Spinal muscular atrophy: cause, types, life expectancy

Definition: what is spinal muscular atrophy?

Spinal muscular atrophy is a genetic disease who touches the motor neuronsthat is to say nerve cells that control muscles. “In the absence of these motor neurons, the disease develops and causes muscle weakness even muscle wasting“, emphasizes Dr. Talal Mahfouz. This disease can start at birth, in childhood or in adulthood. There are three forms of spinal muscular atrophy depending on the muscles affected:

• spinal muscular atrophy proximal
• spinal muscular atrophy distal
• disuse atrophy bulbospinalis

What is Proximal Spinal Muscular Atrophy?

This amyotrophy first affects the proximal muscles, that is to say the muscles close to the trunk such as those of the shoulders and hips. It is due to homozygous deletions of the SMN1 gene encoding the motor neuron survival protein SMN. “A second SMN2 gene has also been identified. It only contributes to produce 10% of the complete SMN protein. Although there are variations, theAmyotrophy is all the less severe as the number of copies of the SMN2 gene is high. Patients with three or four copies more often have amyotrophy type 3 or 4 than type 1 amyotrophy (see below). This influences the severity of the disease.”

What is Distal Spinal Muscular Atrophy?

Distal spinal muscular atrophy affects the distal muscles, i.e. muscles away from the trunk such as the extremities (hands and feet), also due to a deletion of the SMN1 gene. “The majority of distal spinal muscular atrophies are manifested by muscle weakness in the leg and ankle muscles. This is why doctors also speak of tibio-peroneal amyotrophies“, says Dr. Mahfouz.

What is bulbospinal amyotrophy?

Bulbospinal muscular atrophy affects the bulbar muscles, that is to say the muscles of the face and throat. This is the case in the disease of kennedy for instance.

What causes spinal muscular atrophy?

Spinal muscular atrophy is due to a SMN1 gene mutation located on chromosome 5. These mutations are the source of a nerve cell degeneration that we find in spinal cord : them motor neurons. These control muscle movements. During spinal muscular atrophy, the motor nerves are affected and disappear. Thus the muscles atrophy, preventing the movements from being well executed.

How many cases in adults?

Type 4 spinal muscular atrophy, found in adults, is said to be quite rare and affects around uno one out of 300,000.

How many cases in children?

Infantile spinal muscular atrophy (type 1, 2 and 3) would affect 6,000 to 10,000 children at birth. In France there would be approximately 120 new cases of spinal muscular atrophy per year and there would be between 1,500 and 2,500 sick people, all types combined.

What are the symptoms of spinal muscular atrophy?

Basically, the symptoms of spinal muscular atrophy are:

• muscle weakness
• A lack of reflexes
• Swallowing difficulties
• breathing difficulties
• An inability to crawl or walk.

What are the different types of spinal muscular atrophy?

There are four types of spinal muscular atrophy.

► Type 1 called infantile spinal muscular atrophy or Werdnig-Hoffman disease. This form affects the baby from birth.

► The type 2, shape intermediate of Dubowitz’s disease, affects the baby between 6 and 18 months.

► The type 3called juvenile form or Wohlfart-Kugerlberg-Welander disease, begins between 18 and 24 months of age. “The disease is often discovered during the acquisition of walking.

► The type 4 is, unlike the other three types, a form of spinal muscular atrophy that affects adults. In this case, the disease progresses more slowly.

Diagnosis of the form of amyotrophy is based on “history and clinical examination and can be confirmed by genetic testing. An exam electromyographic and an muscle biopsy can also be done.”

Treatment: can spinal muscular atrophy be treated?

“Unfortunately, there is still no treatment to cure spinal muscular atrophy. On the other hand, physiotherapy and balneotherapy can be useful to relieve the patient”. explains Dr. Mahfouz.

What is the life expectancy in case of spinal muscular atrophy?

Types 1 and 2 amyotrophies significantly reduce the life expectancy of children whose death often occurs because of a respiratory failure. People with amyotrophy type 3 may have a normal life expectancy but much depends on the development of breathing difficulties. Finally, for spinal muscular atrophy type 4, life expectancy would not be reduced.

Thanks to Dr. Talal Mahfouz, neurologist.