LabPLUS is part of Auckland District Health Board
and provides medical laboratory testing
| Services |
MOLECULAR GENETICS TESTS - MYOTONIC DYSTROPHY |
| Background: Myotonic dystrophy is the most common form of adult onset muscular dystrophy. It is an autosomal dominant condition with a prevalence of about 1 in 8000, however, the actual incidence may be higher due to undiagnosed mild cases. Clinical expression is highly variable and is related to age of onset. Features of myotonic dystrophy may include muscle weakness and atrophy, clinical and electromyographic evidence of myotonia, cataracts, intellectual impairment, cardiac conduction defects, testicular atrophy in males and premature balding. In families affected by myotonic dystrophy, "anticipation" is commonly observed. That is, the disease demonstrates earlier onset and greater severity in each successive generation. The molecular basis for anticipation is related to the nature of the mutation for this condition. Children born to women with myotonic dystrophy can have a severe form of the condition called congenital myotonic dystrophy. The overall risk of having a congenitally affected child for any carrier woman is about 10%. If the woman has clinical signs of the condition, the risk of congenital myotonic dystrophy in offspring is 40% and this rises to 50% in subsequent pregnancies if an affected child has previously has been born. The molecular basis of myotonic dystrophy is an expansion of a trinucleotide repeat (CTG) in the 3-prime untranslated region of a protein kinase gene on the long arm of chromosome 19. Normal alleles usually have between 5 and 30 CTG repeats, although repeat counts as high as 37 have been found in unaffected individuals. Affected individuals typically have 50 to several thousand repeats. The increase in clinical severity in successive generations is generally paralleled by an increase in the length of the CTG repeat sequence. Additionally, the risk for congenital myotonic dystrophy increases with increasing CTG repeat size in the mother. Current evidence suggests that the majority of individuals with myotonic dystrophy will have an expansion within the CTG repeat. However, there have been a few patients diagnosed with this disorder who have not demonstrated the presence of a CTG-repeat amplification. Although currently undetermined, the chance of having another type of mutation is not likely to be greater than 1%. However, because of this risk, documentation of a CTG amplification in an affected family member is recommended prior to the testing of at risk individuals. Analysis:
Blood: 3 x 4.0mL whole blood into EDTA tubes . Invert several times to mix. Forward within 24-48 hours at ambient temperature. Paediatric Samples: Minimum of 3mL whole blood in EDTA tubes. Prenatal Samples:
6 weeks once sample is received. Cost of test: MD - $676.11 Note: Prices excludes GST (12.5%) and may change without notice. Please click on a link below to view information about another specific test.
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