Spinal muscular atrophy (SMA)

Out of every 10,000 births, one baby is born with spinal muscular atrophy (SMA). SMA is a genetic disease that affects the part of the nervous system that controls voluntary muscle movement. It is cause by malfunctioning genes that affect how nerve cells and muscle cells communicate across the neuromuscular junction—the place where nerve cells meet muscle cells. Patients with SMA can have severe muscle weakness in the upper arms, legs, and shoulders and in some instances SMA may lead to paralysis or death.1

SMA is broken into 4 groups, based on the patient’s age and disease severity1:

  • SMA type 1
  • SMA type 2
  • SMA type 3
  • SMA type 4
SMA type 1 (Werdnig-Hoffmann disease)

This is the most common and severe type of SMA—approximately 50% of SMA diagnoses are SMA type I. SMA I affects the very young, mainly, infants under 6 months of age. These children cannot sit up by themselves and without medical assistance they seldom live beyond 2 years of age.1

SMA type 2

SMA II affects children between the ages of 7 and 18 months. These children can sit up without assistance, and some of them may even stand, but they will never walk. These children may have slight hand or arm tremors and often develop scoliosis in the first year of life. Weak jaw muscles can affect their ability to chew food, and sometimes they have difficulties swalowing.1

SMA type 3 (Kugelberg-Welander disease)

SMA III affects people between 2 and 18 years of age. In SMA III, the disease is generally milder, and most of those affected can typically reach all major motor milestones, including walking independently. Still, the disease severity may vary by person—some might need a wheelchair in childhood, while others may continue to walk and live productive adult lives. Patients who lose the ability to walk often develop scoliosis and other medical problems related to poor mobility, such obesity and osteoporosis.1

SMA type 4

This is a relatively new classification of adult-onset SMA, in patients older than 18 years of age. These patients have a milder form of the disease and include patients who are able to walk in adulthood and who are without respiratory and nutritional problems.1



Since SMA is rare, it is important to rule out other diseases that it may be mistaken for, such as Prader-Willi syndrome, acute hypoxic ischemic encephalopathy, neonatal sepsis, and dyskinetic or metabolic conditions. To confirm SMA, often a family history is needed, a physical examination is done, and other tests may be performed, including:

  • Testing of reflexes
  • Brain MRI
  • Muscle biopsy
  • Metabolic testing
  • Electromyography (EMG), to test the electrical activity of skeletal muscles
  • Genetic testing to identify the affected genes and SMA type


Currently, nusinersen is the only FDA-approved treatment for SMA in children and adults. Before initiating any therapy, patients or parents of children with SMA should consult with a board-certified neurologist who specializes in neuromuscular diseases.

Catalyst Pharmaceuticals is conducting clinical trials into amifampridine phosphate for the treatment of SMA.

It’s important that note that parents who have a child affected by SMA have a 1 in 4 chance that their next child will inherit the disease. It is recommended that those parents undergo prenatal screening for SMA.


  1. D’Amico A, Mercuri E, Tiziano FD, Bertini E. Spinal muscular atrophy. Orphanet J Rare Dis. 201;6:71.


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