Spinal muscular atrophy (SMA) most commonly affects infants and children, making it difficult for them to use their muscles. When your child has SMA, there is a breakdown in nerve cells in the brain and spinal cord. The brain stops sending signals to the body for muscle action. You can find more information below.

What is SMA disease?

SMA disease, also known as spinal muscular atrophy , is a neuromuscular disease that progressively destroys motor neurons (nerve cells in the brain stem and spinal cord), leading to muscle weakness and atrophy by controlling basic skeletal muscle activity such as speaking, walking, breathing, and swallowing .

Motor neurons control movement in the arms, legs, chest, face, throat, and tongue. When the signals between motor neurons and muscles are interrupted, the muscles gradually weaken, begin to weaken, and twitching (fasciculation) develops.

What causes SMA disease?

The most common form of SMA disease is caused by defects in both copies of the SMN1 gene on chromosome 5q. This gene produces the SMN protein, which maintains the health and normal function of motor neurons.

Individuals with SMA disease have insufficient levels of SMN protein. This causes motor neuron loss in the spinal cord, weakening and collapse of skeletal muscles. This weakness is usually more severe in the trunk, upper leg and arm muscles than in the hand and foot muscles.

There are many types of spinal muscular atrophy caused by changes in the same genes. Less common forms of SMA disease are caused by mutations in other genes, such as the VAPB gene on chromosome 20, the DYNC1H1 gene on chromosome 14, the BICD2 gene on chromosome 9, and the UBA1 gene on the X chromosome.

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Types differ in age of onset and severity of muscle weakness; however, there is overlap between species.

How is it inherited?

Except in rare cases caused by mutations in the UBA1 gene, SMA is inherited in an autosomal recessive fashion. This means that the affected person usually has two mutated genes, one inherited from each parent. People who carry only one mutated gene are carriers of the disease without showing any symptoms. Autosomal recessive diseases can affect more than one person (siblings or cousins) in the same generation.

What are the types and symptoms of SMA disease?

A wide variety of disorders, from severe to mild weakness, are seen in SMA, which is caused by defects in the SMN1 gene together with respiratory distress before birth. Accordingly, the most common forms of SMA disease can be divided into five types.

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SMA type 0 is the most severe form of the disease and is characterized by decreased fetal movement, joint abnormalities, difficulty swallowing, and respiratory failure.

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SMA type 1 (Werdnig Hoffman disease) is the most common type of SMA and is also a severe form of the disease. Babies with SMA type 1 experience severe weakness before 6 months of age and are never able to sit independently. Muscle weakness, lack of motor development, and weak muscle tone are the main clinical manifestations of SMA type 1. Babies with a severe prognosis have problems sucking or swallowing.

Some show abdominal breathing during the first few months of life. Muscle weakness occurs on both sides of the body and the ocular muscles are not affected. Twitching of the tongue is common. Intelligence is normal. Most affected children die before the age of two, but survival may depend on the degree of respiratory function.

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The onset of weakness in SMA type 2 patients is usually seen between 6 and 12 months. Affected children are able to sit independently early in their development but cannot walk independently even 3 meters. Finger tremors are almost always seen in SMA type 2. About 70% of those affected do not have deep tendon reflexes. Those affected by SMA type 2 are often unable to sit up independently in their mid-teens or later.

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Patients with SMA type 3 (Kugelberg-Welander syndrome) learn to walk, but often fall and have difficulty going up and down stairs at the age of 2-3 years. Legs are more severely affected than arms. The long-term prognosis depends on the degree of motor function achieved as a child.

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The onset of muscle weakness in SMA type 4 patients is after the age of 10; these patients can usually stand up to age 60.

How is SMA disease diagnosed?

A blood test is available to look for deletions or mutations of the SMN1 gene. This test identifies at least 95 percent of SMA types 0, 1, 2, 3, and 4 and can also reveal whether a person is a carrier of a defective gene that can be passed on to children.

Other diagnostic methods may be used if the SMN1 gene is not found to be abnormal or if the person’s history and examination are not typical for SMA. Other diagnostic methods include electromyography (which records the electrical activity of muscles during contraction and rest), nerve conduction velocity studies, muscle biopsy (used to diagnose many neuromuscular disorders), and other blood tests.

How is SMA disease treated?

There is no complete cure for SMA. Treatment consists of managing symptoms and preventing complications.

In December 2016, the U.S. Food and Drug Administration (FDA) approved nusinersen as the first drug approved to treat children and adults with SMA. The drug is given by injection into the fluid surrounding the spinal cord. It is designed to increase the production of the full-length SMN protein, which is critical for the maintenance of motor neurons.

The drug shows its effect best in infants and children, especially when started early. Several other therapeutic treatments are under investigation and may become available for affected individuals in the near future.

In May 2019, the FDA approved the Onasemnogene abeparovec-xioi gene therapy for children younger than 2 years of age with infantile-onset SMA. A safe virus delivers a fully functional human SMN gene to the targeted motor neurons, which improves muscle movement and function while also increasing the chances of survival. In August 2020, the FDA approved the orally administered drug risdiplam to treat those with SMA disease two months of age and older.

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Physical therapy, occupational therapy, and rehabilitation can help improve posture, prevent joint immobility, and slow muscle weakness and atrophy.

Stretching and strengthening exercises can help reduce contractures, increase range of motion, and keep circulation flowing. Some people need additional treatment for speech and swallowing difficulties.

Supports or assistive devices such as braces, orthoses, speech synthesizers, and wheelchairs can be helpful to develop functional independence.

Proper nutrition and calories are essential to maintain weight and strength by avoiding prolonged starvation. It may be necessary to use a feeding tube for people who cannot chew or swallow.

Non-invasive ventilation at night can improve breathing during sleep, and some people may also need to use assisted ventilation during the day due to muscle weakness in the neck, throat, and chest.

Research for SMA disease

The National Institute of Neurological Disorders and Stroke (NINDS), a component of the National Institutes of Health (NIH), conducts clinical research on SMA in laboratories at the NIH and also supports large medical institutions in research centers through grants. Cellular and molecular studies seek to understand the mechanisms that trigger the degeneration of motor neurons.

Scientists have analyzed human tissue and developed a wide range of model systems in animals and cells to investigate disease processes and accelerate testing of potential treatments. Among these efforts:

  • Gene therapy and specific drugs have been shown to arrest motor neuron destruction and slow disease progression in mouse models and individuals with SMA disease. NINDS supported research to establish these methods and provide a pathway to clinical testing in patients. Clinical trials for gene therapy in SMA are ongoing.
  • Animal models of SMA represent critical tools in the discovery and development of new treatments for SMA. The scientists are also examining samples of zebrafish, mice and pigs with less severe SMA types 2 and 3, which could greatly help identify new therapeutic targets and candidate treatments.

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