Spinal muscular atrophy is a genetic condition that gradually weakens muscles throughout the body, affecting people in different ways depending on when symptoms first appear and how quickly the disease progresses.

What is Spinal Muscular Atrophy?

Spinal muscular atrophy, commonly called SMA, is a group of inherited disorders that cause muscles to become weak and waste away over time. The condition affects specialized nerve cells in the spinal cord called motor neurons, which are cells that send signals from the brain and spinal cord to muscles, telling them when and how to move. When these motor neurons break down or don’t work properly, the muscles they control stop receiving the signals they need to function.^[1]

Without these vital nerve signals, muscles cannot contract and move as they should. Over time, the muscles become progressively weaker and eventually shrink, a process called atrophy. This muscle wasting gives the condition its name. The weakness typically affects muscles closest to the center of the body more severely than those farther away. For example, muscles in the shoulders, hips, and back often weaken before muscles in the hands and feet.^[1]

SMA primarily affects voluntary muscles, which are the muscles you consciously control to perform movements like walking, sitting, crawling, or holding your head up. In more severe cases, it can also affect the muscles used for breathing, swallowing, and coughing. Despite these physical challenges, SMA does not affect a person’s intelligence, senses, or ability to learn and think.^[2]

Types of Spinal Muscular Atrophy

Healthcare providers classify SMA into five main types based on when symptoms begin and how severe they are. The earlier symptoms appear, the more severe the condition tends to be. Each type affects people differently and has different impacts on life expectancy and quality of life.^[1]

Type 0 SMA is the rarest and most severe form. It affects babies before birth, and parents may notice decreased fetal movements during pregnancy. Infants born with type 0 have severe muscle weakness from birth, often with joint deformities, and typically experience respiratory failure. Most infants with this type do not survive beyond the first month of life.^[1]

Type 1 SMA, also called Werdnig-Hoffman disease or infantile-onset SMA, is the most common form, accounting for about 60 percent of all SMA cases. Symptoms appear within the first six months of life. Babies with type 1 have limited head control, appear “floppy” due to decreased muscle tone, and cannot sit without support. They have difficulty swallowing and breathing, which can lead to feeding problems and frequent respiratory infections. Without breathing support and other interventions, children with type 1 historically did not survive past their second birthday, though new treatments are changing these outcomes.^[1][7]

Type 2 SMA, sometimes called Dubowitz disease or intermediate SMA, begins to affect children between 6 and 18 months of age. Children with type 2 may be able to sit without support at some point but cannot stand or walk independently. The muscle weakness tends to affect their legs more severely than their arms. Many develop curvature of the spine and may have breathing difficulties. Around 70 percent of people with type 2 survive until age 25, with some living into their 30s and beyond, especially with modern medical care.^[1]

Type 3 SMA, also known as Kugelberg-Welander disease or juvenile SMA, causes symptoms that appear after 18 months of age and can begin as late as adulthood. Children with type 3 can usually walk on their own initially, though many eventually have difficulty walking and may need a wheelchair later in life. This type tends to progress more slowly, and people with type 3 typically don’t develop breathing problems. Life expectancy is generally normal or near-normal.^[1]

Type 4 SMA is the mildest and rarest form, affecting less than 5 percent of people with SMA. It doesn’t typically appear until after age 21 or even later in adulthood, sometimes not until age 30 or older. Muscle weakness progresses slowly, so most people with type 4 remain mobile throughout their lives. This type typically does not affect life expectancy.^[1][8]

How Common is Spinal Muscular Atrophy?

Although many people consider SMA an uncommon condition, it is actually the second most common severe hereditary disease affecting infants and children, following only cystic fibrosis. The condition affects approximately 1 in every 8,000 to 10,000 live births worldwide, though estimates vary by region. Some sources report incidence rates ranging from 1 in 6,000 to 1 in 11,000 babies born.^[1][8]

SMA affects people of all ethnic backgrounds, but the frequency varies somewhat among different populations. Research shows that SMA is about twice as common in people of white and Asian descent compared to Black and Hispanic populations. Despite these variations, SMA remains a significant health concern across all racial and ethnic groups.^[1]

Approximately 1 in 40 to 1 in 50 people carry a copy of the altered gene that causes SMA, even though they don’t have the condition themselves. These individuals are called carriers. Because carriers are relatively common in the general population, many couples may not know they both carry the gene until they have a child with SMA or undergo genetic testing.^[4][11]

Causes of Spinal Muscular Atrophy

The most common form of SMA is caused by changes in a specific gene called SMN1, which stands for survival motor neuron 1. This gene provides instructions to cells on how to produce a protein called SMN protein. Motor neurons need adequate amounts of this protein to survive and function properly. When the SMN1 gene is mutated or missing, the body cannot produce enough functional SMN protein, causing motor neurons to gradually break down and die.^[2][8]

