Symptoms of Spinal Muscular Atrophy (SMA)

Spinal muscular atrophy (SMA) is a neurological condition characterized by muscle weakness and atrophy (shrinking of the muscles) that affects one in 8,000–10,000 people. You can also develop breathing problems or have a physical injury due to the muscle weakness of SMA. It's important to be aware of potential medical complications and to seek help immediately if you have a medical emergency.

There are several types of SMA, ranging from type 0 to type 4. The effects can begin as early as in utero (when a baby is still growing during the mom’s pregnancy), during childhood, or adulthood. In general, the later the symptoms begin, the milder the disease. Very early-onset SMA is associated with only a few months of survival. People who develop later-onset SMA usually need assistance with a wheelchair or walker but can survive normal life expectancy.

Ways to stay active with spinal muscular atrophy (SMA)
Verywell / Lara Antal

Frequent Symptoms

Impaired muscle strength is the primary feature of SMA. All types of SMA involve weakness and atrophy of the proximal skeletal muscles—the large muscles located close to the body, such as those of the upper arms, thighs, and torso—that worsen over time. These effects make it difficult to stand, support your body in a seated position, hold up your neck, and use your arms.

Less Muscle Control

In SMA, muscle strength and power are diminished. With milder forms of SMA, strength may remain normal or nearly normal in the hands and feet, while weakness in the shoulders and thighs makes it difficult (or impossible) to climb stairs or lift large objects.


Hypotonia, or diminished muscle tone, is common in SMA. It results in loose and limp arms and legs. The untoned muscles may develop a puffy, yet thin, appearance.

Over time, muscle atrophy can cause a change in posture or may result in the development of muscle contractures. Muscle contractures are tight, stiff muscles that cannot be easily relaxed and may remain in an uncomfortable position.


Difficulty breathing can occur due to weak respiratory muscles. When this begins at an early age, it can lead to a bell-shaped chest, a result of using the abdominal muscles to inhale and exhale.

Low energy and fatigue can develop due to a combination of muscle weakness and low oxygen (from struggling to breathe).

Rare Symptoms

Less common effects of SMA include:

  • Pain from muscle contractures or an uncomfortable body position
  • Dysphagia (trouble swallowing), rare in mild forms of SMA but can occur in severe forms of the disease
  • Tremors
  • Respiratory infections due to weak breathing
  • Distal muscle weakness of the hands, feet, fingers, or toes in severe cases
  • Bedsores due to decreased physical movement and prolonged skin pressure

Bowel and Bladder

Bowel and bladder control rely on voluntary skeletal muscles as well as involuntary smooth muscles of the internal organs. In mild forms of SMA, bowel and bladder control is not typically impaired. In the more advanced forms of the condition however, it may be challenging to maintain control of bowel and bladder.

Constipation is common, especially if you or your child has a type of SMA that impairs walking ability.


If you or your child is affected by SMA, periods of sadness or anxiety can occur. These effects are normal reactions to the impaired mobility and uncertainty of the condition, but the disease process does not cause them.

SMA is not associated with any cognitive (thinking) problems, personality issues, vision changes, hearing loss, or impairment of physical sensation.

Five Subgroups of SMA

Types that begin earlier in life are characterized by more extensive weakness, a more rapid progression, more complications, and a shorter life expectancy. The SMA types that begin later in life involve physical limitations but do not always affect self-care or cause health complications.

SMA types are described as type 0 through type 4. Several of the types also have other medical names, usually associated with researchers who described them. SMA types used to be named based on the age of onset, such as “infantile onset” and “late-onset,” but these terms are not used as commonly as they used to be.

SMA is a disease continuum, so while there are defined types, there is some overlap in the different categories of SMA. You or your child will likely be diagnosed with a specific type of SMA, but because the distinction is not always clear-cut, you may notice features of more than one SMA type.

Type 0

This is the most severe SMA type, and it can cause diminished movement of a baby while still in utero. Newborn babies with SMA have hypotonic muscles and do not move as expected for their age, leading to joint deformities. They're generally unable to eat and suck, which can cause severe malnutrition.

Babies with this condition often have severe respiratory weakness and may need mechanical support for breathing. Heart defects are associated with this SMA type.

Life expectancy for this type of SMA typically usually doesn't surpass infancy.

Type 1 (Werdnig-Hoffmann Disease)

This is the most common type of SMA. It begins within the first few months of life. Babies with type 1 SMA have diminished muscle tone and can't hold their head up without support. They can have respiratory weakness, which may cause the development of a bell-shaped chest and in severe cases, life-threatening effects.

Children with this type of SMA may have trouble eating. Without treatment, babies with type 1 SMA do not gain the ability to stand, walk, or support their bodies in a sitting position.

