Clinical features: Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) are two types of muscular dystrophy associated with the dystrophin gene on the X-chromosome. DMD is the most severe and usually presents in early childhood with delayed motor milestones (delay in sitting and standing independently, and walking late). DMD progresses rapidly and affected boys are often in a wheelchair by 12 years of age. A cardiomyopathy is seen in virtually all affected individuals over the age of 18 years. Survival beyond 30 years is unusual. BMD is of later onset and most affected individuals remain ambulatory into their 20�s. A dilated cardiomyopathy is common in BMD and is the most common cause of death. Mean age of death is in the mid-40�s
Incidence: The birth incidence of DMD in a study in northern England was about 1 in 5,600 males, BMD is less common with an incidence of 1 in 18,500 males.
Inheritance : X-linked recessive
Gene: Duchenne and Becker muscular dystrophy are caused by mutations in the dystrophin gene on the X-chromosome. Mutations that lead to complete absence of the dystrophin protein product tend to cause DMD, whereas those that lead to a change in quality or quantity lead to BMD. Virtually all males with DMD and at least 85% of males with BMD have identifiable mutations.
Prenatal diagnosis : Possible in the majority of families. Please refer to your local clinical genetics service prior to a pregnancy. This ensures family members are offered appropriate advice and investigations, to confirm whether or not prenatal diagnosis is possible.
Carrier testing: It may be possible to deduce the risk of a woman being a carrier depending on the family structure and her relationship to the boys who are affected. A mother of a boy with DMD and no previous family history of DMD has a 2/3 chance of being a carrier. Accurate carrier testing is available if the mutation has been identified in affected family members. Otherwise an assessment needs to be made from the family tree, the results of DNA linkage studies and the results of serum creatine kinase (CK) levels. Carrier testing is not wholly reliable using CK levels alone as the normal and carrier distributions overlap. CK levels are not reliable as a carrier test in pregnancy.
UK Support Group: Duchenne Family Support 37a Highbury New Park London N5 2EN Tel (Helpline): 0870 606 1604 Tel: (Office) 0870 241 185
Muscular Dystrophy Campaign 7-11 Prescott Place Clapham London SW4 6BS Tel: 020 7720 8055 Fax: 020 7498 0670 Web: www.muscular-dystrophy.org
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