How Common Is Spinal Muscular Atrophy?

What is spinal muscular atrophy?

Spinal muscular atrophy (SMA) is a hereditary (running in families) disorder that progressively destroys the motor nerve cells (neurons) leading to muscle weakness and wasting (atrophy).
Spinal muscular atrophy (SMA) is a hereditary (running in families) disorder that progressively destroys the motor nerve cells (neurons) leading to muscle weakness and wasting (atrophy).

Spinal muscular atrophy (SMA) is a hereditary (running in families) disorder that progressively destroys the motor nerve cells (neurons) leading to muscle weakness and wasting (atrophy). This makes it hard for children to use their muscles. Because of the breakdown of the brain and spinal cord nerve cells, the brain stops sending messages that control essential skeletal muscle activity such as speaking, walking, breathing, and swallowing.

This muscle weakness usually worsens as the baby grows older. SMA is divided into subtypes (Types 0, 1, 2, 3, and 4) based on the age of onset, severity of disease, and maximum function achieved.

How common is spinal muscular atrophy?

Spinal muscular atrophy (SMA) affects 1 in 8,000-10,000 live-born children worldwide. SMA type 1 is the most common, accounting for about half of the cases. Types 2 and 3 are also common, whereas types 0 and 4 are rare.

What causes spinal muscular atrophy?

Spinal muscular atrophy (SMA) is generally passed down through families. The child may get it through

  • Faulty genes: A child may get SMA through the two copies of a broken gene, one from each parent. SMA types 0, 1, 2, 3, and 4 are inherited (running in families), and this genetic trait (condition) can be passed down from the parent to child, known as an autosomal recessive genetic disorder. If a child gets faulty genes from just one of the parents, they may not get SMA but will be a carrier of the disease. Therefore, when these children grow up, they could pass the broken faulty genes to their children and those children get SMA.
  • Mutation: Abnormal changes occur in SMN 1 and SMN 2 genes that are located on chromosome 5 (microscopic threadlike part of the cell that carries hereditary information). This may happen because of a lack of a specific type of protein called the survival motor neuron protein. Its absence may cause the death of the neurons (nerve cells) that control the muscles.


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What are the symptoms of spinal muscular atrophy?

In spinal muscular atrophy (SMA), the child’s muscles get weak and contract. The child can have difficulty in controlling their head movement, sitting without help, and walking. Some children may have swallowing difficulty and breathing problems.

Symptoms of SMA may vary a lot, depending on the types of SMA and severity:

Type 0: SMA type 0 is mostly diagnosed before birth in the mother’s womb and is the rarest and most severe form of SMA. Infants with SMA move less in the womb and therefore, are often born with joint deformities such as contractures or hypotonia (extremely weak muscle tone). They often do not survive because of respiratory failure or heart defects since birth.

  • Type 1: This SMA type is also called Werdnig-Hoffmann disease. This is the most common and severe form of the disorder with muscle weakness being evident at birth or within the first few months of life. Most affected children cannot control their head movement and have swallowing problems, breathing problems, and poor growth. They may have floppy limbs. They may have an abnormal bell-shaped chest that prevents the lungs to fully expand. Most children with SMA type 1 do not survive past the age of 2 years because of breathing difficulty.
  • Type 2: This SMA type is also called Dubowitz disease or chronic infantile SMA. It is most common in children between 6 and 18 months of age. These children can sit without support, although they may need help to get to a seated position. Muscular weakness may worsen later in childhood, and they may need support to sit. They cannot stand or walk without support and may also have involuntary tremors, scoliosis (side-to-side curves on a spine), and breathing difficulty that can be fatal. The life span of a person with SMA type 2 varies. Mostly, they live up to their 20s or 30s. Symptoms range from moderate to severe. Usually, the legs are more involved than their arms Therefore, the child may be able to sit and walk or stand with help.
  • Type 3: It is also called Kugelberg-Welander disease or juvenile SMA. It typically causes muscle weakness after early childhood at the age of 2-17 years. It is the mildest form of the disease. The child with this type of SMA can stand and walk unaided, but overtime, walking and climbing stairs may become increasingly difficult. Many children may require wheelchair assistance later in life. However, they may have a normal life expectancy.
  • Type 4: This is a rare form of SMA and often begins in early adulthood. Affected people usually have mild to moderate muscle weakness, tremors, twitching, and mild breathing difficulty. Usually, only the upper arm or legs are affected. These people generally have these symptoms throughout their life. However, they can live a normal life with exercise and physiotherapy. People with SMA type 4 mostly have a normal life expectancy.

There are many variations, types, and severity of SMA. There is no cure, but some symptoms may improve, helping your child live longer. Every child or adult with SMA will have different symptoms and severity. However, this disease doesn’t affect their intelligence, and they can socialize normally. Consult your doctor or neurologist for your child’s condition.

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NIH: U.S. National Library of Medicine


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