Spinal muscular atrophy (SMA) is a group of disorders of the motor neurons (motor cells). These disorders are passed down through families (inherited) and can appear at any stage of life. The disorder leads to muscle weakness and atrophy.
SMA is a collection of different motor nerve diseases. Grouped together, it is the second leading cause of hereditary neuromuscular disease, after Duchenne muscular dystrophy.
Most of the time, a person must get the defective gene from both parents to be affected. The most severe form is SMA type I, also called Werdnig-Hoffman disease. Infants with SMA type II have less severe symptoms during early infancy, but they become weaker with time. SMA type III is a less severe form of the disease.
In rare cases, SMA begins in adulthood. This is the mildest form of the disease.
A family history of SMA in an immediate family member (such as brother or sister) is a risk factor for all types of the disorder.
Symptoms of SMA are:
- Infants with SMA type I are born with very little muscle tone, weak muscles, and feeding and breathing problems.
- With SMA type II, symptoms may not appear until age 6 months to 2 years.
- Type III SMA is a milder disease that starts in childhood or adolescence and slowly gets worse.
- Type IV is even milder, with weakness starting in adulthood.
Often, weakness is first felt in the shoulder and leg muscles. Weakness gets worse over time and eventually becomes severe.
Symptoms in an infant:
- Breathing difficulty with shortness of breath and labored breathing, leading to a lack of oxygen
- Feeding difficulty (food may go into the windpipe instead of the stomach)
- Floppy infant (poor muscle tone)
- Lack of head control
- Little movement
- Weakness that gets worse
Symptoms in a child:
- Frequent, increasingly severe respiratory infections
- Nasal speech
- Posture that gets worse
With SMA, the nerves that control feeling (sensory nerves) are not affected. So, a person with the disease can feel things normally.
Exams and Tests
The health care provider will take a careful history and perform a brain/nervous system (neurologic) examination to find out if there is:
- A family history of neuromuscular disease
- Floppy (flaccid) muscles
- No deep tendon reflexes
- Twitches of the tongue muscle
Tests that may be ordered include:
- Aldolaseblood test
- Erythrocyte sedimentation rate (ESR)
- Creatine phosphate kinase blood test
- DNA testing to confirm diagnosis
- Electromyography (EMG)
- MRI of the brain, spine, and spinal cord
- Muscle biopsy
- Nerve conduction study
- Amino acid blood tests
- Thyroid-stimulating hormone (TSH) blood test
There is no treatment to cure the weakness caused by the disease. Supportive care is important. Breathing complications are common in the more severe forms of SMA. To help with breathing, a device or machine called a ventilator may be needed.
People with SMA also need to be watched for choking. This is because the muscles that control swallowing are weak.
Physical therapy is important to prevent contractions of muscles and tendons and abnormal curvature of the spine (scoliosis). Bracing may be needed. Surgery may be needed to correct skeletal deformities, such as scoliosis.
Two recently approved treatments for SMA areonasemnogene abeparvovec-xioi (Zolgensma) and nusinersen (Spinraza).These medicines are used to treat certain forms of SMA. Talk with your health care provider to see if either of these medicines is right for you or your child.
Children with SMA type I rarely live longer than 2 to 3 years because of respiratory problems and infections. Survival time with type II is longer, but the disease kills most of those who are affected while they are still children.
Children with type III disease may survive into early adulthood. But, people with all forms of the disease have weakness and debility that gets worse over time. Adults who develop SMA often have a normal life expectancy.
Complications that may result from SMA include:
- Aspiration (food and fluids get into the lungs, causing pneumonia)
- Contractions of muscles and tendons
- Heart failure
When to Contact a Medical Professional
Call your provider if your child:
- Appears weak
- Develops any other symptoms of SMA
- Has difficulty feeding
Breathing difficulty can rapidly become an emergency condition.
Genetic counseling is recommended for people with a family history of SMA who want to have children.
Werdnig-Hoffmann disease; Kugelberg-Welander disease
Fearon C, Murray B, Mitsumoto H. Disorders of upper and lower motor neurons. In: Daroff RB, Jankovic J, Mazziotta JC, Pomeroy SL, eds. Bradley's Neurology in Clinical Practice. 7th ed. Philadelphia, PA: Elsevier; 2016:chap 98.
Haliloglu G. Spinal muscular atrophies. In: Kliegman RM, St. Geme JW, Blum NJ, Shah SS, Tasker RC, Wilson KM, eds. Nelson Textbook of Pediatrics. 21st ed. Philadelphia, PA: Elsevier; 2020:chap 630.2.
NIH Genetics Home Reference website. Spinal muscular atrophy. ghr.nlm.nih.gov/condition/spinal-muscular-atrophy. Updated October 15, 2019. Accessed November 5, 2019.
Review Date 10/2/2019
Updated by: Alireza Minagar, MD, MBA, Professor, Department of Neurology, LSU Health Sciences Center, Shreveport, LA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.