What is SMA?

The Key Facts

Spinal Muscular Atrophy (SMA) is a genetic disorder that affects nerve cells so that muscles can't be used and become wasted, or atrophied.

SMA affects the nerves in an area of the spinal cord called the anterior horn. The nerve cells become damaged, breaking the link between the brain and the muscles. As a result, the muscles can't be used and waste away. This can lead to problems with breathing as well motor activities such as crawling, walking, feeding and head control.

SMA is a genetic disease and types I, II and III are autosomal recessive conditions meaning that if both your parents are carriers, you have a 1 in 4 chance of having the disease.

After cystic fibrosis, (SMA) is the second most common lethal autosomal recessive disease in Caucasians.

Approximately one million people in the UK are SMA carriers (about one in 50 people)

Approximately one in 20,000 babies is affected with severe SMA.

SMA type II is the most common form.

Boys and girls are affected equally


SMA generally presents with muscle weakness and wasting. The limbs, respiratory and bulbar muscles and brainstem can be affected.

Intellect is preserved and people with SMA often have above average IQ

General clinical signs are that of lower motor neurone weakness:

  • Flaccid weakness (muscles soft and floppy)
  • Hypotonia
  • Reduced or absent tendon reflexes
  • Normal or absent plantar reflexes
  • Muscle fasciculation
  • Muscle atrophy

The severity of the symptoms depends on the type of SMA but within a type group there is much variation between those affected.

SMA type I

Age of onset: under 6 months

Features: the most severe form. Severe muscle weakness, hypotonia (no support of head when pulled up from lying to sitting; floppy when held in ventral suspension), poor suck and swallow reflexes, respiratory failure. Ocular and facial muscles and cerebral function are preserved. May be deformities of limbs/joints at birth from in-utero hypotonia. May be a history of reduced fetal movements in-utero.

Mortality/morbidity: median survival 7 months - 95% die before 18 months.

SMA type II

Age of onset: 6-18 months

Features: developmental motor delay (delay in sitting, standing). Can usually eventually sit unsupported. Some can crawl or stand but these abilities may reduce as body weight increases. May be finger tremor. Musculoskeletal deformities, respiratory failure. Pseudohypertrophy of gastrocnemius muscle.

Mortality/morbidity: can survive into adulthood.  Respiratory infections are a common problem and respiratory support may be required at some point.

SMA type III

Age of onset: over 18 months

Features: a milder disorder. Slowly progressive proximal weakness. Difficulty with more complex motor skills, e.g. climbing stairs. May have gastrocnemius pseudohypertrophy. Chewing and swallowing may be affected later.

Mortality/morbidity: can have a normal lifespan.

SMA type IV

Age of onset: usually mid-30s.

Features: similar to type III but tends to be less severe.

Mortality/morbidity: can have normal lifespan.

SMA with respiratory distress type 1 (SMARD1)

Age of onset: 1-6 months

Features: similar to SMA types I-IV but the predominant symptom is severe respiratory distress due to involvement of the diaphragm muscles. Respiratory problems are generally the first symptoms. Distal muscle weakness. Sensory and autonomic nervous systems may also be involved.


Currently, there's no cure or treatment to repair the nerve damage but research into gene therapy is ongoing.

Drugs including valproate, salbutamol and phenylbutyrate have been shown to stimulate SMN2 gene activity and seem to increase muscle function and help to improve symptoms.

A Multi-Disciplinary approach to Supportive care is very important, especially physio and attention to respiratory function.

Within affected families, once the abnormal gene has been identified, carriers can be detected by a blood test, and antenatal screening using CVS (chorionic villus sampling) is available.