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Spinal muscular atrophy (SMA) is a genetic disorder that affects the central nervous system, peripheral nervous system, and voluntary muscle movement. SMA involves the loss of lower motor neurons, a type of nerve cell in the spinal cord that controls muscle movement. As a result, the muscles do not receive nerve signals, leading to muscle weakness and atrophy (wasting). SMA has several subtypes, which differ in severity and age of onset, with symptoms typically worsening over time. While there is currently no cure for SMA, treatments such as physical therapy, medications, and assistive devices can help manage symptoms and improve quality of life.
| Characteristics | Values |
|---|---|
| Type of Disorder | Genetic disorder |
| Affected Body Part | Motor neurons (nerve cells that control voluntary muscle movement) |
| Location of Motor Neurons | Spinal cord |
| Muscle Movement | Without motor neurons, muscles don't receive nerve signals that make them move |
| Muscle Weakness | More severe in muscles closer to the center of the body (proximal muscles) than in muscles farther away (distal muscles) |
| Muscle Wasting | Muscles weaken and shrink from inactivity |
| Severity | Varies across subtypes, but generally, the earlier the symptoms begin, the more severe the disorder |
| Age of Onset | Varies across subtypes, but can strike children at any age |
| Diagnosis | Genetic testing, electromyography, muscle biopsy |
| Treatment | Physical therapy, occupational therapy, speech therapy, assistive devices, medications |
| Prevalence | Affects 1 per 8,000 to 10,000 people worldwide |
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What You'll Learn
SMA is a genetic disorder
Spinal muscular atrophy (SMA) is a genetic disorder. SMA is a neuromuscular disease that affects the central nervous system, peripheral nervous system, and voluntary muscle movement (skeletal muscle). SMA involves the loss of nerve cells called motor neurons in the spinal cord, which control muscle movement. Without these motor neurons, the muscles do not receive the nerve signals that make them move, causing the muscles to weaken and shrink from inactivity. This is known as atrophy, the medical term for getting smaller.
There are five subtypes of SMA, classified based on the age of onset, severity, and life expectancy. SMA type 0 is the rarest and most severe form, affecting foetuses in the womb and often resulting in respiratory failure. SMA type 1, also known as Werdnig-Hoffmann disease, is the most common and severe form, with symptoms evident within the first six months of life, including severe muscle weakness and trouble breathing, coughing, and swallowing. SMA type 2, or intermediate SMA, is usually first noticed between six and 18 months of age, with children experiencing muscle weakness and respiratory challenges. SMA type 3, or juvenile SMA, emerges in children 18 months old or older, with muscle weakness present but most patients able to walk and stand for limited periods. SMA type 4 is a rare form that develops in adulthood, often with mild to moderate muscle weakness and a normal life expectancy.
SMA is typically inherited, with most cases resulting from an altered SMN1 gene being passed on to a child by their parents. In most cases, for SMA to be inherited, both parents must carry the altered gene. SMA can be diagnosed through genetic testing, which involves a blood test to detect mutations in the SMN1 gene. While there is currently no cure for SMA, treatments such as disease-modifying therapy and medications can help manage symptoms and improve quality of life.
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SMA affects the motor neurons
Spinal muscular atrophy (SMA) is a genetic disorder that affects the motor neurons, a type of nerve cell in the spinal cord that controls voluntary muscle movement. SMA involves the loss of these motor neurons, which results in muscles not receiving nerve signals and subsequently weakening and shrinking from inactivity. This leads to muscle weakness, which is typically more severe in the muscles closer to the centre of the body, such as the shoulders, hips, thighs, and upper back.
The primary symptom of SMA is progressive muscle weakness, with lower limbs generally being more affected than upper limbs. The severity of the condition depends on the number of copies of the SMN2 gene a person has. The SMN2 gene produces a small amount of SMN protein, which is essential for the survival and function of motor neurons. If there is insufficient SMN protein, the motor neurons shrink and die, leading to a loss of muscle control.
SMA is classified into several types based on the age of onset and severity of symptoms. Type 0 SMA is the rarest and most severe form, affecting infants before birth and often resulting in respiratory failure. Type 1 SMA, also known as Werdnig-Hoffmann disease, is the most common and severe form, with symptoms evident within the first six months of life, including severe muscle weakness and respiratory problems. Type 2 SMA, or intermediate SMA, is characterised by muscle weakness developing between six and 18 months of age, with children being able to sit but requiring assistance to stand or walk.
Type 3 SMA, or juvenile SMA, typically emerges in children 18 months or older and can manifest as late as the teenage years. Muscle weakness is present, but most patients can walk and stand for limited periods, especially early on. Type 4 SMA, or adult SMA, develops in adulthood and is rarely severe enough to impact lifespan. It is characterised by mild to moderate leg muscle weakness.
While there is currently no cure for SMA, treatments are available to improve quality of life. These include medications such as risdiplam (Evrysdi™), which increase the concentration of SMN protein in the body, and nusinersen (Spinraza®), which is injected into the spinal canal. Physical therapy, occupational therapy, and assistive devices can also help improve posture, prevent joint immobility, and slow muscle weakness.
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SMA causes muscle weakness and atrophy
Spinal muscular atrophy (SMA) is a genetic disorder that causes muscle weakness and atrophy. SMA affects the motor neurons, which are nerve cells in the spinal cord that control voluntary muscle movement. When these motor neurons are lost, the muscles do not receive signals from the nerves, leading to muscle weakness and atrophy.
