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Spinal Muscular Atrophy
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Genetics Birth Defects

Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a group of genetic diseases that damages and kills motor neurons, leading to progressive muscle weakness and atrophy.

What is spinal muscular atrophy (SMA)?

Spinal muscular atrophy (SMA) is a group of genetic diseases that damages and kills motor neurons. Motor neurons are a type of nerve cell in the spinal cord and lower part of the brain that control movement in your arms, legs, face, chest, throat, and tongue.

As the motor neurons die off, your muscles start to weaken and atrophy (waste away). The muscle damage gets worse over time and can affect speaking, walking, swallowing, and breathing.

What are the types of spinal muscular atrophy (SMA) and what are their symptoms?

There are different types of SMA, categorized by the severity of the disease and the age of symptom onset:

  • Type l (Werdnig-Hoffman disease or infantile-onset SMA): The most severe and common type. Signs usually appear before 6 months of age (or even before/just after birth for Types 0 or 1A). Symptoms include trouble swallowing and breathing, limited movement, and chronic shortening of muscles or tendons (contractures). Affected infants usually cannot sit up without help and, without treatment, often die before 2 years of age.
  • Type ll: A moderate to severe type typically noticed between 6 and 18 months of age. Most children can sit without support but cannot stand or walk independently. They may experience breathing difficulties and can usually live into adolescence or young adulthood.
  • Type lll (Kugelberg-Welander disease): The mildest childhood form, with signs appearing after 18 months of age. Children can walk independently but may struggle with running, rising from a chair, or climbing stairs. They may also develop scoliosis, contractures, and respiratory infections. With treatment, most have a normal lifespan.
  • Type IV: A rare, often mild form with symptoms appearing after age 21. Symptoms include mild to moderate leg muscle weakness, tremors, and mild breathing problems that worsen slowly over time. Individuals with this type have a normal lifespan.

What causes spinal muscular atrophy (SMA)?

Most types of SMA are caused by a change in the SMN1 gene, which is responsible for producing a protein necessary for motor neuron health. When this gene is missing or abnormal, the resulting protein deficiency causes motor neurons to die.

SMA is typically inherited when both parents pass on a changed copy of the gene. If only one copy is inherited, there are usually no symptoms, though the gene can still be passed to children. Some less common types of SMA may be caused by changes in other genes.

How is spinal muscular atrophy (SMA) diagnosed?

Health care providers may use the following tools to diagnose SMA:

  • A physical exam
  • A medical history, including family history
  • Genetic testing to check for the gene changes that cause SMA
  • Electromyography, nerve conduction studies, and muscle biopsy (especially if no gene changes are found)

Parents with a family history of SMA may opt for prenatal testing via amniocentesis or chorionic villi sampling (CVS). In some states, genetic testing for SMA is included in newborn screening.

What are the treatments for spinal muscular atrophy (SMA)?

While there is no cure for SMA, treatments focus on managing symptoms and preventing complications:

  • Medicines to help the body produce more of the necessary proteins
  • Gene therapy for children under 2 years of age
  • Physical, occupational, and rehabilitation therapy to improve posture, joint mobility, blood flow, and muscle strength
  • Assistive devices such as braces, orthotics, speech synthesizers, and wheelchairs
  • Nutritional support, including a balanced diet or the use of a feeding tube
  • Breathing support, ranging from daytime devices to prevent sleep apnea to the use of a ventilator

Frequently Asked Questions

Yes, SMA is an autosomal recessive condition, meaning both parents must be carriers of one mutated copy of the SMN1 gene to have a child with SMA. Carriers are typically asymptomatic and healthy but have a 25% chance of passing the condition to each child. Genetic counseling and carrier screening are recommended for couples with a family history of SMA or those from populations with higher carrier frequencies to assess risks and discuss options such as preimplantation genetic testing or prenatal diagnosis.
Diagnosis is primarily confirmed through genetic testing that identifies mutations in the SMN1 gene, which is responsible for producing the survival motor neuron protein. Doctors typically perform a blood test to analyze DNA, often followed by a quantitative PCR test to determine the number of SMN2 gene copies, which helps predict disease severity. In some cases, a muscle biopsy or nerve conduction studies may be used to rule out other neuromuscular disorders, but genetic testing is the gold standard for a definitive diagnosis.
Recent advancements have introduced disease-modifying therapies such as nusinersen (Spinraza), onasemnogene abeparvovec (Zolgensma), and risdiplam (Evrysdi), which aim to restore the production of the survival motor neuron protein. These treatments can significantly improve motor function, survival rates, and quality of life, particularly when administered early in the disease course. Additionally, supportive care including respiratory support, physical therapy, and nutritional management remains essential to maintain overall health and prevent complications like contractures and respiratory failure.
Early signs often include hypotonia (low muscle tone), which may be described as the baby being "floppy," along with delayed achievement of motor milestones such as rolling over, sitting, or crawling. Infants may also exhibit difficulty feeding, weak cry, and limited movement of the limbs. If a parent notices that their baby is not meeting expected developmental milestones or appears unusually weak compared to peers, they should consult a pediatrician immediately for a neurological evaluation and potential genetic testing.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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