An accurate diagnosis is important to assist with appropriate medical monitoring and management of symptoms. In addition, genetic counseling should be made available to all people 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. Classification
There are two main types of myotonic dystrophy. 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. This disease is most often in the facial muscles, levator palpebrae superioris, 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 Pagets disease and frontotemporal dementia. Genetic testing
Genetic tests, including prenatal testing, are available for both confirmed forms. Molecular testing is considered the gold standard of diagnosis. Prenatal testing
Testing at pregnancy to determine whether an unborn child is affected is possible if genetic testing in a family has identified a DMPK mutation.
Anticipation tends to be less severe than in cases of maternal inheritance.The RNA from the expanded trinucleotide repeat region forms intranucleoplasmic hairpin loops due to the extensive hydrogen bonding between C-G base pairs, and it has been demonstrated that these sequester the splicing regulator MBNL1 to form distinctive foci.A severe form of DM1, congenital myotonic dystrophy, may appear in newborns of mothers who have DM. Congenital myotonic dystrophy can also be inherited via the paternal gene, although it is said to be relatively rare. Congenital means that the condition is present from birth. DM2
The microsatellite expansion responsible for DM2 is of cytosine-cytosine-thymine-guanine (CCTG) repeats, classifying it as a tetranucleotide repeat disorder. This expansion occurs in the first intron CNBP gene on chromosome 3.The repeat expansion for DM2 is much larger than for DM1, ranging from 75 to over 11,000 repeats. Like DM1, the size of the microsatellite repeat array lengthens from generation to generation. Unlike DM1, anticipation does not result, as the degree of repeat expansion beyond 75 repeats does not affect the age of onset or disease severity.The repeat expansion produces an RNA transcript that binds to RNA-binding proteins such as MBNL1, as in DM1. Also, repeat expansion likely reduces expression of CNBP, loss of which causes muscle toxicity. Pathophysiology
Molecular
Mutations of DM1 and DM2 cause production of RNA that sequesters RNA-binding proteins, causing dysregulated RNA splicing. This dysregulated RNA splicing is particularly toxic to skeletal, cardiac, and smooth muscle. One example in DM1 involves the chloride channel ClC-1.