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Muscle Weakness in Spinal Muscular Atrophy

Updated on May 25, 2021
Medically reviewed by
Evelyn O. Berman, M.D.
Article written by
Victoria Menard

Spinal muscular atrophy (SMA) is an inherited neuromuscular disease. SMA causes progressive muscle weakness (hypotonia) in both skeletal muscles and smooth muscles due to the loss of motor neurons in the spinal cord. The motor neurons in the spinal cord are responsible for controlling muscle movements throughout your body.

There are five main types of spinal muscular atrophy — SMA types 0, 1, 2, 3, and 4 — as well as several additional forms of SMA. In general, a higher number indicates better motor function. A person living with SMA type 4, for example, has much greater motor function than a person with SMA type 1 (also called Werdnig-Hoffmann disease).

What Causes Spinal Muscular Atrophy?

SMA types 0-4 are genetic conditions caused by a mutation of the survival motor neuron 1 or SMN1 gene. This genetic disorder inhibits the production of SMN proteins, which are essential to the proper functioning of motor neurons, the nerve cells that send signals from the brain and spinal cord to the muscles.

When motor neurons can’t send signals to the muscles, they shrink (or atrophy). This inhibits everyday physical activity and, in some cases, impacts life expectancy. People living with SMA may need help to bathe, dress, and eat.

Spinal muscular atrophy types 0-4 account for as many as 95 percent of all SMA cases.

Muscle Weakness in Spinal Muscular Atrophy

Muscle weakness and low muscle tone are the primary symptoms of all forms of SMA. The type of SMA is determined by the age of onset, the severity of muscle weakness, and the underlying genetic mutation.

In most types of SMA, muscle weakness affects the proximal muscles (those closer to the midline of the body), as well as the lower extremities (legs and feet). Muscle weakness is usually observed on both sides of the body and, in most cases, worsens over time.

SMA can impact motor function in a variety of ways. Infants and toddlers diagnosed with SMA may not develop the ability to sit independently, stand, or walk. SMA can also affect functions like breathing and swallowing. Respiratory problems can negatively impact life expectancy.

How Does Spinal Muscular Atrophy Impact Motor Function?

SMA affects motor function in different ways. The severity of a person’s SMA depends on the age of onset, the type of SMA they have, and which body parts and muscle groups are affected by the disease. People living with SMA may experience a combination of the following symptoms.

Difficulty Chewing or Swallowing

Problems with sucking and swallowing are caused by weak muscles in the mouth and throat. Difficulty chewing and swallowing can lead to inadequate nutrition and may require feeding support. This may also increase the risk for aspiration (choking) and developing pneumonia.

Difficulty Sitting up Without Back Support

Babies and toddlers with symptoms of SMA types 0 and 1 may never be able to sit up unsupported. Babies and toddlers with SMA type 2 usually develop the muscle strength necessary to sit up in early childhood. However, since SMA is degenerative, people with type 2 often lose the ability to sit up unsupported as they grow older.

Inability To Stand Without Support

Babies and toddlers with SMA type 2 are rarely able to stand on their own. Those with type 3 SMA (Kugelberg-Welander disease) will usually learn to stand. Sometimes, people with Kugelberg-Welander disease will have difficulty standing up from a seated position.

Inability To Walk Without Support

Babies and toddlers with SMA types 0-2 will never develop the ability to walk on their own. People with SMA type 3 often learn to walk unaided. A person with SMA type 4 might not experience symptoms or receive a diagnosis until after the age of 35. Since SMA is a degenerative disease, difficulty with stairs, foot deformities, and abnormal gaits can develop over time.

How Does Spinal Muscular Atrophy Affect Breathing?

Muscle weakness not only impacts the muscles necessary for motor function, but can also affect muscles that govern breathing. People living with SMA often experience breathing problems as a result. Weak respiratory muscles can lead to fatal complications and a reduced life expectancy for people living with SMA.

People living with SMA may experience the following respiratory problems:

Difficulty Breathing

The muscles most involved with breathing are the intercostal muscles, which cause the chest to expand and contract, as well as the diaphragm, which allows the lungs to expand vertically. SMA typically weakens the intercostal muscles, which forces the diaphragm to overcompensate. This may lead to abnormal breathing and underdeveloped lungs.

Ineffective Coughing

People cough to clear lung obstructions, such as mucus. SMA makes it difficult to cough with enough force to properly clear the lungs. This, in turn, can make breathing even more difficult. Ineffective coughing can lead to chest infections and, in severe cases, complete or partial lung collapse.

