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Is Spinal Muscular Atrophy a Motor Neuron Disease?

Spinal muscular atrophy (SMA) is a type of genetic condition that affects the nerve cells
Spinal muscular atrophy (SMA) is a type of genetic condition that affects the nerve cells

Spinal muscular atrophy (SMA) is a type of genetic condition that affects the nerve cells regulating the muscles that help move around (motor neurons), resulting in weakness and wasting (atrophy) of these muscles. This motor neuron disease varies in its onset and severity. Accordingly, it has been classified into five types:

  1. SMA type 0
  2. SMA type 1
  3. SMA type 2
  4. SMA type 3 
  5. SMA type 4

The characteristic feature of all these types is the same, muscle weakness. The muscle weakness is more pronounced in the muscles that are closer to the center of the body, such as the neck, hips, and back. As a result, the child finds it difficult to perform activities, such as holding the neck, walking, sitting, and climbing stairs.

What causes spinal muscular atrophy?

Spinal muscular atrophy (SMA) is a genetic condition, and it is typically inherited from the parents. The problem happens in the SMN (1 and 2) gene. This gene is responsible for controlling the motor neurons that play a vital role in contracting and relaxing the muscle. Once the brain decides what to do, the motor neurons receive this information from the brain and transmit it to the spinal cord. The spinal cord then transmits the impulse to the muscle to make movements. Problems in the gene cause the motor neurons to break down, leading to disruption in these signals and difficulty/inability in moving the muscles. The muscles of the arms, legs, and trunks get affected. In severe cases, muscles involved in breathing and swallowing may also get affected.

The child can get the condition only if both the parents have a problem with the SMN gene. However, even if the parents have this genetic issue, there is only a 25% chance that the child will inherit the condition from them.


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How is spinal muscular dystrophy diagnosed?

If an expecting couple, both of whom are affected with spinal muscular dystrophy wants to know whether their fetus (baby in the womb) has inherited the condition from them, their doctor will ask for amniocentesis. Amniocentesis involves removing a sample of amniotic fluid from the womb of the mother and checking it for spinal muscular dystrophy through molecular genetic testing. Molecular genetic testing is a test done on the blood, hair, or skin to check if the baby has spinal muscular dystrophy. It may also be done if the doctor suspects the baby’s movements have slowed down.

Other tests that the doctor can use to arrive at the diagnosis include:

  • Muscle biopsy (removal of a small piece of muscle to examine it under a microscope)
  • Electromyography (checking the electrical activity of the muscle in response to nerve stimulation)

What is the treatment of spinal muscular dystrophy?

The United States Food and Drug Administration has approved three medications for the treatment of spinal muscular dystrophy. These are:

  • Spinraza (nusinersen): An injection administered into the fluid surrounding the spinal cord.
  • Zolgensma (onasemnogene abeparvovec-xioi): Gene therapy that is given in the form of intravenous injection.
  • Evrysdi (risdiplam): Available in the form of an oral pill.

With the use of these medications, doctors have seen improvements in the muscle weakness of the affected children. However, the long-term effects of these medicines are unknown and research is on to know the same.

Other supportive treatments help the patients with spinal muscular dystrophy by providing comfort and helping them lead better lives. These include:

  • Powered wheelchairs
  • Ventilator (breathing device)
  • Occupational therapy
  • Physical therapy (water therapy, wheelchair sports)
  • Modified access to computers

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