Spinal muscular atrophy (SMA) is an inherited neuromuscular disorder. It affects the nerves that control muscle movement. There are five main types of SMA: types 0, 1, 2, 3, and 4 — along with a few rare forms. These types are based on when symptoms first appear, how serious they are, and which mutations (gene changes) cause them.

Note that the types of SMA were established before disease-modifying therapies became available. Disease-modifying therapies are treatments that change the course of the disease rather than just easing symptoms. Early treatment can improve symptoms and outcomes. As research continues, the way SMA types are described may change based on how treatment affects people throughout their lives.

In this article, we’ll cover the different types of SMA and how they can affect daily life, mobility, and overall health. If you have any questions about your own or a loved one’s SMA diagnosis, talk to a pediatric neurologist or genetic counselor.

SMA Types 0 to 4

Types 0 through 4 of 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 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.

Understanding the specific type of SMA that affects you or your loved one is important for tailoring care and managing the condition effectively.

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.

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, the prognosis (outlook) for children with SMA is improving.

Type 0: Prenatal SMA

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). These infants may have physical deformities due to limited movement in the womb. They may also have weak respiratory systems, making it difficult for them to breathe on their own. Infants with type SMA type 0 will not reach motor milestones, such as lifting their heads.

Types of SMA were established before disease-modifying SMA treatments became available. Early treatment improves symptoms and outcomes.

If untreated, the life expectancy for an infant with SMA type 0 is short, with most not surviving beyond a few months. Early screening and identification are important and may affect the prognosis.

Type 1: Werdnig-Hoffmann Disease

SMA type 1 is also called Werdnig-Hoffmann disease. This type accounts for approximately 45 percent of cases of SMA. Type 1 affects babies, and symptoms begin in the first six months of life. They may have one or two copies of the SMN2 gene, which makes it harder to produce the SMN protein needed for nerve cells that control movement.

Babies with SMA type 1 may struggle to suck, swallow, and breathe. This makes them more likely to have respiratory infections. Because of their poor muscle tone, these infants will not be able to lift their heads or sit up by themselves.

Life expectancy for babies with SMA type 1 is often related to how well they can breathe. Without disease-modifying therapies, 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 reach motor skill milestones, such as sitting or standing independently. However, intellectual development is not affected.

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.

Before SMA treatment became available, individuals with SMA type 2 lived longer than those with types 0 and 1 — often into their 20s or 30s. Treatment with disease-modifying therapies has improved outcomes.

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. Type 3b includes children and adolescents who develop symptoms between age 3 and early adulthood. 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, unlike untreated children with types 0 to 2. However, the first signs of the condition may include frequent falls, difficulty climbing stairs, and general muscle weakness. People with SMA type 3 usually do not have serious breathing problems, which are more common in types 0 to 2.

Many people with type 3 can still walk, although they may have an unusual walking pattern and foot deformities. Some may eventually need to use a wheelchair or require help with getting dressed or using the bathroom. However, life expectancy for those with type 3 is typically the same as that of the general population.

Type 4: Adult-Onset SMA

Type 4 SMA is the mildest form of SMA and accounts for approximately 5 percent of SMA cases. People 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.

Most individuals with type 4 are able to stay mobile throughout their lives.

Life expectancy for people with type 4 is about the same as for most people. Most individuals with type 4 are able to stay mobile throughout their lives.

Other Types of Spinal Muscular Atrophy

The following SMA types are rare and are associated with different genetic mutations than those found in types 0 to 4. Each type can differ widely in how severe it is, how it affects daily life, and how long a person may live.

SMA With Respiratory Distress

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

SMARD tends to progress quickly in the first two years of life. After this period, the disease may stabilize. However, SMARD still often results in a reduced life expectancy.

Distal SMA

Distal SMA can be passed down in different ways, depending on the gene involved. Some types are inherited from one parent, and others need a gene change from both parents. It mainly affects the hands and feet. Hand cramping in cold weather is often the first sign of distal SMA. 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. According to the National Organization for Rare Disorders, it mostly affects males. The first symptoms usually appear between the ages of 20 and 50, often including hand tremors and muscle twitching. As the disease progresses, muscle weakness develops in the arms and legs, which can make it harder to move around. 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. However, some may develop serious complications later in life, such as aspiration pneumonia.

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. Seizures often start after muscle symptoms appear.

SMA-PME progresses rapidly in early childhood, leading to seizures, falls, tremors, and loss of mobility. Most children with SMA-PME do not live beyond late childhood or early adulthood because of respiratory complications.

SMA With Lower Extremity Predominance

SMA with lower extremity predominance (SMA-LED) causes muscle weakness in the lower limbs, especially the quadriceps (the muscle at the front of the thigh). It also leads to muscle wasting, or loss of muscle tissue. This weakness can cause difficulty walking and joint deformities in the lower body. Some people may also experience weakness in their arms.

Symptoms of SMA-LED typically appear in infancy or early childhood. About 25 percent of those affected develop symptoms in adulthood. Unlike other forms of SMA, SMA-LED is inherited in an autosomal dominant pattern. This means it’s 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 usually appears in infancy. Babies with XL-SMA experience significant muscle weakness, absent reflexes, and joint deformities like contractures, which cause stiffness in the joints. 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 caused by respiratory failure. Most children do not survive beyond early childhood, but in rare cases, some have lived into their teen years.

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.

If you have questions about a diagnosis or the best care plan, talk with your healthcare professional. They can guide you on your journey with SMA.

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