Spinal muscular atrophy type 1 (SMA type 1) is the most severe form of a genetic disease that destroys the nerve cells controlling voluntary muscle movement. It typically appears before 6 months of age and causes profound weakness that affects breathing, swallowing, and the ability to move. It is also the most common form of SMA, sometimes called Werdnig-Hoffmann disease.
Until recently, most children with SMA type 1 did not survive past early childhood. The arrival of three approved treatments over the past several years has fundamentally changed that trajectory, though outcomes depend heavily on how early treatment begins.
The Genetic Cause
SMA type 1 is caused by mutations in a gene called SMN1, which provides instructions for making a protein that motor neurons need to survive. Without enough of this protein, the motor neurons in the spinal cord gradually break down and die, cutting off signals from the brain to the muscles. The disease is inherited in an autosomal recessive pattern, meaning a child must receive a faulty copy of SMN1 from both parents to develop the condition.
About 95% of people with SMA are missing a critical section (exon 7) of the SMN1 gene on both copies of chromosome 5, either through deletion or a process called gene conversion. The remaining 5% carry a deletion on one chromosome and a smaller point mutation on the other.
A closely related gene called SMN2 acts as a partial backup. It produces a small amount of functional protein, and the number of SMN2 copies a person has directly influences disease severity. More copies generally mean milder disease. People can carry anywhere from one to eight copies of SMN2. Children with SMA type 1 typically have only one or two copies, which is why their symptoms are the most severe.
Signs and Symptoms
The hallmark of SMA type 1 is severe muscle weakness that appears early in life. Most children show symptoms before 8 months of age, and some mothers notice unusually weak fetal movements during the last trimester of pregnancy. The weakness is most obvious in the muscles closest to the trunk of the body, including the shoulders, hips, and chest wall, though it quickly affects the limbs as well.
Specific signs include:
- Hypotonia: severely reduced muscle tone, often described as “floppy” when the infant is held
- Inability to hold the head up or reach motor milestones like sitting independently
- Feeding and swallowing difficulties due to weakness in the muscles that control chewing and swallowing
- Tongue fasciculations: a visible quivering or twitching of the tongue
- Weak cry and cough
- Breathing problems: a bell-shaped chest develops because the rib muscles are weak while the diaphragm is relatively spared, creating a characteristic breathing pattern
Infants with SMA type 1 typically move very little. They do not develop the ability to sit unsupported without treatment, and progressive weakness in the respiratory muscles makes them increasingly vulnerable to pneumonia and respiratory failure.
How SMA Type 1 Is Diagnosed
Many babies in the United States are now identified through newborn screening, which uses a small blood sample from a heel prick to look for the absence of exon 7 in the SMN1 gene. This screening catches the vast majority of cases, though an estimated 2 to 5 percent of SMA cases involve a different type of genetic change that current newborn screening does not detect.
When screening returns an abnormal result, or when an infant develops symptoms before screening catches the condition, follow-up genetic testing confirms the diagnosis and determines the number of SMN2 copies. That copy number is important because it helps predict severity and guides treatment decisions. The genetic test is the definitive diagnostic tool. Additional tests like electromyography or muscle biopsy, once commonly used, are now rarely needed.
Treatment Options
Three disease-modifying treatments are now approved for SMA type 1 in the United States. All work by increasing the amount of functional motor neuron protein available to nerve cells, but they do so through different mechanisms and routes of delivery.
Gene Replacement Therapy
A one-time intravenous infusion delivers a working copy of the SMN1 gene directly into motor neurons using a modified virus as a carrier. The version called Zolgensma is approved for children under 2 years of age with confirmed mutations in both copies of SMN1. A newer formulation, Itvisma, was approved in 2025 for patients 2 years of age and older. Both are single-dose treatments, which is a significant practical advantage for families. Early treatment, ideally before symptoms appear, produces the best motor outcomes.
Spinal Injections
Nusinersen (Spinraza) is injected into the fluid surrounding the spinal cord. Treatment starts with four loading doses over about two months: three doses given two weeks apart, then a fourth dose 30 days later. After that, maintenance injections are given once every four months for the duration of the patient’s life. Each injection requires a lumbar puncture, which can be challenging in young children with spinal changes from SMA.
Oral Medication
Risdiplam (Evrysdi) is a liquid taken by mouth once daily, making it the least invasive option. In a clinical trial of infants with SMA type 1, 44% of treated children could sit without support for at least 30 seconds after 24 months of treatment. That milestone is essentially never reached by untreated children with type 1. None of the treated infants in the trial were able to stand or walk independently at the 24-month mark, but motor function continued to improve throughout the study period.
The single most important factor in treatment success is timing. Children treated before they lose motor neurons see dramatically better outcomes than those treated after significant nerve damage has already occurred. This is why newborn screening has been so consequential: it identifies affected babies before symptoms appear, creating a window for early intervention.
Respiratory and Nutritional Support
Even with disease-modifying treatment, many children with SMA type 1 need respiratory support. Weakness in the chest wall muscles leads to shallow breathing, poor cough, and difficulty clearing mucus from the airways. Noninvasive ventilation, typically a mask that delivers pressurized air during sleep, is often introduced when nighttime breathing becomes inadequate. Caregivers are trained in cough-assist techniques to help clear secretions when respiratory muscle strength drops below half of what’s expected.
Feeding is the other major area of supportive care. Weakness in the swallowing muscles raises the risk of aspiration, where food or liquid enters the airway instead of the stomach. Aspiration pneumonia is a serious and potentially fatal complication. Early strategies include thickening liquids and adjusting food textures, but many children with SMA type 1 ultimately benefit from a gastrostomy tube (G-tube) placed surgically through the abdominal wall directly into the stomach. This ensures adequate nutrition while reducing aspiration risk. If gastroesophageal reflux is also present, a procedure to tighten the junction between the esophagus and stomach may be performed at the same time.
Optimal care for SMA type 1 involves a coordinated team of neurologists, pulmonologists, physical therapists, nutritionists, and home ventilation specialists. The complexity of this care is significant, and families often work with a specialized neuromuscular center to coordinate it.
What Outcomes Look Like Now
The natural history of SMA type 1 without treatment is severe. Most untreated children never sit independently, and respiratory failure historically occurred within the first two years of life. That picture has shifted substantially with current therapies. Many treated children achieve motor milestones that were previously impossible for this form of SMA, including head control, assisted sitting, and in some cases independent sitting.
Outcomes vary widely depending on the number of SMN2 copies, how much motor neuron damage occurred before treatment started, and which therapy was used. Children identified through newborn screening and treated in the first weeks of life consistently show the strongest responses. Some of these pre-symptomatically treated children develop near-normal motor function, though long-term data beyond several years is still accumulating as these therapies are relatively new.
SMA type 1 remains a serious condition that requires lifelong medical management. But for families receiving this diagnosis today, the landscape is vastly different from what it was even a decade ago.