Often, new parents aren’t sure what to expect in terms of a baby’s movements and abilities. As a parent, it's important to trust your instincts and seek medical advice if you have any concerns. If you notice that your child isn't moving as expected, or if their motor control is declining, be sure to talk to your child’s pediatrician right away because it can be the sign of SMA or another neuromuscular condition.

Type 2 (Dubowitz Disease)

This type of SMA causes muscle weakness, usually beginning after four months of age and before the age of one. Babies who previously had been able to use and control their muscles lose some of their abilities and become unable to sit up without assistance, and cannot stand or walk. Tremors of the muscles, such as muscles in the fingers, can develop.

It can also cause respiratory muscle weakness, which interferes with breathing, can lead to severe shortness of breath and a tendency to develop respiratory infections.

Scoliosis, a curvature of the spine, is common in type 2 SMA. Scoliosis occurs because the back is usually slouched, which causes the spine to curve. Untreated scoliosis can affect the spinal nerves and/or spinal cord, exacerbating muscle weakness, and potentially causing sensory loss as well.

Type 3 (Kugelberg-Welander Disease)

This type of SMA develops during late childhood and is characterized by trouble walking, declining motor skills throughout childhood, and potential for wheelchair assistance in later years.

With support and treatment, children can survive and participate in many activities and are expected to have normal life expectancy but may have persistent proximal muscle weakness and atrophy.

Type 4

This is the least common type of SMA, with symptoms usually beginning during adulthood. If you have this type of SMA, you would have proximal muscle weakness and atrophy. The weakness can make it difficult to climb stairs, lift heavy objects, or participate in sports.

You may experience mild tremors when moving weak muscles. Breathing problems, which are usually mild, can develop as well. Life expectancy might not be affected by this type of SMA.

When to See a Healthcare Provider

If you or your child has SMA, it's essential that you're aware of the issues and emergencies that require medical attention.

Call your healthcare provider if you experience any of the following:

  • Fever: Respiratory infections or infected pressure sores may need treatment.
  • Non-healing wound: If you have a wound or a non-healing or painful sore or ulcer, it can worsen or become infected if left untreated.
  • Puffiness of the arm or leg: This can be a sign of a deep vein thrombosis (DVT) that could travel elsewhere in the body—like to the lungs—causing severe problems.
  • Trouble eating or swallowing: Difficulty keeping food down can cause weight loss and malnutrition. You may need an evaluation from a dietitian or a speech and swallow therapist.
  • Cramps or abdominal discomfort: This may arise due to constipation.
  • Recurrent coughing: A persistent cough could be a sign that you're at risk of aspiration, a potentially life-threatening situation in which food or saliva becomes lodged in the lungs.
  • Aspiration pneumonia: It can also cause aspiration pneumonia, a lung infection that requires medical treatment.
  • Weakening muscles: If you notice your muscle strength is weakening, you should talk to your healthcare provider.

Get urgent medical attention if you experience any of the following:

  • A high fever
  • Severe abdominal pain
  • Trouble breathing or shortness of breath
  • Severe or persistent coughing 
  • Falling, especially if you hit your head

Prevention is an important part of care, so close contact with your medical team is important. With treatment, many complications and their serious effects can be alleviated.

Frequently Asked Questions

  • What are the causes of SMA?

    One of the main causes of SMA is a mutation in the SMN1 gene. If you have a missing piece of the SMN1 gene, signals aren't transmitted between the brain and muscles. This leads the skeletal muscles to weaken and shrink. The number of copies you have of the related SMN2 gene (which can make up for the lost function of SMN1) determines how severe the condition is.

  • How many people have SMA?

    About 10,000 to 25,000 children and adults have SMA in the United States.

  • How is SMA diagnosed?

    To help diagnose SMA, your healthcare provider may order a blood test to look for mutations of the SMN1 gene. This test can identify 95% of SMA cases. Other tests may include nerve conduction velocity studies, which measure how well your nerves send electrical signals, and a muscle biopsy, which can help diagnose neuromuscular disorders.

9 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  4. McGraw S, Qian Y, Henne J, Jarecki J, Hobby K, Yeh W. A qualitative study of perceptions of meaningful change in spinal muscular atrophyBMC Neurol. 2017;17(1):68. doi:10.1186/s12883-017-0853-y

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  9. National Institute of Neurological Disorders and Stroke. Spinal muscular atrophy fact sheet.

By Heidi Moawad, MD
Heidi Moawad is a neurologist and expert in the field of brain health and neurological disorders. Dr. Moawad regularly writes and edits health and career content for medical books and publications.