Spinal muscular atrophy

Spinal muscular atrophy
Name	Spinal muscular atrophy
Synonyms	SMA
Field	Neurology, Pediatrics
Symptoms	Progressive muscle weakness, atrophy
Complications	Scoliosis, Respiratory failure, Dysphagia
Onset	Infancy to adulthood
Duration	Lifelong
Types	Type 0, I, II, III, IV
Causes	Mutations in the SMN1 gene
Risks	Family history
Diagnosis	Genetic testing, Electromyography
Differential	Muscular dystrophy, Amyotrophic lateral sclerosis
Treatment	Nusinersen, Onasemnogene abeparvovec, Risdiplam, supportive care
Prognosis	Varies by type
Frequency	~1 in 10,000 live births

Spinal muscular atrophy is a genetic neuromuscular disorder characterized by the degeneration of motor neurons in the anterior horn of the spinal cord, leading to progressive muscle weakness and atrophy. It is caused by mutations in the SMN1 gene, which is critical for the survival of motor neurons. The disease spectrum is broad, ranging from severe infantile-onset forms to milder adult-onset presentations, with management focusing on supportive care and recently developed gene-targeting medications.

Signs and symptoms

The primary manifestation is symmetrical, progressive muscle weakness, most severely affecting the proximal muscles, particularly those of the shoulders, hips, and back. Infants with the most severe form, historically classified as Werdnig-Hoffmann disease, present with profound hypotonia, a weak cry, and an inability to sit independently. Weaker intercostal muscles lead to paradoxical breathing, where the abdomen protrudes during inspiration. Common complications include Scoliosis, joint contractures, and Respiratory failure due to involvement of the diaphragm and accessory muscles. Bulbar weakness can result in Dysphagia, poor weight gain, and Aspiration pneumonia. In milder forms, symptoms such as hand tremors and difficulty with tasks like climbing stairs may be the initial presentation.

Causes

The disorder is almost exclusively caused by homozygous deletions or mutations in the SMN1 gene located on chromosome 5q13. This gene encodes the survival motor neuron (SMN) protein, which is essential for the health and function of alpha motor neurons. Disease severity is primarily modulated by the copy number of a nearly identical backup gene, SMN2. The SMN2 gene produces mostly an unstable, truncated protein isoform due to alternative splicing; a higher number of SMN2 copies generally correlates with a milder phenotype. The condition is inherited in an autosomal recessive pattern, meaning an affected individual inherits a non-functional copy of SMN1 from each parent, who are typically asymptomatic carriers.

Diagnosis

Diagnosis is confirmed through molecular genetic testing that identifies the homozygous absence of exon 7 of the SMN1 gene. Electromyography may show evidence of denervation and reinnervation, while muscle biopsy, now less common, typically reveals grouped atrophy of type I fibers. Serum creatine kinase levels are usually normal or mildly elevated. Prenatal testing via chorionic villus sampling or amniocentesis is available for families with a known history. Differential diagnoses include other neuromuscular conditions such as Duchenne muscular dystrophy, congenital myopathies, and Amyotrophic lateral sclerosis.

Management

Management is multidisciplinary, involving neurologists, pulmonologists, orthopedic surgeons, and physical therapists. Disease-modifying therapies include Nusinersen, an antisense oligonucleotide administered via intrathecal injection; Onasemnogene abeparvovec, a one-time adeno-associated virus-mediated gene therapy; and Risdiplam, a daily oral small molecule that modifies SMN2 splicing. Supportive care is critical and may involve non-invasive ventilation for respiratory support, gastrostomy tube placement for nutritional support, and spinal fusion surgery for progressive Scoliosis. Regular monitoring by teams at specialized centers like Boston Children's Hospital is standard.

Prognosis

Prognosis is heavily dependent on the clinical type, which correlates with the age of onset and the number of SMN2 copies. Without treatment, Type I historically led to death from Respiratory failure often before age two. Type II individuals can often sit independently but typically do not walk and have a reduced lifespan. Types III (Kugelberg-Welander disease) and IV are associated with normal life expectancy, though individuals may eventually require mobility aids like wheelchairs. The advent of Nusinersen, Onasemnogene abeparvovec, and Risdiplam has significantly altered the natural history, with treated infants achieving motor milestones previously unattainable.

Epidemiology

It is one of the most common genetic causes of infant mortality, with an incidence of approximately 1 in 10,000 live births and a carrier frequency of about 1 in 50 in most populations. The disorder is pan-ethnic, though some studies suggest a slightly lower carrier frequency in the Ashkenazi Jewish population. Universal newborn screening programs, implemented in many U.S. states and countries like Germany and Taiwan, allow for presymptomatic diagnosis and early intervention, which is critical for optimal outcomes with new therapies.

Category:Neurological disorders Category:Rare diseases Category:Genetic disorders