What is Spinal Muscular Atrophy (SMA)?

What is Spinal Muscular Atrophy (SMA)?

Spinal muscular atrophy (SMA) is a group of hereditary diseases that affects the central nervous system, peripheral nervous system, and skeletal muscle movement. In SMA, the nerves in your brain and spinal cord that control muscle strength and movement break down. As a result, they can’t send signals from your brain to your muscles to make them move, making muscles weaken and shrink. This causes problems with speaking, walking, breathing, and swallowing.

Types of Spinal Muscular Atrophy


There are five different types of SMA. They are broken down by severity and the age when symptoms begin.

  • Type 0. Called prenatal SMA. The first sign is decreased fetal movement in the weeks before delivery. At birth, the baby has trouble breathing, eating, and may have joint and heart problems. The infant typically requires breathing and feeding tubes and usually only survives a few months.
  • Type 1. Also known as Werdnig-Hoffmann disease or infantile-onset SMA, SMA Type 1 is the most common condition. It affects about 60% of babies born with SMA. It’s usually diagnosed during an infant’s first six months. Babies with SMA Type 1 face many physical challenges, including trouble breathing, coughing, and swallowing, and may require breathing assistance or a feeding tube.
  • Type 2. This intermediate form of SMA affects about 30% of people with the condition. It’s usually diagnosed between six months and two years old. The first sign is that a baby is delayed in meeting motor milestones or misses them entirely. People with SMA Type 2 can usually sit up alone but can’t stand or walk independently.
  • Type 3. Also known as Kugelberg-Welander disease or juvenile SMA, Type 3 is usually diagnosed between 18 months and three years old, although sometimes it’s not picked up until adolescence. It affects about 10% of people with SMA. Children with Type 3 can walk initially, but lose this skill as they grow older and eventually require a wheelchair.
  • Type 4. Also known as late-onset SMA, this condition is the rarest form and affects less than 1% of patients. It usually begins in adulthood, after the age of 35. It causes mild motor impairment, and most people can still walk.

There are some conditions that are similar to SMA but are caused by mutations in different genes. They include:

  • SMA Respiratory Distress (SMARD). This affects the upper spinal cord and causes severe respiratory distress.
  • Kennedy’s disease. Since this is an X-linked genetic disease, it only affects males. It typically appears during adulthood.

Signs & Symptoms of Spinal Muscular Atrophy


The signs and symptoms of spinal muscular atrophy (SMA) vary based on the stage and severity of the disease. These symptoms typically develop without treatment or when a patient receives only supportive care.

Type O

Mothers of children who have SMA Type 0 often report a decrease in fetal movement in late pregnancy and give birth early. Symptoms for SMA Type 0 in their babies include:

  • Severe weakness
  • Respiratory failure
  • Significant contractures (arthrogryposis)
  • Hypotonia (low muscle tone)
  • Heart defects
  • Facial paralysis
  • Lack of reaction to stimuli

Type 1

These severe symptoms and signs are usually present at or shortly after birth for children with SMA Type 1:

  • Weakness
  • Difficulty breathing
  • Difficulty feeding (sucking and swallowing)

Babies with SMA Type 1 are rarely able to sit up on their own. With new medications and technology, however, the survival rate of babies with SMA Type 1 is improving. Children can survive for a number of years with supportive care like ventilators and feeding tubes, as well as gene therapy.

Type 2

Babies with SMA Type 2 have normal cognition and can sit independently, but are rarely able to stand or walk. Other symptoms include:

  • Muscle weakness close to the center of the body, particularly in the lower limbs
  • Trouble breathing during sleep

Type 3

SMA type 3 usually shows up in older childhood and adolescence and includes:

  • Trouble walking and running, climbing steps, and rising from a chair
  • Hand tremors
  • Scoliosis
  • Joint contractures, or chronic shortening of muscles and tendons around joints
  • Respiratory infections

Type 4

Most SMA Type 4 symptoms begin to develop after the age of 35 and include:

  • Muscle weakness in the legs and hips
  • Muscle twitching and aching
  • Hand and finger tremors

Most people with SMA Type 4 remain mobile into their 60s, after which they may need walking aids like a cane.

What Causes Spinal Muscular Atrophy?


Spinal muscular atrophy is a genetic condition. SMA is caused by a gene mutation of the survival motor neuron 1 gene (SMN1) on the fifth chromosome. This prevents the gene from producing enough of a protein called survival motor neuron (SMN), which helps your muscles function normally by allowing them to receive nerve signals. When you have low levels of the SMN protein, you lose motor neurons in your brain and spinal cord, which causes muscles to weaken and shrink. This happens mostly in your trunk, upper arm, and leg muscles, but can also occur in your hands and feet.

SMA is an autosomal recessive disease, which means that a child with this condition inherits a mutated SMN1 gene from both parents. Those who have just one mutated gene are carriers of SMA. This means that they won’t have any symptoms of the disease, but they can pass it on to their children.

Risk Factors

Risk Factors

A child is at increased risk of SMA if there is a family history of the condition. However, you can still develop SMA even if you have no family history. A parent or parents can be silent carriers of the SMA gene mutation, and thus have no symptoms. If both parents are carriers, they have a one in four chance for a child with SMA with each pregnancy. About one in 50 people are genetic carriers for SMA.



Even if it’s well-controlled, SMA can lead to complications. These include:

  • Pneumonia. Children with SMA often have weak, underdeveloped lungs. As a result, it’s much harder for them to clear their airways of mucus. Chest physical therapy, and devices like the Cough Assist, which forces a cough, can help.
  • Scoliosis. More than half of all patients with SMA Type 1 or SMA Type 2 will go on to develop scoliosis, a sideways curvature of the spine. Treatment often involves physical therapy, spinal bracing, and occasionally surgery.
  • Bone fractures. Muscle atrophy and chronic immobility may lead to low bone mineral density. One study of children with SMA found that about two-thirds had low bone density, and about 2.5% had osteoporosis.
  • Heart problems. Children with more severe forms of SMA, like SMA Type 1, are more likely to have heart defects, while those with Type 2 or 3 are more likely to have heart rhythm problems.



SMA is an inherited disease. If you and your partner carry the SMA gene mutation, then each child you conceive will have a one in four risks of being born with the condition. A genetic counselor can explain the chances of your child having SMA, or of being a carrier.

If both you and your partner are carriers, you can talk to your medical provider about preimplantation genetic diagnosis (PGD), which identifies embryos that don’t have the mutated gene. The healthy embryos are then implanted during in vitro fertilization (IVF). You can learn more about genetic testing and counseling at New York-Presbyterian here.

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