SMA life expectancy and disease onset

Last updated Sept. 6, 2023, by Susie Strachan
Fact-checked by Patrícia Silva, PhD

Spinal muscular atrophy (SMA) is a genetic, progressive disease that affects the nervous system and muscles. The life expectancy for people with SMA depends on the type of their disease and symptom severity.

For some types, SMA life expectancy is very short — from soon after birth to about 2 years old. But disease-modifying therapies have shown potential to improve SMA life expectancy.

SMA is caused by inherited mutations in the SMN1 gene, which provides instructions for making the survival motor neuron, or SMN, protein. When mutations exist in the SMN1 gene, little or no SMN protein is made, leading to progressive muscle weakness.

Genetic testing generally is needed to confirm an SMA diagnosis. Many U.S. states and countries offer prenatal and/or newborn screening.

There are five types of SMA — 0, 1, 2, 3 and 4. The first two are the most severe, and people with these types have the shortest life expectancy. Adult-onset type 4 SMA is the mildest form and generally does not affect life expectancy.

Still, the question of how long someone can live with SMA is complex.

Starting SMA treatment early is thought to improve life expectancy. Ideally, SMA should be treated before symptoms start. Disease-modifying therapies can work to slow or halt progression of the various symptoms of SMA.

Two medications — Evrysdi (risdiplam) and Spinraza (nusinersen) — and the gene therapy Zolgensma (onasemnogene abeparvovec-xioi) may help infants in reaching otherwise unattainable motor milestones, like sitting and crawling. Evrysdi and Spinraza also may help older patients maintain or improve motor, mobility, breathing, and swallowing abilities.

Living with SMA requires the use of adaptive devices, specialized equipment and technologies, along with physical therapy. Good nutrition also supports overall health.

SMA type 0 life expectancy

SMA type 0 is rare. It is the most severe form of the disease, with symptom onset before birth.

SMA type 0 life expectancy is short. Infants typically die at birth or within six months of life.

Babies with SMA type 0 have severe muscle weakness, poorer muscle tone, and are unable to develop age-appropriate motor skills. Severe respiratory problems are common in infants with type 0 SMA.

New medications and gene therapy are helping to prolong life in other types of SMA. But they tend to work best when disease symptoms are not already severe, as they are in infants with SMA type 0.

SMA type 1 life expectancy

Also known as Werdnig-Hoffmann disease, SMA type 1 is usually diagnosed in the first six months of life. It is the most common form of SMA.

Without medical support, SMA type 1 life expectancy is short, with infants having an average lifespan of about 2 years of age.

Infants can’t sit independently, and when lying down, their legs may adopt a frog-like position.

Along with lack of muscle tone, SMA type 1 babies may develop bulbar weakness and be unable to suck and swallow, potentially leading to aspiration (food or liquid in the lungs) and nutritional deficiencies.

Disease-modifying therapies have changed the prognosis of infants born with SMA type 1. These treatments have been shown to improve motor function — allowing children to reach certain motor milestones — and to increase their life expectancy.

SMA type 2 life expectancy

Also known as Dubowitz disease, SMA type 2 is usually diagnosed between ages 6 and 12 months. It is the second most common form of SMA.

Around 70% of the people with SMA type 2 live until age 25. Their life expectancy often is shortened by respiratory problems.

Children reach motor milestones, including being able to sit with assistance. But by their teenage years, many have increased muscle weakness. They also may develop progressive respiratory weakness and scoliosis, a sideways curvature of the spine.

Disease-modifying therapies can lengthen survival and improve the motor abilities of people living with SMA type 2.

SMA type 3 life expectancy

Also known as juvenile SMA or Kugelberg-Welander syndrome, SMA type 3 is a milder form of SMA. Its symptoms may appear anywhere between 18 months of age and the end of adolescence.

People with SMA type 3 have a normal life expectancy.

Toddlers may start walking, but progressive muscle weakness, particularly in the legs, causes some children to eventually lose the ability to walk independently. Generally, a later age at disease onset correlates with less severe symptoms.

Spinraza, Evrysdi, and Zolgensma have been shown to slow disease progression in people living with SMA type 3.

SMA type 4 life expectancy

SMA type 4 is the mildest form of the disease and one of the rarest, affecting fewer than 1% of everyone living with SMA. Type 4 symptoms often begin in adulthood, usually after the age of 35.

Life expectancy with SMA type 4 is normal.

SMA type 4 is characterized by muscle weakness, with the legs usually being affected first. People are able to walk, but may require a cane or wheelchair in the latter decades of life.