In more than 95 percent of SMA cases, the mutation involves a deletion of a large segment of the SMN1 gene, specifically a section called exon 7. Less commonly, the gene has spelling errors called point mutations or other types of genetic variations. Regardless of the specific type of mutation, the result is the same: not enough SMN protein is produced to keep motor neurons healthy.^[11][16]

There is another gene very similar to SMN1 called SMN2. Everyone with SMA has at least one copy of the SMN2 gene, and some people have multiple copies. While the SMN2 gene produces much less functional SMN protein compared to a healthy SMN1 gene, having extra copies of SMN2 can help compensate somewhat for the missing or faulty SMN1 gene. People with more SMN2 gene copies tend to have milder forms of SMA because they produce more SMN protein overall. This explains why some people have type 3 or type 4 SMA while others have the more severe type 1 or type 2.^[2][11]

How Spinal Muscular Atrophy is Inherited

SMA is an autosomal recessive genetic condition. This means that a child must inherit two copies of the mutated SMN1 gene—one from each parent—to develop the condition. If both parents carry one mutated copy of the gene, they are called carriers, and they typically don’t show any signs or symptoms of SMA themselves.^[11]

When both parents are carriers, with each pregnancy there is a 25 percent chance (1 in 4) that their child will have SMA. There is a 50 percent chance (2 in 4) that their child will be a carrier like the parents but will not have SMA. There is also a 25 percent chance (1 in 4) that their child will neither have SMA nor be a carrier. These probabilities apply to each pregnancy independently.^[11]

In approximately 2 percent of cases, the mutation occurs during early development rather than being inherited from parents. These are called de novo mutations, meaning they happen spontaneously. If someone in your family has SMA, your chance of being a carrier increases significantly, making genetic counseling and testing particularly important for family members.^[6][11]

Risk Factors for Spinal Muscular Atrophy

The primary risk factor for SMA is having two parents who both carry a mutated copy of the SMN1 gene. Since about 1 in 40 to 1 in 50 people are carriers without knowing it, many couples discover they are both carriers only after having a child diagnosed with SMA or through genetic screening.^[4]

Certain populations have slightly higher carrier rates. People of European and Asian descent have approximately twice the risk compared to those of African and Hispanic ancestry, though SMA can affect anyone regardless of ethnic background. If you have a family history of SMA—such as a sibling, cousin, aunt, uncle, or more distant relative with the condition—your risk of being a carrier increases substantially.^[1]

It’s important to understand that being a carrier does not mean you will develop SMA. Carriers have one working copy of the SMN1 gene and one mutated copy. The working copy produces enough SMN protein to keep their motor neurons healthy. However, carriers can pass the mutated gene to their children. If a carrier has children with another carrier, their children face the risks described earlier.^[5]

Signs and Symptoms of Spinal Muscular Atrophy

The symptoms of SMA vary widely depending on the type and when the condition begins. The hallmark symptom across all types is progressive muscle weakness that typically affects muscles closest to the center of the body more severely. However, each type presents with its own characteristic pattern of symptoms.^[1]

Babies with type 0 SMA may show decreased movements before birth. At birth, they have severe muscle weakness, extremely low muscle tone, and severe breathing difficulties. Some may be born with joint deformities called contractures, where joints become permanently fixed in bent or straight positions.^[1]

Infants with type 1 SMA appear “floppy” because of severely decreased muscle tone, a condition called hypotonia. They have limited head control and cannot lift their heads when lying on their stomachs. They are unable to sit without support and lack normal reflexes. Many have a weak cry, difficulty swallowing, and trouble sucking, which leads to feeding problems and slow growth. Their breathing is often affected, causing an abnormal breathing pattern and a bell-shaped chest. They may have frequent respiratory infections because weak breathing muscles make it hard to cough effectively.^[1][7]

Children with type 2 SMA develop symptoms after they’ve reached some developmental milestones. They may learn to sit independently but cannot stand or walk on their own. They experience progressive muscle weakness that affects their legs more than their arms. Many develop curvature of the spine, and some have involuntary trembling or shaking movements in their fingers. Breathing problems can develop as the condition progresses, particularly during sleep or when lying down.^[1]

People with type 3 SMA initially develop normally and learn to walk. Symptoms appear later in childhood or even in adolescence. They experience lower limb muscle weakness that makes walking, running, climbing stairs, and rising from a chair increasingly difficult over time. Some eventually need to use a wheelchair. Unlike the more severe types, type 3 usually doesn’t affect breathing muscles significantly.^[1]

Adults with type 4 SMA typically experience mild to moderate muscle weakness, often beginning in the legs. They may notice tremors and some breathing difficulties, though these are usually mild. Most people with type 4 can continue walking and performing daily activities with minimal assistance.^[8]

