Spinal muscular atrophy
Werdnig-Hoffmann disease; Kugelberg-Welander diseaseSpinal 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.
Causes
SMA is a collection of different motor nerve (or neuron) diseases. The disease is caused by a lack of a protein (SMN) due to defective genes.
Most of the time, a person must get one copy of 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
Symptoms of SMA may vary depending on the SMA type.
- Infants with SMA type I are born with very little muscle tone, weak muscles, and feeding and breathing problems.
- In infants 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:
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Aldolase blood test
Aldolase blood test
Aldolase is a protein (called an enzyme) that helps break down certain sugars to produce energy. It is found in high amount in muscle and liver tiss...
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Erythrocyte sedimentation rate (ESR)
Erythrocyte sedimentation rate
ESR stands for erythrocyte sedimentation rate. It is commonly called a "sed rate. "It is a test that indirectly measures the level of certain protei...
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Creatine phosphate kinase blood test
Creatine phosphate kinase blood test
Creatine phosphokinase (CPK) is an enzyme in the body. It is found mainly in the heart, brain, and skeletal muscle. This article discusses the test...
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Electromyography
(EMG)
Electromyography
Electromyography (EMG) is a test that checks the health of the muscles and the nerves that control the muscles.
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MRI of the brain, spine, and spinal cord
MRI
A magnetic resonance imaging (MRI) scan is an imaging test that uses powerful magnets and radio waves to create pictures of the body. It does not us...
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Muscle biopsy
Muscle biopsy
A muscle biopsy is the removal of a small piece of muscle tissue for examination.
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Nerve conduction study
Nerve conduction study
Nerve conduction velocity (NCV) is a test to see how fast electrical signals move through a nerve. This test is done along with electromyography (EM...
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- Lung function tests
- Swallowing study
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Thyroid-stimulating hormone (TSH) blood test
Thyroid-stimulating hormone
A TSH test measures the amount of thyroid stimulating hormone (TSH) in your blood. TSH is produced by the pituitary gland. It prompts the thyroid g...
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Treatment
There is no treatment to cure SMA. However, there are now three drugs approved by the FDA that slow how fast the muscle weakness progresses:
- Onasemnogene abeparvovec-xioi (Zolgensma)
- Risdiplam (Evrysdi)
- Nusinersen (Spinraza)
These drugs work by increasing the amount of the SMN protein produced. Talk with your provider to see if either of these medicines is right for you or your child.
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.
Scoliosis
Scoliosis is an abnormal curving of the spine. Your spine is your backbone. It runs straight down your back. Everyone's spine naturally curves a b...
Read Article Now Book Mark ArticleOutlook (Prognosis)
Without treatment, 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. New therapies that increase the amount of SMN protein have resulted in infants and children living much longer.
Children with type III disease may survive into early adulthood. But, people with all forms of the disease have weakness and disability that gets worse over time. Adults who develop SMA often have a normal life expectancy.
Possible Complications
Complications that may result from SMA include:
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Aspiration
(food and fluids get into the lungs, causing pneumonia)
Aspiration
Aspiration means to draw in or out using a sucking motion. It has two meanings:Breathing in a foreign object (for example, sucking food into the air...
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- Heart failure
- Scoliosis
When to Contact a Medical Professional
Contact your provider if your child:
- Appears weak
- Develops any other symptoms of SMA
- Has difficulty feeding
Breathing difficulty can rapidly become an emergency condition.
Breathing difficulty
Breathing difficulty may involve:Difficult breathing Uncomfortable breathingFeeling like you are not getting enough air
Read Article Now Book Mark ArticlePrevention
Genetic counseling is recommended for people with a family history of SMA who want to have children.
Genetic counseling
Genetics is the study of heredity, the process of a parent passing certain genes on to their children. A person's appearance, such as height, hair co...
Read Article Now Book Mark ArticleReferences
Fearon C, Murray B, Mitsumoto H. Disorders of upper and lower motor neurons. In: Jankovic J, Mazziotta JC, Pomeroy SL, Newman NJ, eds. Bradley and Daroff's Neurology in Clinical Practice. 8th ed. Philadelphia, PA: Elsevier; 2022:chap 97.
Manzur AY. Evaluation and investigation of neuromuscular disorders. In: Kliegman RM, St. Geme JW, Blum NJ, Shah SS, Tasker RC, Wilson KM, eds. Nelson Textbook of Pediatrics. 22nd ed. Philadelphia, PA: Elsevier; 2025:chap 647.
NIH-National Institute of Neurological Disorders and Stroke website. Spinal muscular atrophy. www.ninds.nih.gov/health-information/disorders/spinal-muscular-atrophy. Updated November 28, 2023. Accessed March 14, 2024.
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Superficial anterior muscles - illustration
Superficial muscles are close to the surface of the skin. Muscles which lie closer to bone or internal organs are called deep muscles.
Superficial anterior muscles
illustration
-
Scoliosis - illustration
Abnormal curvature in the spine is known as scoliosis, and generally begins just at the onset of puberty and progresses during the period of rapid growth. Most junior high schools routinely screen for scoliosis because, if caught early, progressive spine curvature can be prevented. Scoliosis affects girls much more frequently than boys.
Scoliosis
illustration
-
Superficial anterior muscles - illustration
Superficial muscles are close to the surface of the skin. Muscles which lie closer to bone or internal organs are called deep muscles.
Superficial anterior muscles
illustration
-
Scoliosis - illustration
Abnormal curvature in the spine is known as scoliosis, and generally begins just at the onset of puberty and progresses during the period of rapid growth. Most junior high schools routinely screen for scoliosis because, if caught early, progressive spine curvature can be prevented. Scoliosis affects girls much more frequently than boys.
Scoliosis
illustration
Review Date: 12/31/2023
Reviewed By: Joseph V. Campellone, MD, Department of Neurology, Cooper Medical School at Rowan University, Camden, NJ. Review provided by VeriMed Healthcare Network. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.