Spinal Muscular Atrophy (SMA)

Overview

Spinal muscular atrophy (SMA) is a genetic disease that affects the motor neurons, the specialized nerve cells in the spinal cord and brainstem that carry signals telling muscles to move. When these nerve cells are lost, the muscles no longer receive proper signals and gradually become weak and waste away (atrophy). The muscles closest to the center of the body, such as those of the shoulders, hips, thighs, and upper back, are usually affected most.

SMA varies widely in severity and age of onset, ranging from a severe form that appears in infancy to milder forms that begin in childhood or adulthood. The most common forms are caused by a fault in a gene called SMN1. Importantly, the outlook for SMA has changed dramatically in recent years: new disease-modifying treatments, along with supportive care and, in some countries, newborn screening, have transformed what was once a uniformly grim prognosis into a condition where many people can do far better than in the past, especially when treatment starts early.

Symptoms

Symptoms depend on the type and age of onset, but all forms share progressive muscle weakness, usually most pronounced in the muscles near the center of the body.

• Muscle weakness and reduced muscle tone (floppiness), especially in infants
• Difficulty reaching motor milestones such as sitting, standing, or walking in children
• Trouble with movements such as climbing stairs, rising from the floor, or lifting the arms
• Muscle wasting and sometimes fine tremors of the hands
• In more severe forms, difficulty breathing, feeding, and swallowing
• A weak cry or weak cough in affected infants

Breathing and swallowing difficulties can be serious, particularly in infants. Signs of breathing trouble, choking, or a child struggling to breathe require urgent medical attention.

Causes

Most cases of SMA are caused by a change in the SMN1 gene, which provides instructions for a protein that motor neurons need to survive.

• SMN1 gene loss or change: Without enough working SMN protein, motor neurons gradually die, leading to muscle weakness.
• Role of the SMN2 gene: A related backup gene, SMN2, makes a small amount of the protein; having more copies of SMN2 generally leads to a milder form of the disease.
• Inheritance: The common forms are inherited in an autosomal recessive pattern, meaning a child must inherit a changed gene from both parents, who are usually unaffected carriers.

Rarer forms of SMA are caused by other genes. Genetic counseling helps families understand inheritance and carrier status.

Risk Factors

• Both parents being carriers of an SMN1 gene change
• A family history of spinal muscular atrophy
• Belonging to a family where carrier status is known

Because carriers usually have no symptoms, many families have no warning until a child is affected or until carrier or newborn screening identifies the risk.

Diagnosis

Diagnosis is based on the pattern of muscle weakness and confirmed with genetic testing.

• Genetic testing: A blood test that looks for changes in the SMN1 gene confirms the diagnosis and can count SMN2 copies, which helps predict severity.
• Newborn screening: In some regions, SMA is now included in newborn screening, allowing diagnosis and treatment before symptoms appear.
• Clinical evaluation: Assessment of muscle strength, tone, and reflexes.
• Other tests: Occasionally electromyography (a nerve and muscle test) or other studies are used when the diagnosis is unclear.

Treatment

Treatment has advanced greatly and now combines disease-modifying therapies with supportive care from a specialized team.

• Disease-modifying treatments: Newer therapies aim to increase the amount of SMN protein, which can slow or change the course of the disease, especially when started early. These include medicines given by injection into the spinal fluid, oral medication, and gene therapy for eligible patients.
• Respiratory support: Help with breathing and clearing secretions, ranging from techniques to assist coughing to breathing support when needed.
• Nutrition and feeding support: Assistance with feeding and swallowing to maintain growth and prevent choking.
• Physical and occupational therapy: To maintain strength, mobility, and function and to prevent joint contractures.
• Orthopedic care and equipment: Braces, wheelchairs, and management of spine curvature (scoliosis) to support posture and mobility.

Care is best delivered by a multidisciplinary team, and early treatment offers the best opportunity to preserve function.

Prevention

SMA cannot be prevented once inherited, but families have options to understand and plan for risk.

• Carrier screening for prospective parents to learn whether they carry an SMN1 gene change
• Genetic counseling to understand inheritance and reproductive options
• Newborn screening, where available, to allow early diagnosis and treatment before symptoms appear
• Early specialist referral if SMA is suspected, since prompt treatment improves outcomes

When to See a Doctor

See a doctor if an infant or child shows unusual floppiness, weak movements, delayed motor milestones, or difficulty feeding, or if an older child or adult develops progressive muscle weakness. Seek urgent or emergency care for:

• Difficulty breathing or signs of respiratory distress
• Choking or trouble swallowing
• A sudden worsening of weakness or breathing during an illness

Early diagnosis matters, because starting treatment promptly offers the best chance to preserve muscle function.

Related Topics

Frequently Asked Questions

References

1. National Institute of Neurological Disorders and Stroke (NINDS). Spinal Muscular Atrophy.
2. Muscular Dystrophy Association. Spinal Muscular Atrophy (SMA).
3. MedlinePlus, U.S. National Library of Medicine. Spinal muscular atrophy.
4. Cure SMA. About SMA.