Therapy and Support for Spinal Muscular Atrophy

There are several types of therapies available to people with SMA to counteract muscle wasting, improve motor function, and provide support. Physical therapy, respiratory therapy, assistive devices, and feeding support are all options — depending on the severity of a person’s muscle weakness.

Respiratory Support for Spinal Muscular Atrophy

Respiratory distress and respiratory failure are the most common causes of illness and death for children with SMA types 1 and 2. Several types of respiratory therapy can help. As an example, chest physiotherapy (manual or mechanical percussion) involves clapping on or vibrating the chest wall to loosen mucus. Ventilation devices can also extend the life of people with SMA who experience breathing problems.

Feeding Support for Spinal Muscular Atrophy

Some people with SMA may have difficulty chewing and swallowing. When the muscles that govern these actions weaken, it becomes difficult to eat. Infants and children with SMA, for example, may need to use feeding tubes. People with feeding tubes typically consume liquid food replacements, but may also eat by mouth for pleasure, if the risk of choking is not high.

Physical Therapy for Spinal Muscular Atrophy

Physical therapy aims to help people preserve or improve movement ability, as well as prevent future disabilities. Physical therapy is often a key treatment for people with SMA or other neuromuscular disorders. Several types of physical therapy may be effective at treating people with SMA, depending on how their motor function is affected.

Aquatic Therapy

Aquatic therapy, or hydrotherapy, usually takes place in a pool. Water buoyancy supports mobility and makes it easier for people with SMA to perform specific movements that are difficult on land.

Occupational Therapy

Occupational therapy aims to help people with day-to-day activities, such as dressing or using a computer. Occupational therapy can help people with SMA learn new ways to accomplish certain skills as their muscle strength changes.

Gait Training

Gait training is designed to improve walking ability and can help people with SMA reduce walking risks (such as poor balance) while using up less energy. Walking irregularly for a long time can lead to balance issues, as well as knee, ankle, and foot problems.

Braces and Mobility Devices

Braces can support weak muscles while maintaining flexibility. Braces can be used to support standing and prevent contractures in people with SMA who can stand but not walk. Mobility devices are also an important part of many physical therapy regimens. A physical therapist may recommend a walker, wheelchair, or stander.

Have you explored therapies to assist with SMA muscle weakness? What have you found helpful? Comment below or start a conversation on mySMAteam.

References
  1. Symptoms, Diagnosis & Effects of 5q Spinal Muscular Atrophy — Spinal Muscular Atrophy UK
  2. Spinal Muscular Atrophy: Diagnosis and Management in a New Therapeutic Era — Muscle & Nerve (NCBI)
  3. Spinal Muscular Atrophy — Genetics Home Reference
  4. Spinal Muscular Atrophy Causes/Inheritance — Muscular Dystrophy Association
  5. SMA Added to National List of Disorders to Screen for at Birth — Muscular Dystrophy Association
  6. FDA Approves First Drug for Spinal Muscular Atrophy — U.S. Food and Drug Administration
  7. Proximal Spinal Muscular Atrophy: Current Orthopedic Perspective — The Application of Clinical Genetics
  8. Spinal Muscular Atrophy — Orphanet Journal of Rare Diseases
  9. Spinal Muscular Atrophy Medical Management — Muscular Dystrophy Association
  10. Spinal Muscular Atrophy — National Organization for Rare Disorders
  11. Kugelberg Welander Syndrome — National Organization for Rare Disorders
  12. Spinal Muscular Atrophy Signs and Symptoms — Muscular Dystrophy Association
  13. Breathing Basics: Respiratory Care for Children With Spinal Muscular Atrophy — Cure SMA
  14. Spinal Muscular Atrophy Fact Sheet — American Physical Therapy Association
  15. Qualification of Aquatic Interventions in Children With Disabilities: A Systematic Literature Review — International Journal of Aquatic Research and Education
  16. Therapy for Muscular Dystrophy — NYU Langone Health
  17. Mobility Aids & Orthotic Devices for Muscular Dystrophy — NYU Langone Health
  18. Physical/Occupational Therapy — Spinal Muscular Atrophy Clinical Research Center (Columbia University Department of Neurology)
All updates must be accompanied by text or a picture.
Evelyn O. Berman, M.D. is a neurology and pediatric specialist and treats disorders of the brain in children. Review provided by VeriMed Healthcare Network. Learn more about her here.
Victoria Menard is a writer at MyHealthTeam. Learn more about her here.

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