Sections View Full Chapter Figures Tables Videos Annotate Full Chapter Figures Tables Videos Supplementary Content +++ Key Features ++ An autosomal recessive neuromuscular disorder Degeneration of anterior horn cells in the spinal cord, leading to progressive atrophy of skeletal muscle Incidence is approximately 1 in 12,000, with the majority of the cases presenting in infancy Carrier frequencies approach 1 in 40 in populations with European ancestry +++ Clinical Findings ++ Three clinical subtypes are recognized based on age of onset and rate of progression SMA I is the most devastating Mild weakness may be present at birth but is clearly evident by 3 months and is accompanied by loss of reflexes and fasciculations in affected muscles Progression of the disorder leads to eventual respiratory failure, usually by age 1 year Symptoms of SMA II begin later, with weakness and decreased reflexes generally apparent by age 2 years. Children affected with SMA III begin to become weak as they approach adolescence +++ Diagnosis ++ Prenatal diagnosis is available through genetic testing Careful molecular analysis of the proband and demonstration of carrier status in parents is advised Homozygous deletion of exon 7 of SMN1 is detectable in approximately 95–98% of cases of all types of SMA and confirms the diagnosis +++ Treatment ++ Supportive Your Access profile is currently affiliated with '[InstitutionA]' and is in the process of switching affiliations to '[InstitutionB]'. Please click ‘Continue’ to continue the affiliation switch, otherwise click ‘Cancel’ to cancel signing in. Get Free Access Through Your Institution Learn how to see if your library subscribes to McGraw Hill Medical products. Subscribe: Institutional or Individual Sign In Username Error: Please enter User Name Password Error: Please enter Password Forgot Password? Forgot Username? Sign in via OpenAthens Sign in via Shibboleth