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Myotonic muscular dystrophy type 1 - Description
Other Names
Congenital myotonic muscular dystrophy, Steinert Disease, dystrophia myotonica, DM. In this module, we have used the abbreviation MMD for myotonic muscular dystrophy. Although this is not the most common abbreviation, it cannot be confused with diabetes mellitus.Description
There are two kinds of myotonic muscular dystrophy (MMD) caused by different gene mutations, named MMD1 and MMD2. MMD2 does not have a congenital form and is not discussed further here. MMD1 can be classified into three overlapping types, but it is really a continuum between mild and severe disease. It is a multisystem disorder that affects brain, skeletal and smooth muscle, eyes, the heart, and the endocrine system. Individuals with mild disease do not develop symptoms until adolescence or older, and may display mild muscle weakness, cataracts, and myotonia, but have normal life spans. Individuals with the more classic form of the disease likewise have onset of symptoms in the teenage years, but have progressive muscle weakness, cataracts, conduction abnormalities, and may have a shortened life span and require a wheelchair for mobility. The most severe form, the congenital form, may be present at birth or become obvious over the first few years of life.MMD is an autosomal dominant disease that shows "anticipation", meaning it has an earlier onset and more severe symptoms in successive generations, particularly if passed on through the mother (See the Genetics section below). Because of this, an infant being diagnosed with the congenital form may lead to the recognition of milder symptoms in parents and other relatives.
Children with congenital MMD may present in early infancy with hypotonia, breathing and feeding problems, and frequently have persistent drooling and/or swallowing problems. Global developmental delays are often observed in the first few years of life. Distal muscle atrophy and weakness, and the hallmark myotonia, delayed relaxation of muscles after contraction, may not be noted until school age or later. IQ may be borderline or low, learning disabilities may be present, and attention deficit disorder or autism spectrum disorder characteristics may also be present.
Individuals with MMD often have cardiac arrhythmias, cataracts, speech and swallowing problems, and early onset of Type II diabetes. Fatigue and sleep disorders, including central and obstructive sleep apnea, are very common in children and adults with MMD. Although children with congenital MMD can make a lot of developmental progress, they can't outgrow the disease.
Genetics
MMD1 is due to a trinucleotide repeat (CTG) in the DMPK gene region on chromosome 19. It is one of the many examples of neurologic diseases being caused by an increased number of repeats including Huntington disease and fragile X syndrome. In MMD1, a CTG repeat length of 35 or more is abnormal. The number of repeats can be in the thousands. Myotonic dystrophy 1 (GeneReviews) In the case of MMD1, these repeats are present in the RNA, but are not translated into the protein. The disease mechanism for these untranslated repeats is called the “toxic RNA hypothesis”. In this disease model, the excessive CTG repeats bind proteins important in splicing (editing) other RNAs besides DMPK. This leads to RNA splicing errors, which have been detected in many different proteins, in the brain (amyloid precursor protein), heart (troponin), muscle (chloride ion channel), and endocrine systems (insulin receptor). This could explain why MMD1 is a multisystemic disorder. [Wheeler: 2007]Molecular genetic testing is available for diagnosis and is relatively inexpensive, and often obviates biopsy and electromyography. Children with congenital MMD have a high number of repeats, often greater than 1000. Myotonic dystrophy 1 (GeneReviews)
MMD is inherited in an autosomal dominant fashion. The disease-causing allele found in a parent with MMD may lengthen during the process of gametogenesis and the resulting offspring may have more severe disease and earlier onset than seen in the parent. This phenomenon is called anticipation. Anticipation resulting in large expansions (to greater than 1,000 repeats) and hence congenital MMD, is much more common when the mother is the affected parent than the father; but, anticipation with smaller or moderate expansions can occur from either parent. [Zeesman: 2002]
Prognosis
Congenital MMD is much more severe than MMD that presents in adulthood, and life expectancy is reduced. Complications of breathing and swallowing problems in infancy can lead to death in some cases. In older children and adolescents with MMD, cardiac arrhythmias may be a cause of death. [Mathieu: 1999]Prevalence
MMD is the most common muscular dystrophy. [Norwood: 2009] MMD affects about 5 per 100,000 people worldwide, but differs in prevalence in different ethnic populations. Type 1 is the most common form of the condition, accounting for about 98 percent of all cases. The remaining 2 percent of cases consist of myotonic dystrophy, type 2. The prevalence of the two types of myotonic dystrophy varies among different ethnic populations.Helpful Articles
PubMed search for articles on Myotonic Dystrophy in children for the last 5 years.
Menezes J, Patel P, Dussault H, Joncas J, Leibold W.
Effect of interferon on lymphocyte transformation and nuclear antigen production by Epstein-Barr virus.
Nature.
1976;260(5550):430-2.
PubMed abstract
Hsu DT.
Cardiac manifestations of neuromuscular disorders in children.
Paediatr Respir Rev.
2010;11(1):35-8.
PubMed abstract
Angeard N, Gargiulo M, Jacquette A, Radvanyi H, Eymard B, Héron D.
Cognitive profile in childhood myotonic dystrophy type 1: is there a global impairment?.
Neuromuscul Disord.
2007;17(6):451-8.
PubMed abstract
Myotonic muscular dystrophy type 1 Module Authors
| Authors: | Jacinda B Sampson MD, PhD, 5/2010 Lynne M Kerr MD, PhD, 3/2010 |
| Content Last Updated: | 5/2010 |
The authors listed above are responsible for the overall Myotonic muscular dystrophy type 1 Module. Authors contributing to individual pages in the module are listed on those pages.
Page Bibliography
Mathieu J, Allard P, Potvin L, Prévost C, Bégin P.
A 10-year study of mortality in a cohort of patients with myotonic dystrophy.
Neurology.
1999;52(8):1658-62.
PubMed abstract
Norwood FL, Harling C, Chinnery PF, Eagle M, Bushby K, Straub V.
Prevalence of genetic muscle disease in Northern England: in-depth analysis of a muscle clinic population.
Brain.
2009;132(Pt 11):3175-86.
PubMed abstract
Wheeler TM, Thornton CA.
Myotonic dystrophy: RNA-mediated muscle disease.
Curr Opin Neurol.
2007;20(5):572-6.
PubMed abstract
Zeesman S, Carson N, Whelan DT.
Paternal transmission of the congenital form of myotonic dystrophy type 1: a new case and review of the literature.
Am J Med Genet.
2002;107(3):222-6.
PubMed abstract