Spinal muscular atrophy | |
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Other names | Autosomal recessive proximal spinal muscular atrophy, 5q spinal muscular atrophy |
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Location of neurons affected by spinal muscular atrophy in the spinal cord | |
Specialty | Neurology |
Symptoms | Progressive muscle weakness[1] |
Complications | Scoliosis, joint contractures, pneumonia[2] |
Usual onset | Mutation is congenital, symptoms start varies by type |
Duration | Lifelong |
Types | Type 0 to type 4[2] |
Causes | Mutation in SMN1[2] |
Diagnostic method | Genetic testing[1] |
Differential diagnosis | Congenital muscular dystrophy, Duchenne muscular dystrophy, Prader-Willi syndrome[2] |
Treatment | Supportive care, medications[1] |
Medication | Nusinersen, onasemnogene abeparvovec, Risdiplam |
Prognosis | Varies by type[2] |
Frequency | 1 in 10,000 people[2] |
Spinal muscular atrophy (SMA) is a rare neuromuscular disorder that results in the loss of motor neurons and progressive muscle wasting.[3][4][5] It is usually diagnosed in infancy or early childhood and if left untreated it is the most common genetic cause of infant death.[6] It may also appear later in life and then have a milder course of the disease. The common feature is progressive weakness of voluntary muscles, with arm, leg and respiratory muscles being affected first.[7][8] Associated problems may include poor head control, difficulties swallowing, scoliosis, and joint contractures.[2][8]
The age of onset and the severity of symptoms form the basis of the traditional classification of spinal muscular atrophy into a number of types.[4]
Spinal muscular atrophy is due to an abnormality (mutation) in the SMN1 gene[1][2] which encodes SMN, a protein necessary for survival of motor neurons.[8] Loss of these neurons in the spinal cord prevents signalling between the brain and skeletal muscles.[8] Another gene, SMN2, is considered a disease modifying gene, since usually the more the SMN2 copies, the milder is the disease course. The diagnosis of SMA is based on symptoms and confirmed by genetic testing.[9][1]
Usually, the mutation in the SMN1 gene is inherited from both parents in an autosomal recessive manner, although in around 2% of cases it occurs during early development (de novo).[1][10] The incidence of spinal muscular atrophy worldwide varies from about 1 in 4,000 births to around 1 in 16,000 births,[11] with 1 in 7,000 and 1 in 10,000 commonly quoted for Europe and the US respectively.[2]
Outcomes in the natural course of the disease vary from death within a few weeks after birth in the most acute cases to normal life expectancy in the protracted SMA forms.[8] The introduction of causative treatments in 2016 has significantly improved the outcomes. Medications that target the genetic cause of the disease include nusinersen, risdiplam,[12] and the gene therapy medication onasemnogene abeparvovec. Supportive care includes physical therapy, occupational therapy, respiratory support, nutritional support, orthopaedic interventions, and mobility support.[1]
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