Spinal muscular atrophy (SMA) is an inherited neuromuscular disorder — meaning it affects the nerves that control muscle movement. It’s characterized by progressive muscle weakness (hypotonia). There are five main types of SMA — types 0, 1, 2, 3, and 4 — along with a few rarer forms. These types differ based on the age at which symptoms appear, the severity of those symptoms, and the underlying genetic cause.

In this article, we’ll cover the different types of SMA and how they impact daily life, mobility, and overall health. If you have any questions about your own or a loved one’s SMA diagnosis, a neurologist or genetic counselor who specializes in SMA can provide valuable guidance and support.

SMA Types 0-4

Types 0 through 4 of spinal muscular atrophy (SMA) belong to a group called 5q-SMA, which accounts for up to 95 percent of all SMA cases. These types of SMA are caused by mutations (variations) in both copies of the survival motor neuron 1 (SMN1) gene, which leads to a shortage of SMN proteins. These proteins are crucial for motor neurons — the nerve cells that help the brain and spinal cord communicate with the muscles.

A second gene, SMN2, produces a small amount of SMN protein, but the SMN1 gene is responsible for most of it. The number of SMN2 copies a person has can influence how severe their form of SMA is. More copies generally mean more SMN protein is produced, which can result in a milder form of the disease.

It's important to note that much of what we know about life expectancy and SMA symptom progression comes from studies done before the development of newer treatments for the condition. With the introduction of disease-modifying therapies (DMTs), the prognosis (outlook) for children with SMA is improving. Ongoing research and long-term studies will help us understand how these treatments may improve prognosis and slow disease progression.

Type 0

Type 0 is among the rarest and most severe forms of SMA. It affects babies before birth, while they are still in the uterus. Babies with SMA type 0 may have only one copy of the SMN2 gene, which produces very little of the SMN protein.

At birth, babies with SMA type 0 have hypotonia (extremely weak muscle tone) and may show physical deformities due to limited movement in the womb. These infants are often born in a breech position (feet facing downward, rather than the typical head-down position). They may have weak respiratory systems, making it difficult for them to breathe on their own. They may also experience facial paralysis and show minimal response to outside stimuli, such as sound, touch, or light. Infants with type 0 will not achieve motor milestones, such as lifting their heads.

The life expectancy for an infant with SMA type 0 is short, with most not surviving beyond 6 months of age.

Birth can be a medical emergency for these infants. If the baby is unable to breathe independently, a breathing tube may be required, and they will be admitted to a neonatal intensive care unit (NICU). If SMA type 0 is diagnosed before birth, a detailed birth plan can help prepare for immediate medical care. If the diagnosis is unknown at birth, further testing will be needed to determine why breathing difficulties are present.

Type 1 (Werdnig-Hoffmann Disease)

SMA type 1 — also called Werdnig-Hoffmann disease — accounts for approximately 45 percent of cases of SMA. Type 1 impacts babies and is usually diagnosed in the first six months of life. They may have one or two copies of the SMN2 gene, which limits their ability to produce the SMN protein needed for motor neuron function.

Babies with SMA type 1 have muscle weakness on both sides of their bodies, which affects their ability to suck, swallow, and breathe. This makes them more prone to respiratory infections, such as pneumonia. Because of their poor muscle tone, these infants are sometimes described as “floppy.” They will not be able to lift their heads or sit up independently.

Despite their severe physical challenges, cognitive development (how their brains work) in babies with type 1 is not affected. Many of these infants are alert, able to recognize their surroundings, and can express emotions through facial expressions.

Life expectancy for babies with SMA type 1 is often related to how well they can breathe. Without advanced respiratory support, many infants with type 1 do not survive beyond age 2.

Type 2 (Intermediate SMA/Dubowitz Disease)

Type 2 SMA, also known as intermediate SMA or Dubowitz disease, accounts for about 20 percent of SMA cases. Type 2 is diagnosed between the ages of 6 months and 18 months. The first noticeable sign is often a failure to meet motor skill milestones, such as sitting or standing independently. However, intellectual development is not affected, and babies with type 2 usually respond to stimulation and engage with those around them.

Children with SMA type 2 experience muscle weakness, especially in the lower limbs, and often lack reflexes. Most children with type 2 develop scoliosis (a curvature of the spine), which can make breathing more difficult as they grow. They are also prone to pneumonia and may have difficulty chewing and swallowing.

With proper care and therapies, individuals with SMA type 2 can live into adulthood. The median survival rate for people with SMA type 2 is 25 years, meaning half of people with the condition live to age 25 or older. Advanced medical support can help extend a person’s life.

Type 3 (Juvenile SMA/Kugelberg-Welander Disease)

Also known as juvenile SMA or Kugelberg-Welander disease, type 3 is a milder form of SMA that typically develops later in childhood or young adulthood. Approximately 30 percent of SMA cases are type 3. This type can be divided into two subgroups based on age of onset: Type 3a includes children who develop symptoms between 18 months and 3 years old, while type 3b includes children and young adults who develop SMA between 3 and 30 years old. Individuals with type 3 usually have three or four copies of the SMN2 gene.

Children and young adults with type 3 are able to stand and walk, although the first signs of the condition may include frequent falls, difficulty climbing stairs, and general muscle weakness. Muscle weakness typically affects both sides of the body and is more noticeable in the muscles closest to the center of the body, particularly in the legs. Unlike those with types 0-2, people with SMA type 3 generally do not experience significant breathing problems. Cognitive development is also unaffected.