Some SMA treatments may slow the progression of SMA type 4 and delay new symptoms from developing.

Other types

In addition to the five main types of SMA that are caused by mutations in the SMN1 gene, several other rare types of SMA are known. Life expectancy varies based on the type.

X-linked infantile SMA

X-linked infantile SMA mainly affects boys. It is caused by mutations in UBA1, a gene located on the X chromosome.

X-linked infantile SMA symptoms include difficulty breathing, along with muscle weakness and contracture, a tightening of muscle, tendons, or skin leading to misshapen joints.

Life expectancy for those with X-linked infantile SMA is poor, with most not surviving beyond early childhood.

SMA with respiratory distress type 1 (SMARD1)

SMA with respiratory distress type 1, or SMARD1, is caused by mutations in the gene IGHMBP2. 

Babies with this very rare form of SMA usually have difficulty breathing and begin to show muscle weakness in the first months of life.

Without mechanical ventilation, infants with SMARD1 usually die in the first year of life. Those who live beyond that age usually require a tracheostomy, an opening into the windpipe to facilitate breathing, and are dependent on a ventilator to breathe.

SMA with lower extremity predominance (SMA-LED)

SMA with lower extremity predominance (SMA-LED), caused by mutations in either the DYNC1H1 or BICD2 gene, usually starts in infancy or early childhood. It progresses slowly, weakening the lower limbs, especially the muscles of the thigh.

Most people with SMA-LED have a normal life expectancy. Later in life, some may need support to walk and get around, such as braces or a wheelchair.

Finkel type SMA

Finkel type SMA (FSMA) is an adult-onset form of SMA that is caused by mutations in the VAPB gene.

People with FSMA generally have a normal life expectancy. They may need support or assistance with daily activities — such as walking — as they age and muscles start to weaken.

Kennedy's disease

Spinal and bulbar muscular atrophy, also known as Kennedy’s disease, is a rare adult-onset form of SMA that causes muscle weakness and atrophy in the face and throat, as well as the limbs. Kennedy's disease is associated with a defect in the AR gene, located on the X chromosome, and as such, it usually affects males.

With treatment, someone with Kennedy’s disease can have a nearly normal life expectancy.

Risk factors affecting prognosis

Some risk factors can affect a person's prognosis, or the likely course of their SMA.

For example, an early diagnosis and the use of disease-modifying treatments early in the disease course are key for someone with SMA, especially types 1 and 2.

These treatments may help extend life expectancy and change the natural history of the disease.

However, treatments generally do not help everyone in the same way, as a patient’s response to them can differ. Those who respond well to treatment typically have a better prognosis.

SMA also is associated with medical complications such as respiratory failure, gastrointestinal problems, and orthopedic issues, which can affect a person's quality of life and potentially be life-threatening. The management of these health complications is important for a better prognosis.

Respiratory failure, in fact, is the most common cause of death in SMA types 1 and 2. Noninvasive ventilation, including a BiPap machine, cough assist machine, and high-frequency chest wall oscillation, can be used to help clear mucus from the lungs. Sometimes mechanical ventilation and a tracheostomy may be needed.

Gastrointestinal issues may include difficulty swallowing, which can lead to a failure to thrive in babies. Other complications include constipation, delayed gastric emptying, and reflux.

Orthopedic complications can include scoliosis, hip subluxation, and being prone to fractures. Treatment for these conditions may require physiotherapy, orthotics such as braces, and mobility aids.

Studies also suggest that factors such as being ambulatory and having a higher copy number of the SMN2 gene are associated with a better prognosis and life expectancy.

Improving life expectancy

Treatment with gene therapy and disease-specific medications has the potential to extend SMA life expectancy and improve quality of life. Starting treatment as early as possible, preferably before symptoms begin, is key for achieving the best outcome.

Biogen’s Spinraza was the first disease-modifying therapy approved to treat the underlying cause of SMA. It works by boosting the ability of the SMN2 gene to make enough of a functional SMN protein.

Pharmaceutical company Roche and subsidiary Genentech’s Evrysdi is a disease-modifying medication, taken daily at home, that’s approved for children and adults. Also working through the SMN2 gene, it has the potential to restore the production of functional SMN throughout the body.

Zolgensma is a one-time gene therapy originally developed by AveXis, now owned by Novartis. Zolgensma delivers a working copy of the SMN1 gene to the body’s cells. It is used to treat children with the main types of SMA up to the age of 2.

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