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Spinal muscular atrophy (SMA)

Clinical features: Spinal muscular atrophy (SMA) is characterised by progressive muscle weakness due to degeneration and loss of the anterior horn cells (the lower motor neurons) in the spinal cord and the brain stem nuclei. The condition is extremely variable and the onset of weakness ranges from before birth to early adulthood. Prior to DNA testing SMA was classified into distinct subgroups based upon the age of onset of symptoms. Whilst it is now recognised that SMA associated with mutations of the SMN1 gene (see below) shows no clear delineation of subtypes, the clinical classification remains useful for both prognosis and management. The classification includes: SMA I (Werdnig- Hoffman disease) - onset before six months of age; SMA II - onset between six and 12 months; SMA III (Kugelberg-Welander syndrome) - onset in childhood after 12 months; and SMA IV - adult onset. Other forms of SMA which are not associated with mutations in SMN1 are also described.

Incidence: Approximately 1 in 25,000 in the United Kingdom, with a carrier frequency of 1 in 80.

Inheritance : Autosomal recessive, but 2% of affected individuals have one de novo mutation, meaning that only one parent is a carrier.

Gene: Survival of motor neuron 1 gene (SMN1) on chromosome 5. About 95% of individuals with SMA share a common mutation.

Prenatal diagnosis: Usually possible. If a family is considering prenatal diagnosis referral should be made to the 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: Possible if mutations identified in affected family members.

Patient support groups

Jennifer Trust for SMA
Elta House
Birmingham Road
Stratford-on-Avon
Warwickshire
CV37 0AQ
Tel (Helpline): 0800 975 3100
Tel: 0870 774 3651
Web: www.jtsma.org.uk

If you would like to access specialist information on spinal muscular atrophy please go to:-
Genetests - Spinal muscular atrophy