Spinal Muscular Atrophy (SMA)

What is spinal muscular atrophy?

Spinal muscular atrophy (SMA) is a rare hereditary genetic condition in which muscles throughout the body are weakened because nerve cells in the spinal cord and brainstem do not work properly. SMA is the number one genetic cause of infant mortality.

There are four types of SMA:

  • Type 1 is the most common and severe form of SMA. It’s sometimes called Werdnig-Hoffmann disease or infantile-onset SMA. Children with type 1 have limited movement, can’t sit without support, and have trouble breathing, feeding, and swallowing. Symptoms begin at birth or within the first six months of life. Many children with type 1 do not live past age 2.
  • Type 2 is an intermediate form of SMA. Children with this type may sit without support at some point but cannot walk on their own. Symptoms typically start between 6 and 18 months of age. Depending on the severity of symptoms, children with type 2 may have a normal life span.
  • Type 3 is a milder form of SMA. It’s also known as Kugelberg-Welander disease and resembles muscular dystrophy. While children with type 3 usually are able to walk, most have some difficulty walking. Some may eventually need to use a wheelchair. Symptoms usually appear around 18 months of age or in early childhood. Children with this type of SMA generally have an almost normal life expectancy.
  • Type 4 is very rare. It usually starts in young adulthood, and causes mild motor impairment.

What are the symptoms of spinal muscular atrophy?

Symptoms of spinal muscular atrophy (SMA) vary greatly depending on the type. Children with the more serious form of SMA (Type 1) usually have more severe symptoms that are noticeable within the first six months of life. Those with milder forms have less severe symptoms that may not be noticeable until the child is 18 months or older.

Symptoms of SMA may include:

  • muscle weakness and decreased muscle tone
  • limited mobility
  • breathing problems
  • problems eating and swallowing
  • delayed gross motor skills
  • spontaneous tongue movements
  • scoliosis (curvature of the spine)

What are the causes of spinal muscular atrophy?

SMA is an autosomal recessive disease. This means that (most of the time) both parents must carry the genetic mutation for a child to have the condition.

The gene affected in SMA is the “survival of motor neuron” gene (SMN1 and SMN2). In 95 percent of SMA cases, both copies of the SMN1 gene are missing. All people with SMA have a number of copies of the SMN2 gene. But the SMN2 gene produces only a small amount of functional SMN protein; the more copies of the SMN2 gene a child has, the milder the disease.

If someone in your family has SMA, your chance of being an SMA carrier significantly increases. When both parents are carriers, there is a 1 in 4 (25 percent) chance with each pregnancy that they will have a child with SMA.

How we care for spinal muscular atrophy

The Spinal Muscular Atrophy Program at Boston Children’s Hospital brings together a team of experts from different specialties experienced in caring for children with SMA. We hold a dedicated SMA clinic once or twice a month so that your child can receive all of their SMA-related care in a single visit. This approach means that our various specialists can all work closely with your family to make sure your child’s care is well coordinated. Our program offers several therapy choices, including gene therapy, for SMA.