Spinal Muscular Atrophy - Medical Experts

Spinal muscular atrophy (SMA) is the second most common recessively inherited disorder. Approximately one in every 6,000 newborns is affected. It affects boys and girls equally. 

One in every 35 to 40 people in the general population is a carrier of the SMA gene. These individuals have no symptoms because, in addition to the defective gene, they also have a second, intact gene. This intact gene compensates for the defect.

If both parents carry this gene and pass it on to their child, the child will develop SMA. The risk is 25 percent.

Spinal muscular atrophy is characterized by a loss of motor neurons in the anterior horn of the spinal cord. When these cells in the spinal cord are no longer functional, the muscles also cease to function properly. This leads to progressive muscle weakness and muscle wasting.

Patients with SMA suffer from muscle wasting, which can vary in severity @ Framestock /AdobeStock

The different types of spinal muscular atrophy are primarily distinguished by the age at which the disease begins:

• SMA Type I

50 to 60 percent of SMA patients have the severe infantile form of SMA (SMA Type I). With this form, the first symptoms appear as early as the first few months of life—and in some cases, even within the first few weeks. These children are generally unable to lift their heads and will never be able to sit up on their own. Without treatment, most die within their first year of life or before the age of 2. The cause is respiratory muscle failure.

• SMA Type II

The onset of the intermediate form (SMA Type II) occurs later. Children learn to sit independently but are unable to walk. Doctors usually make the diagnosis at the beginning of the second year of life.

• SMA Type III

Children with the juvenile form (SMA Type III) exhibit a highly variable pattern of muscle wasting. They initially learn to walk independently but subsequently lose some of their motor skills. Due to the highly variable course of the disease, specialists now subdivide the juvenile form into two further subgroups: Type IIIa, with symptoms beginning before age 3, and Type IIIb, with symptoms beginning only after age 3.

SMA is first suspected when a baby or toddler fails to reach certain developmental milestones. Particularly in Types I and II, muscle reflexes do not function properly and do not respond as they should. Some patients also experience a slight tremor in the tongue. To confirm the suspicion, doctors perform a genetic test (SMN1 gene).

Since July 2017, a drug has been approved that treats the underlying cause of the disease. The drug is called nusinersen. Nusinersen partially compensates for the genetic defect. As a result, the spinal cord cells are able to produce a functional SMN protein.

A drawback of this treatment is that doctors can only administer the drug via the cerebrospinal fluid. This requires a lumbar puncture.

Clinical studies show that the use of nusinersen significantly improves motor skills. Nusinersen enables infants and toddlers to sit, crawl, stand, and walk. Previously—without nusinersen—they would have died or would never have been able to move on their own. The new drug offers hope to SMA patients and their families.

Previously, treatment for SMA patients was limited to managing symptoms. Especially in the early-onset form of SMA, survival was only possible with continuous mechanical ventilation. 

Specialists in the treatment of SMA include:

• Pediatric neurologists 
• Specialists in pulmonary diseases, 
• Specialists in gastrointestinal disorders 
• Orthopedic surgeons

Treatment primarily involves physical therapy and respiratory therapy. In addition, doctors must continually adjust the provision of assistive devices. An international group of experts has developed detailed guidelines for treatment standards in spinal muscular atrophy (www.treat-nmd.de).

The prognosis for patients with spinal muscular atrophy depends heavily on which type the children have:

• Until now, most children with SMA Type I had a short life expectancy (months to 2 years). 
• Children with SMA Type II were able to reach adulthood—albeit with significant impairments. 
• With SMA Type III, life expectancy is usually normal, though it depends on the progression of muscle weakness.

Once adulthood is reached, the disease continues to progress, but generally at a slower rate @ sofiko14 /AdobeStock

So far, doctors cannot yet predict how nusinersen will affect the life expectancy of SMA patients. The drug has not been on the market long enough for that. 

However, study results show that the earlier patients begin treatment with Nusinersen, the better their prognosis.