The loss of motor neurons results in insufficient levels of the survival motor neuron (SMN) protein, which is essential for the health and function of motor neurons. The SMN protein is produced by the SMN1 gene, and mutations or alterations in this gene can lead to reduced production of the protein. The severity of SMA is influenced by the number of copies of the SMN2 gene, which also produces a small amount of SMN protein.
The muscle weakness associated with SMA tends to be more severe in the muscles closer to the centre of the body, such as the shoulders, hips, thighs, and upper back. The lower limbs are often more affected than the upper limbs. As SMA progresses, muscle weakness tends to worsen over time.
The symptoms of SMA vary and can range from mild to disabling. The specific symptoms depend on the subtype of SMA, which is classified based on the age of onset, severity, and life expectancy. There are five subtypes of SMA, including type 0 (congenital SMA), which is the rarest and most severe form, and type 1 (infantile-onset SMA), which is the most common form.
While there is currently no cure for SMA, treatments are available to help manage symptoms and improve quality of life. These treatments include medications, physical therapy, occupational therapy, and assistive devices. Early treatment is generally more beneficial, as it can help preserve motor neurons and improve muscle function.
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SMA has no cure, but treatments are available
Spinal muscular atrophy (SMA) is a genetic disease that affects the central nervous system, peripheral nervous system, and voluntary muscle movement. SMA involves the loss of nerve cells called motor neurons in the spinal cord, which results in muscles not receiving signals from these nerve cells and therefore weakening and shrinking. SMA has no cure, but treatments are available to manage symptoms and improve the prognosis.
SMA is caused by insufficient levels of the SMN protein, which is essential for normal motor function as it enables muscles to receive signals from nerves. The SMN protein is produced by the SMN1 gene, and individuals with SMA have a mutation in this gene that results in reduced production of the protein. Treatment options aim to increase the amount of SMN protein in the body.
There are several treatments available for SMA, including medications, physical therapy, occupational therapy, and assistive devices. Medications such as nusinersen (Spinraza) and risdiplam (Evrysdi) have been approved by the U.S. Food and Drug Administration (FDA) and can stimulate the production of SMN protein. These medications are injected into the space around the spinal canal or administered orally, respectively. Physical therapy can help improve posture, prevent joint immobility, and slow muscle weakness, while occupational therapy can improve the ability to perform daily tasks. Assistive devices such as orthopedic braces, crutches, walkers, and wheelchairs can also be used to improve mobility and independence.
In addition to these treatments, gene therapy offers a promising approach to treating SMA. Gene therapy, such as Zolgensma®, aims to replace the defective or missing SMN1 gene with a new working SMN gene. This therapy is delivered through an intravenous (IV) infusion and has been shown to improve muscle movement, function, and survival in children under two years old with infantile-onset SMA (Type I). Clinical trials are also ongoing to test the effectiveness of these treatments and develop new therapeutic approaches.
While there is currently no cure for SMA, the development of these treatments has improved the expected course of the disease. Early diagnosis and treatment are critical for the best outcomes, as motor neuron loss progresses over time. Individuals with SMA and their healthcare providers can work together to determine the most appropriate treatment plan based on their specific needs, goals, and values.
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SMA affects 1 in 6,000 babies
Spinal muscular atrophy (SMA) is a genetic disorder that affects nerve cells in the spinal cord, known as motor neurons, which control voluntary muscle movement. SMA is a neuromuscular disorder, causing muscles to become weak and waste away (atrophy) due to insufficient nerve signals. The muscles closest to the body's core, such as those in the shoulders, hips, thighs, and upper back, are the most affected. The lower limbs are generally impacted more than the upper limbs.
- SMA type 0 (congenital SMA): This is a rare subtype that affects a fetus before birth, causing severe muscle weakness and respiratory failure.
- SMA type 1 (infantile-onset SMA or Werdnig-Hoffman disease): This is the most severe form of SMA, affecting infants within the first six months of life. It causes limited head control, decreased muscle tone, and difficulties with swallowing and breathing.
- SMA type 2 (intermediate SMA): This type affects children between six and 18 months old. Children may be able to sit up but have respiratory challenges and worsening muscle weakness, particularly in the legs.
- SMA type 3 (juvenile SMA or Kugelberg-Welander syndrome): This subtype emerges in children 18 months old or older and can even manifest during the teenage years. Muscle weakness is present, but most patients can walk and stand for limited periods.
- SMA type 4 (adult SMA): This form of SMA develops in adulthood and typically presents as mild leg weakness.
The different types of SMA are classified based on the age of onset, severity, and life expectancy. SMA is a progressive condition, meaning the symptoms worsen over time. While there is currently no cure for SMA, treatments such as physical therapy, occupational therapy, assistive devices, and disease-modifying therapies can help manage symptoms and improve quality of life.
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Frequently asked questions
Spinal muscular atrophy (SMA) is a genetic disorder characterised by weakness and wasting (atrophy) in muscles used for movement (skeletal muscles). It is caused by a loss of specialised nerve cells, called motor neurons, in the spinal cord, which control muscle movement.
The symptoms of SMA vary and may be mild or disabling, but they involve a weakness of the muscles that control movement. The muscles closest to the centre of the body, such as the shoulders, hips, thighs, and upper back, tend to be affected more than those farther away from the body's centre. SMA can also cause breathing problems due to weakness of the respiratory muscles and an abnormally bell-shaped chest that prevents the lungs from fully expanding.
There is currently no cure for SMA, but there are treatments and support available to improve quality of life. Treatments include physical therapy, occupational therapy, and medications that target the altered genes that cause SMA.
