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A toddler learning how to walk with its parent holding its hands

Everything You Need to Know About the Different Types of Spinal Muscular Atrophy

What is Spinal Muscular Atrophy?

Spinal muscular atrophy (SMA) is a genetic motor neuron disease that makes it difficult to utilize the body’s muscles. Most often found in babies and small children, SMA involves the nerve in the spinal cord that directs muscle movement.

In SMA, a breakdown in communication between the nerve cells in the spinal cord and the muscles occurs, preventing them from moving appropriately. Children with SMA may have trouble with major developmental milestones like sitting up or walking. Due to SMA, the child’s muscles can become weak and start to become smaller, which can lead to muscle atrophy.

Causes of Spinal Muscular Atrophy


Because SMA is a genetic disease, it is inherited from a child’s parents. The severity of the disease depends on the various gene combinations that may occur with motor neuron genes.

There are genes that cause the deficiency of the “survival of motor neuron” gene (SMN). The SMN protein is vital to motor neuron function, which is what causes muscles to move correctly. Genetic mutations can cause varying degrees of SMA.

Symptoms and Classification

The most common type of SMA is chromosome 5 SMA, or SMN-related SMA. Because there is a wide range of differences in symptoms and disease progression, a classification system is used. Below we will outline types 0 to 4.

Type 0

At birth, a child with SMA shows severe weakness, lack of reaction to stimuli and facial paralysis. Sadly, children with type 0 SMA die by the time they are six months old.

Type 1

SMA symptoms seen at birth or within six months are classified as having type 1 SMA, which is also known as infantile-onset SMA or Werdnig-Hoffman disease. These infants show muscle weakness throughout their body, have difficulty breathing and cannot cry loudly. They are also unable to sit up without assistance and may have trouble while feeding.

Type 2

When SMA symptoms start between 3 to 5 months, it is classified as type 2 SMA. Typically, the lower limbs are more affected than the upper limbs. The face is not commonly affected though, which allows these infants to have full muscle control of their face and eyes.

Type 3

Type 3 SMA accounts for 30% of all diagnosed cases of SMA. Symptoms start to show at about 18 months of age. Children with type 3 SMA usually find climbing stairs difficult and may fall often. These patients may require a wheelchair as they get older, and may develop respiratory muscle weakness and scoliosis.

Type 4

Accounting for less than 5% of all cases, people with type 4 SMA are considered to have mild forms of SMA. For these people, their lifespans are average, and they can move independently throughout their entire life.

SMA Not Linked to Chromosome 5

Other forms of SMA exist which are not related to a lack of SMN protein. These SMA disorders vary significantly in terms of severity and muscle response. An example is SMA with respiratory distress (SMARD), which is a rare form of SMA caused by gene defects. Babies with SMARD may have respiratory problems and muscle weakness.

Treatment Options

Treatment options for SMA depend on the classification level and type of symptoms that are displaying.

For example, assisting children with breathing is a priority. Helping them to breathe may require ventilation through a machine. Respiratory secretions must be removed from the respiratory tract to avoid infections. Getting a flu shot is also encouraged every year to minimize the possibility of getting a respiratory infection.

If an infant or child cannot swallow or suck well enough to meet their nutritional needs, a feeding tube may be necessary. A speech-language pathologist can also help develop the ability to swallow or speak. For children with spinal issues, a back brace or spinal surgery can help alleviate symptoms.

Currently, there is no nutritional treatment that has been identified to treat SMA. However, a nutritious diet, in moderation, can help keep weight under control. Often, the lack of exercise due to muscle weakness can lead to weight gain. Because of this, the patient’s caloric intake may need to be adjusted in order to keep weight gain to a minimum.

For those with SMA who can move around independently, maintaining mobility is a priority. Assistive devices and mobility aids can help people of all ages function independently. Physical therapy is another treatment that may help children and adults strengthen and utilize the muscle control they do have.

In recent years, drug companies have developed gene modifying therapies, such as Spinraza (nusinersen) and Zolgensma (onasemnogene abeparvovac-xioi). These drugs directly address the gene component of the disease. They provide hope for parents and those struggling with SMA by improving both the quality and length of life.

Genetic Counseling

Despite having a genetic cause at its core, not all forms of SMA have the same genetic inheritance patterns. Knowing what genes are affected and how they can affect offspring may impact family planning decisions.

Consulting with a physician and genetic counselor may provide peace of mind.