Spinal muscular atrophy type 1

Last updated Feb. 2, 2023, by Marisa Wexler, MS
Fact-checked by Ana de Barros, PhD

Type 1 spinal muscular atrophy (SMA), also called infantile-onset SMA or Werdnig-Hoffmann disease, is the most common form of SMA, accounting for about 60% of cases.

SMA is a rare genetic disease characterized by the progressive loss of motor neurons, the nerve cells that control voluntary movement. The condition is divided into five types based on how symptoms manifest.

What are the symptoms of SMA type 1?

SMA type 1 symptoms are apparent at birth or develop within the first 6 months of life. The hallmark of SMA is muscle weakness and wasting due to the death of motor neurons. In young infants, this results in symptoms such as:

• low muscle tone, resulting in floppy legs and arms
• failure to reach motor milestones like rolling over or sitting up unsupported
• a lack of reflexes.

In addition to impacting the muscles used to move the body around, SMA type 1 commonly affects muscles in the chest, throat, and mouth. This can lead to symptoms including:

• difficulty feeding, due to weakness in the muscles used for sucking, swallowing, and chewing
• poor head control and involuntary tongue movements
• breathing problems caused by weakness of the muscles around the lungs
• weak cry and cough
• scoliosis, an abnormal and progressive sideways curvature of the spine, caused by weakness in the back muscles that support the spine’s position.

How is SMA type 1 diagnosed?

When symptoms are present and/or there is a history of SMA in the family, a diagnosis can be made through genetic testing. Such testing, generally done on a blood sample, looks for disease-causing mutations in the SMN1 gene.

Genetic testing also can be used to measure the number of copies of the “backup” SMN2 gene a person has. That count influences disease severity — typically, a higher number of SMN2 copies is associated with less severe SMA. Most type 1 patients carry two copies of this backup gene. Combining these genetic results with age of symptom onset and symptom severity helps to confirm the type of disease.

SMA is an autosomal recessive disease, which means that a person will only develop symptoms if both copies of SMN1 — one inherited from each biological parent — contain a disease-causing mutation.

Genetic testing may be used to identify SMA carriers. These individuals will not develop SMA, but carry one mutated SMN1 copy and could pass it to their own biological children.

Babies also can be diagnosed using genetic testing — either while still in the womb or shortly after birth. Testing before birth is known as prenatal screening, while newborn screening for SMA can assess babies for SMA right after birth.

If both parents carry one mutated copy of the SMN1 gene, they will have a 25% chance of having a biological child with SMA, and a 50% risk of each of their biological offspring being a carrier like they are. There’s also a 25% chance that any children will neither have SMA nor be a carrier.

Further tests, such as an electromyogram or muscle biopsy, may be considered if there is any uncertainty about the diagnosis. Generally, however, genetic testing and symptom appearance is sufficient to confirm an SMA diagnosis.

Electromyography assesses the health of muscles and motor neurons that control them; the technique measures muscles’ responses to electrical stimulations by motor neurons. In a muscle biopsy, a small section of muscle tissue is removed, usually from the upper thigh, and examined to see whether it has SMA-associated features.

What are the treatment options for SMA type 1?

There now are three disease-modifying therapies approved to treat SMA in the U.S. These therapies also are approved in other countries, though specific indications may differ.

Depending on each patient’s specific case, SMA type 1 treatment may include:

• Spinraza (nusinersen), administered via regular injections into the spine. The first approved therapy for SMA, Spinraza is indicated for all ages and all main types of SMA.
• Evrysdi (risdiplam), a daily oral therapy approved for all main types of SMA in patients ages 2 months and older.
• Zolgensma (onasemnogene abeparvovec-xioi), a one-time gene therapy for children ages 2 and younger. It is approved for patients with all main types of SMA.

In clinical trials, these treatments have been proven to slow the progression of SMA. Because these medications can slow or stop disease progression, but have limited ability to reverse damage that’s already accumulated, better clinical outcomes are generally seen when treatment is started as early as possible.

Supportive care and assistive devices

In addition to disease-modifying therapies, people living with SMA type 1 commonly benefit from nondrug forms of supportive care. Some of these are:

• physical therapy, exercise, and whole-body vibration, which aim to improve motor function
• occupational therapy, designed to develop strategies for making day-to-day life easier
• speech and language therapy, which can help with talking ability and also may help with chewing and swallowing
• respiratory therapy, for strengthening and maintaining breathing ability
• dietary counseling to ensure adequate nutrition and digestive health
• palliative care, a broad branch of therapy that aims to improve life quality for people with chronic conditions and their families.

Specialized equipment may make living with SMA on a day-to-day basis more manageable. Among available options are:

• mobility equipment, like adaptive strollers or wheelchairs, to help patients get around more easily
• specialized chairs and seats to make activities including bathing and feeding easier
• braces, standers, and other devices that can provide structural support to the body and aid in everyday tasks
• equipment to support lung function, possibly including include a ventilator and/or devices like cough assist machines and high-frequency chest wall oscillation vests, which can help clear secretions from the lungs
• gastrostomy tubes and other strategies to deliver nutrients directly to the digestive system, bypassing the need to chew and swallow food.

What is the life expectancy for people with SMA type 1?

While life expectancy for milder types of the disease is not significantly affected, in the absence of treatment, most babies with SMA type 1 do not survive past early childhood. Historically, nearly all children born with this type of SMA have died before their second birthday.

Today, new disease-modifying therapies have radically changed the prognosis for babies born with SMA type 1, especially when treatment is initiated early. Clinical trials have demonstrated such therapies can improve survival outcomes compared with what’s expected without treatment, and many children treated early in life are still alive today.

However, because clinical trials of these modern DMTs began in the 2010s, the long-term survival outcomes for patients given modern treatment are not yet known.

Disease-modifying treatment can also improve motor development — in fact, many children with SMA type 1 who start on treatment in the first weeks of life, before showing symptoms, have been shown to be able to stand and walk within developmentally normal timeframes.

SMA News Today is strictly a news and information website about the disease. It does not provide medical advice, diagnosis, or treatment. This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website.