetic risks are important to note, so the presence of this disorder should be brought to the attention of all medical providers.
Testing at pregnancy to determine whether an unborn child is affected is possible if genetic testing in a family has identified a DMPK mutation. This can be done at 10–12 weeks gestation by a procedure called chorionic villus sampling (CVS) that involves removing a tiny piece of the placenta and analyzing DNA from its cells. It can also be done by amniocentesis after 14 weeks gestation by removing a small amount of the amniotic fluid surrounding the baby and analyzing the cells in the fluid. Each of these procedures has a small risk of miscarriage associated with it and those who are interested in learning more should check with their doctor or genetic counselor. There is also another procedure called preimplantation diagnosis that allows a couple to have a child that is unaffected with the genetic condition in their family. This procedure is experimental and not widely available. Those interested in learning more about this procedure should check with their doctor or genetic counselor.
It is possible to test someone who is at risk for developing DM1 before they are showing symptoms to see whether they inherited an expanded trinucleotide repeat. This is called predictive testing. Predictive testing cannot determine the age of onset that someone will begin to have symptoms, or the course of the disease.
There is currently no cure for or treatment specific to myotonic dystrophy. Therefore, the focus is on managing the complications of the disease, particularly those relating to the cardiopulmonary system as these account for 70% of deaths due to DM1. Pacemaker insertion may be required for individuals with cardiac conduction abnormalities. Central sleep apnoea or obstructive sleep apnoea may cause excessive daytime sleepiness, and these individuals should undergo a sleep study. Non-invasive ventilation may be offered if there is an abnormality. Otherwise, there is evidence for the use of modafinil as a central nervous system stimulant, although a Cochrane review has described the evidence thus far as inconclusive.
Some small studies have suggested that imipramine, clomipramine and taurine may be useful in the treatment of myotonia. However, due to the weak evidence and potential side effects such as cardiac arrhythmias, these treatments are rarely used.
Altered splicing of the muscle-specific chloride channel 1 (ClC-1) has been shown to cause the myotonic phenotype of DM1 and is reversible in mouse models using Morpholino antisense to modify splicing of ClC-1 mRNA.
Combined strengthening and aerobic training at moderate intensity was deemed safe for individuals with neuromuscular diseases and the combination was found to increase muscle strength. Specifically, aerobic exercise via stationary bicycle with an ergometer was found to be safe and effective in improving fitness in DM1 patients. The strength training or aerobic exercise may promote muscle and cardiorespiratory function, while preventing further disuse atrophy. Cardiovascular impairments and myotonic sensitivities to exercise and temperature necessitate close monitoring of patients and educating patients in self-monitoring during exercise via the Borg scale, heart rate monitors, and other physical exertion measurements.
Muscular weakness of dorsiflexors (dorsiflexion) hinders the ability to clear the floor during the swing phase of gait and patients may adopt a steppage gait pattern or ankle-foot-orthotics may be indicated. Factors such as hand function, skin integrity, and comfort must be assessed prior to prescription. Neck braces can also be prescribed for neck muscle weakness.
Upper and lower limb weakness, visual impairments and myotonia may lead to the need for mobility aids and functional adaptive equipment such as buttonhooks and handled sponges for optimal hand function. If assistive devices and home adaptations are needed, physical therapists may refer on to occupational therapist(s) for further assessment.
DM1 is the most common form of myotonic muscular dystrophy diagnosed in children, with a prevalence ranging from 1 per 100,000 in Japan to 3-15 per 100,000 in Europe. The prevalence may be as high as 1 in 500 in regions such as Quebec, possibly due to the founder effect. In most populations, DM1 appears to be more common than DM2. However, recent studies suggest that type 2 may be as common as type 1 among people in Germany and Finland.
The years since the discovery of the genetic cause of MMD in 1992 have been fruitful ones for MMD research. Scientists are gaining understanding of how the expanded DNA section on chromosome 19 causes so many physiologic changes. In the meantime, scientists are also working to test drug treatments that may help symptoms in MMD. Among these are a drug that can make muscles more sensitive to insulin, one that may help improve muscle function and one that may relieve myotonia. The ultimate ―cure for MMD would probably require finding a way to block the expanded area of DNA on chromosome 19 or chromosome 3 so that it would lose its toxic effect on cells. It is not far-fetched to imagine that, in the future, this expanded section of DNA could be blocked or ―silenced. Scientists around the world are studying the unusual biological mechanisms that underlie MMD and working on pathways to treatment. However, it needs to be understood that such a treatment will take many years to be sufficiently developed to be used on humans.