Across all types, people may experience additional problems including spinal curvature, joint contractures where joints become stiff and difficult to move, and respiratory infections. However, SMA does not affect intelligence, sensation, or the ability to form relationships and engage with others.^[2][9]

Prevention and Early Detection

Because SMA is a genetic condition present from birth, there is no way to prevent it through lifestyle changes or environmental modifications. However, several approaches can help identify the condition early or help families understand their risks before having children.^[5]

Many countries, including all U.S. states, now screen newborn babies for SMA as part of routine newborn screening programs. These blood tests can identify babies with SMA before symptoms appear, allowing treatment to begin as early as possible. Starting treatment before symptoms develop can significantly improve outcomes, particularly for infants who would otherwise develop severe type 1 SMA.^[6]

For couples planning to have children, genetic testing can identify whether one or both partners are carriers of the mutated SMN1 gene. This carrier testing involves a simple blood test. If both partners are carriers, genetic counselors can explain the risks and discuss options, which may include prenatal testing, preimplantation genetic testing for couples using in vitro fertilization, or preparing for the possibility of having a child with SMA.^[5][6]

During pregnancy, two tests can determine whether a fetus has SMA: chorionic villus sampling (CVS), typically performed between 10 and 13 weeks of pregnancy, and amniocentesis, usually done between 15 and 20 weeks. Both tests analyze the baby’s genetic material to look for SMN1 mutations. Knowing a diagnosis before birth allows families to prepare and make informed decisions about care and treatment options.^[5]

If you’re pregnant or planning a pregnancy and someone in your family or your partner’s family has SMA, it’s particularly important to speak with a healthcare provider or genetic counselor about testing options. They can help you understand your specific risks and what steps might be appropriate for your situation.^[5]

Understanding How SMA Affects the Body

To understand SMA’s effects on the body, it helps to know how muscles normally work. When you decide to move—whether to pick up a cup, take a step, or turn your head—your brain sends electrical signals through nerve pathways. These signals travel down the spinal cord to motor neurons, which are specialized nerve cells located in the spinal cord and brainstem. The motor neurons then transmit these signals to muscles, telling them to contract and produce movement.^[3]

In SMA, the lack of sufficient SMN protein causes motor neurons to malfunction and eventually die. As these nerve cells disappear, the communication pathway between the brain and muscles breaks down. Muscles stop receiving the signals they need to contract. Without regular stimulation from motor neurons, muscles become progressively weaker and smaller, a process called atrophy. This muscle wasting happens because muscles need regular nerve signals to maintain their size and strength.^[3]

The muscle weakness in SMA follows a specific pattern. Muscles closer to the center of the body, called proximal muscles, are usually affected more severely and earlier than muscles farther from the body’s center, called distal muscles. This means that shoulder, hip, and trunk muscles typically weaken before hand and foot muscles. This pattern explains why people with SMA often have particular difficulty with activities that require core strength and proximal muscle power, such as sitting up, standing from a seated position, or lifting the arms above shoulder height.^[1]

The severity of motor neuron loss and muscle weakness depends primarily on how much functional SMN protein is available. People with more severe types of SMA produce very little SMN protein, leading to rapid and extensive motor neuron death. Those with milder types produce more SMN protein, usually because they have more copies of the SMN2 gene, which slows the rate of motor neuron loss and results in less severe muscle weakness.^[2]

In more severe cases, the respiratory muscles that control breathing can be affected. The diaphragm and intercostal muscles between the ribs help expand the lungs when you breathe in and push air out when you exhale. When these muscles weaken, breathing becomes less efficient, particularly during sleep. Weak respiratory muscles also make it difficult to cough forcefully, which is important for clearing secretions from the airways. This combination increases the risk of respiratory infections like pneumonia, which can be serious or life-threatening for people with SMA.^[2]

Muscles used for swallowing and eating can also be affected, particularly in more severe types. When the muscles that move food from the mouth to the stomach weaken, eating and drinking become difficult and potentially dangerous. There is a risk of aspiration, where food or liquid enters the airway instead of the esophagus, which can lead to serious lung infections.^[8]

The muscles and soft tissues that support the spine can also weaken, leading to spinal curvature called scoliosis. When the muscles that normally keep the spine straight and well-aligned are weak, the spine can curve sideways. Scoliosis can affect posture, balance, comfort when sitting, and in severe cases, can further compromise breathing by reducing the space available for the lungs to expand.^[2]

Joint contractures can develop when muscles around a joint become weak or tight, pulling the joint into a fixed position. This happens more commonly in people who cannot move certain joints through their full range of motion regularly. Contractures can limit mobility and make care more challenging.^[2]