Many people with type 3 remain mobile, though they may have an abnormal walking pattern (gait) and foot deformities. Some may eventually need to use a wheelchair or require assistance with daily activities such as dressing or using the bathroom. However, life expectancy for individuals with type 3 is typically the same as that of the general population.

Type 4

Type 4 SMA is the mildest form of SMA and accounts for approximately 5 percent of SMA cases. Individuals with type 4 often have four or more SMN2 gene copies. Symptoms usually begin after age 35, although they can sometimes appear as early as 18. Early signs of type 4 may include hand tremors, muscle aches or heaviness, numbness, and cramping.

Life expectancy for people with type 4 is comparable to that of the general population. Most individuals with type 4 are able to stay mobile throughout their lives.

Other Types of Spinal Muscular Atrophy

The following spinal muscular atrophy types are rare and are associated with different genetic mutations than those found in types 0-4. Like types 0-4, each of these forms can vary significantly in terms of severity, quality of life, and life expectancy.

SMA With Respiratory Distress

SMA with respiratory distress (SMARD) causes muscle weakness and severe breathing difficulties in infants. Early signs of SMARD include difficulty feeding, noisy or labored breathing, and frequent cases of pneumonia. Babies with SMARD may also experience diaphragm paralysis, which makes it difficult or impossible to breathe without the help of a ventilator.

SMARD tends to progress rapidly in the first two years of life. After this period, the disease may stabilize, and some children experience mild improvements in muscle strength and respiratory function, possibly due to the regeneration of some muscle fibers. However, SMARD still often results in a reduced life expectancy.

Distal SMA

Distal SMA is inherited from one parent, rather than both, and primarily affects the hands and feet. The first sign of distal SMA is often hand cramping in response to cold weather. Symptoms usually begin during adolescence but can also start in childhood or as late as a person’s 30s. Despite the muscle weakness, people with distal SMA typically have a normal life expectancy.

Kennedy’s Disease

Kennedy’s disease, also known as X-linked spinal and bulbar muscular atrophy, is an adult-onset form of SMA that affects males, according to BetterHealth Channel. The first symptoms usually appear between ages 20 and 50, often including hand tremors and fasciculations (muscle twitching). As the disease progresses, muscle weakness develops in the arms and legs, which can limit mobility. Weakness may also affect the face and tongue, leading to difficulties with speech and swallowing.

Kennedy's disease generally progresses slowly, and individuals with the condition usually have a normal life span.

SMA With Progressive Myoclonic Epilepsy

SMA with progressive myoclonic epilepsy (SMA-PME) is a rare form of SMA that causes muscle weakness and seizures. Most children with SMA-PME develop normally in early childhood before showing signs of lower limb weakness. Over time, muscle weakness spreads and causes breathing difficulties, with seizures often starting after muscle symptoms appear.

The disease progresses rapidly in early childhood, leading to seizures, falls, tremors, and loss of mobility. While some functions, like muscle strength and respiratory activity, may stabilize or mildly improve, most children with SMA-PME do not live beyond late childhood or early adulthood due to respiratory complications.

SMA With Lower Extremity Predominance

SMA with lower extremity predominance (SMA-LED) leads to muscle weakness and wasting in the lower limbs, particularly affecting the quadriceps (the muscle at the front of the thigh). This weakness can cause difficulty walking and lead to joint deformities in the hips, knees, ankles, and feet. Some people may also experience weakness in the upper limbs.

Symptoms of SMA-LED typically appear in infancy or early childhood, though about a quarter of those affected develop symptoms in adulthood. Unlike other forms of SMA, SMA-LED is inherited in an autosomal dominant pattern, meaning it is passed down from one parent. Despite the muscle weakness, people with SMA-LED have a normal life expectancy.

X-Linked Infantile SMA

X-linked infantile SMA (XL-SMA) affects only boys, according to MedlinePlus, and is usually apparent from infancy. Babies with XL-SMA experience significant muscle weakness, areflexia (absent reflexes), and joint deformities like contractures, which cause stiffness in the joints. In severe cases, infants may be born with fractures (broken bones). While some babies may develop motor skills like sitting, they often lose these abilities as the disease progresses.

Muscle weakness in the chest can lead to life-threatening breathing difficulties. Children with XL-SMA often have a shortened life expectancy due to respiratory failure, with many not surviving beyond early childhood, though rare cases have lived into adolescence.

Talk to Your Doctor

Understanding the specific type of SMA that affects you or your loved one is important for tailoring care and managing the condition effectively. Different types of SMA come with unique challenges, symptoms, and treatment options. Knowing the type can help you make informed decisions about medical care, therapies, and support services that improve quality of life.

If you have questions about a diagnosis or the best care plan, it’s essential to speak with a health care professional. They can guide you through treatment options, help manage symptoms, and connect you or your loved one with specialists who focus on SMA care.

Find Your Team

On mySMAteam, the social network for people with spinal muscular atrophy and their loved ones, more than 2,600 members come together to ask questions, give advice, and share their stories with others who understand life with spinal muscular atrophy.

Are you or a loved one diagnosed with SMA? What advice can you offer others? What questions to you have? Share your experience in the comments below, or start a conversation by posting on your Activities page.