Myotonic dystrophy is an inherited disorder that affects muscle tone, and hair loss and can involve varying degrees of impaired cognitive abilities. It is inherited as a dominant disorder, which means that individuals that carry the defective gene have the disease. The amount of symptoms exhibited in persons with myotonic dystrophy varies.
Description
Physical limitations resulting from myotonic dystrophy can be significant, involving muscle weakness and difficulty lifting items and performing certain routine daily tasks. There are many cases in which affected persons experience mental delays, and this usually correlates with the extent of the genetic defect. Myotonic dystrophy is a progressive disorder in terms of muscle weakness and muscle wasting.
Demographics
Myotonic dystrophy is relatively rare, occurring approximately once in 8,000 people. There is also a more rare, severe congenital form that occurs with an incidence of about 1 in 100,000.
Causes and symptoms
Myotonic dystrophy involves many different tissues within the body, including the eye, the heart, the endocrine system, and the central nervous system. The clinical manifestations in myotonic dystrophy span from mild to severe, leading to three separate categories with somewhat overlapping characteristics: mild, classical, and congenital (in which the clinical manifestations are evident at birth).
Mild myotonic dystrophy
In the mild form, persons usually develop cataracts and experience mild muscle tone dysfunction (myotonia). They normally do not experience clinical manifestations until they reach 20 years of age. Some patients do not develop symptoms until 70 years of age.
Classical myotonic dystrophy
In the classical form, patients can have generalized weakness, myotonia that is more severe than the mild form, cataracts, balding, and heart rhythm disturbances. The age of onset can be from 10 years until they are 30 years old.
Congenital myotonic dystrophy
Symptoms in the congenital form of myotonic dystrophy are evident at birth. Affected infants show muscle weakness, respiratory defects, and eventually, mental retardation. There are cases that appear after birth but before 10 years of age, although the symptoms might be slight and remain unnoticed. Congenital myotonic dystrophy is almost always inherited from the mother; however, inheritance from the father has occurred. Mental retardation is thought to be associated with early respiratory failure and the effects of the mutated gene on the brain.
Causes of myotonic dystrophy
Myotonic dystrophy is caused by a DNA alteration the in the Myotonin-protein kinase (DMPK) gene. This gene has been found to localize to specialized structures of the heart and skeletal muscle. Normal function is important for intercellular conduction and impulse transmission. It interacts with a variety of proteins that are important in signaling neurological messages. The abnormal gene product leads to disease but the mechanism is complex and in some tissues, it is relatively unclear. The alteration in the DNA leads to abnormal RNA processing, an important step in the production of proteins. This abnormal processing is felt to result in functional alterations of this protein that can lead to disease.
Diagnosis
Myotonic dystrophy is diagnosed clinically in individuals that have a specific type of muscle weakness. This is confirmed with molecular genetics testing, where the DMPK is analyzed. This gene is located on chromosome 19q13.2-13.3. Within the gene, there is a DNA sequence that is a string of three letters in the DNA alphabet (GTC, which are abbreviations for the nucleotides guanine, thymine, and cytosine) that are normally repeated up to 37 times. CTG repeats repeated greater than 50 times alters the function of the protein and can lead to disease. Individuals that have repeats from 38–49 times are considered to have permutations and in this range they generally do not produce symptoms, but their children are at risk for having repeats that expand into the disease causing range. Patients have more symptoms when they have repeat sizes greater than 50. DNA testing is 100% sensitive (able to determine the defect) and widely available. Prenatal diagnosis to determine if a fetus is affected is also available.
Myotonic dystrophy is suspected by physicians if patients experience muscle weakness in the lower legs, hands, neck, and face. The will experience a characteristic sustained muscle contraction whereby they have difficulty in quickly releasing their hand grip during a handshake. They also develop cataracts. Newborns usually have generalized and facial muscle weakness, club foot, and respiratory difficulties. Their muscles usually appear hypotonic (floppy).
Treatment team
A general practitioner may not see very many cases of myotonic dystrophy during his career, but may be the first physician to observe a patient. Usually, a neurologist and a geneticist are consulted. Depending on the age of onset, the extent of professional help varies. When the age of onset is a birth or infancy, a cardiologist and a pulmonologist will be necessary to evaluate and heart or respiratory deficiencies, respectively. These individuals usually also require special education, depending on the extent of the cognitive deficits.
Treatment
There is no specific treatment that has been identified to help the muscle weakness or prevent muscle wasting in myotonic dystrophy. Ankle and/or leg braces can be used to help support the muscles as the disease worsens. Heart problems, cataracts, and other abnormalities can often be treated. There are also medications that can help relieve the myotonia.
Recovery and rehabilitation
Although patients with this disorder do not recover, occupational and physical therapy is felt to be of benefit in many cases to help maintain optimum function for as long as possible.
Clinical trials
As of May 2004, the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) and the National Institute of Neurological Disorders and Stroke (NINDS) were recruiting patients for a registry that will connect people with myotonic dystrophy with researchers studying these diseases. (Contact information: Colleen M. Donlin-Smith, MA, telephone: 585-275-6372, email: Colleen_DonlinSmith@urmc.rochester.edu; Eileen Eastwood, telephone 585-275-6372; email: Eileen_Eastwood@urmc.rochester.edu).
Prognosis
The prognosis for patients that are diagnosed with the mild form of the disease is quite good. They usually do not have mental retardation and can live a close to normal lifespan. Affected individuals that have the classic form have a more severe prognosis. They have more clinical manifestations and lifespan usually ranges 48–55 years. The congenital form is the most severe, although patients live, on average, until they are 45 years old. They have more severe mental retardation, respiratory deficits, and have clinical manifestations at birth.
Special concerns
As this disorder can be inherited, genetic counseling for at-risk families is recommended. Offspring of an affected individual, regardless of gender, have a 50% chance of inheriting the mutant gene. It is important to recognize that expanded repeats within the gene can expand even more in the gametes (sex cells—sperm or egg) from individuals with expansions, resulting in the transmission of even longer trinucleotide repeat genes. This expansion leading to longer repeats is associated with more severe disease that is observed in the parent. Therefore, affected individuals are more likely to have more offspring with a more serious form of the disorder. Premutation carriers, or individuals that have repeats that do not usually cause disease but are likely to expand in their offspring, should be identified (if possible) in cases where there is a family history of the disorder. These individuals are at risk for having affected offspring, although they may not themselves have the disorder.
BOOKS
Nussbaum, Robert L., Roderick R. McInnes, and Huntington F. Willard. Genetics in Medicine. Philadelphia: Saunders, 2001.
Rimoin, David L. Emery and Rimoin's Principles and Practice of Medical Genetics. London; New York: Churchill Livingstone, 2002.
PERIODICALS
Cobo, A. M., J. J. Poza, L. Martorell, A. Lopez de Munain, J. I. Emparanza, and M. Baiget. "Contribution of molecular analyses to the estimation of the risk of congenital myotonic dystrophy." J Med Genet 32 (1995): 105–108.
Redman, J. B., R. G. Fenwick Jr, Y. H. Fu, A. Pizzuti, C. T. Caskey. "Relationship between parental trinucleotide GCT repeat length and severity of myotonic dystrophy in offspring." JAMA 269 (1993): 1960–1965.
