Spinal muscular atrophy (SMA) is a genetic disease that destroys the nerve cells controlling muscle movement, leading to progressive muscle weakness and wasting. It affects roughly 1 in 10,000 live births, making it one of the leading genetic causes of infant death. SMA ranges widely in severity, from a fatal condition in newborns to a mild form that appears in adulthood, and treatments introduced in recent years have dramatically changed the outlook for many patients.
How SMA Affects the Body
SMA targets motor neurons, the nerve cells in the spinal cord that send signals from the brain to your muscles. These neurons depend on a protein called survival motor neuron (SMN) to stay healthy and function properly. People with SMA have a defective or missing copy of the gene (SMN1) responsible for producing this protein. Without enough SMN protein, motor neurons gradually break down and die.
As motor neurons are lost, the muscles they control stop receiving signals. This causes the muscles to weaken and shrink, a process called atrophy. The weakness is typically “proximal,” meaning it hits muscles closer to the center of the body first: the shoulders, hips, back, and thighs. Muscles used for breathing and swallowing can also be affected, which is why the most severe forms of SMA are life-threatening. Importantly, SMA does not affect thinking or intelligence. The disease is specific to motor neurons.
The Role of the SMN2 Backup Gene
Nearly everyone has a second, similar gene called SMN2. This backup gene produces a small amount of functional SMN protein, but only about 10 to 15 percent of what the body needs. The number of SMN2 copies a person carries is the single biggest factor in determining how severe their SMA will be.
People with only one or two copies of SMN2 typically develop the most severe form of SMA, while those with four or more copies generally have much milder disease. The correlation isn’t perfect, since the gene’s output can vary between individuals, but SMN2 copy number remains the most reliable predictor of how aggressively the disease will progress.
Types of SMA
SMA is classified into five types based on when symptoms appear and the highest level of physical ability a person reaches. These types form a spectrum rather than rigid categories.
- Type 0 is the rarest and most severe form. Symptoms are present at birth, and affected infants typically have zero SMN2 copies. Survival is measured in weeks.
- Type 1 is the most common severe form. Symptoms appear before 6 months of age. Babies may gain some head control but never sit independently. Without treatment, most children with Type 1 do not survive past age 2. These children typically carry one to two copies of SMN2.
- Type 2 appears between 6 and 18 months. Children can learn to sit on their own but never stand or walk independently. With proper care, survival well into adulthood is expected. Most have two to three SMN2 copies.
- Type 3 begins after 18 months, often in early childhood. Children can stand and walk, though many lose the ability to walk as they grow older and muscles weaken over time. They typically carry three to four SMN2 copies and have a normal or near-normal lifespan.
- Type 4 is the mildest form, with symptoms starting in adulthood, usually after age 30. Weakness is mild and progresses slowly. People with Type 4 generally have four or more SMN2 copies and retain all major motor functions.
Symptoms to Recognize
In infants, the earliest signs of SMA are often described as “floppiness,” or low muscle tone. A baby with SMA may feel unusually limp when picked up, have difficulty holding up their head, and show little spontaneous movement of the arms and legs. Feeding and swallowing difficulties are common in severe cases, and a weak cry can be an early clue. Some infants show a fine trembling of the tongue, called fasciculations.
In older children and adults, symptoms tend to appear as difficulty climbing stairs, getting up from a chair, or lifting the arms overhead. Falls become more frequent. Because SMA primarily affects muscles close to the trunk, fine motor skills in the hands are often preserved longer than the ability to walk or raise the arms. Breathing muscles can weaken over time in Types 1 and 2, making respiratory infections more dangerous and sometimes requiring ventilatory support.
How SMA Is Diagnosed
The fastest route to diagnosis today is newborn screening. Every U.S. state now screens newborns for SMA as part of routine blood spot testing, with the last state beginning in early 2024. This is critical because treatment works best when started before symptoms appear. International adoption of newborn SMA screening has also grown rapidly since 2021.
For babies or children who were not caught by newborn screening, diagnosis typically involves a genetic blood test that looks for deletions or mutations in the SMN1 gene and counts SMN2 copies. This test is definitive in the vast majority of cases.
Treatment Options
Three disease-modifying therapies are now available for SMA, each working in a different way. All aim to increase the amount of functional SMN protein in the body, and all have shown the greatest benefit when started as early as possible, ideally before significant motor neuron loss has occurred.
Gene Therapy
A one-time intravenous infusion delivers a working copy of the SMN1 gene directly into the patient’s cells. This therapy is FDA-approved for children under 2 years of age with confirmed SMN1 mutations. Because it is a single treatment rather than an ongoing regimen, it represents a fundamentally different approach from the other two options.
SMN2-Targeting Injections
The first approved SMA treatment works by helping the SMN2 backup gene produce more functional protein. It is delivered as an injection into the spinal fluid. Treatment begins with four loading doses over the first two months, followed by maintenance doses every four months for life. It is approved for patients of all ages.
Oral SMN2-Targeting Therapy
A daily liquid medication taken by mouth also boosts protein production from the SMN2 gene. In a large clinical trial of patients with Type 2 and non-ambulant Type 3 SMA, 32% showed meaningful improvement in motor function after two years of treatment, and 58% maintained stable function. Even patients who started treatment later showed some benefit, though the gains were smaller: 16% improved and 59% stabilized after 12 months on the medication.
Timing Matters
Across all three therapies, earlier treatment produces better outcomes. For Type 1 SMA, data from Thailand showed that children under 2 who received treatment had improved survival compared to untreated children, but this advantage diminished when treatment started after age 2. This is because SMA therapies can slow or stop motor neuron loss, but they cannot restore neurons that have already died. Universal newborn screening exists largely because of this narrow treatment window.
Ongoing Supportive Care
Disease-modifying drugs are only one piece of SMA management. Comprehensive care involves a team of specialists addressing the multiple body systems that SMA can affect.
Respiratory care is often the most important element, especially for Types 1 and 2. Weak breathing muscles make it hard to cough effectively, so secretions build up in the lungs and infections become dangerous. Techniques like assisted coughing devices, non-invasive ventilation (a mask that helps push air into the lungs), and careful monitoring during respiratory illnesses are standard practice.
Nutritional support is equally important. Swallowing difficulties can lead to poor nutrition and aspiration, where food or liquid enters the airway. Some children benefit from feeding tubes to ensure adequate calorie intake and reduce aspiration risk. Bone health is also a concern, since reduced mobility and muscle pull on bones can lead to low bone density and scoliosis.
Physical therapy and rehabilitation help maintain flexibility, prevent joint contractures, and maximize whatever motor function a person has. Bracing, wheelchair fitting, and adaptive equipment all play roles in keeping people with SMA as independent as possible. Updated standards of care published in 2018 by an international group of SMA specialists remain the framework most clinical teams follow, covering everything from orthopedic management to palliative care.
Living With SMA Today
The landscape of SMA has shifted dramatically in the past decade. Before treatments were available, Type 1 SMA was almost universally fatal by age 2. Now, many children with Type 1 who receive early treatment are surviving well beyond that threshold, though the degree of motor function they achieve varies widely. Some treated infants reach milestones like sitting or even standing that were previously unthinkable for their diagnosis.
For people with Types 2 and 3, disease-modifying therapy has helped stabilize or improve motor function that would otherwise slowly decline. Adults with SMA, particularly those diagnosed before treatments existed, are also seeing benefit from these therapies, though the improvements tend to be more modest since significant motor neuron loss has already occurred. The combination of early genetic diagnosis, effective therapies, and coordinated multidisciplinary care means that the prognosis for someone born with SMA today is fundamentally different from what it was even 10 years ago.