Adult-onset spinal muscular atrophy


Spinal muscular atrophy type 4
SMA type 4


Adult-onset spinal muscular atrophy atrophy ( also known as Spinal muscular atrophy type 4, SMA type 4) is a disorder that affects the control of muscle movement. It is caused by a loss of specialized nerve cells, called motor neurons, in the spinal cord and the part of the brain that is connected to the spinal cord (the brainstem). The loss of motor neurons leads to weakness and shrinkage (atrophy) of muscles used for activities such as crawling, walking, sitting up, and controlling head movement. In severe cases of spinal muscular atrophy, the muscles used for breathing and swallowing are affected.


The symptoms of Adult-onset spinal muscular atrophy  include:
• Mild weakness and wasting – typically only the muscles close to the centre of the body such as the upper arms and legs are affected
• Muscle aching and joint overuse symptoms
• Fine shaking of the fingers and hands (twitches and tremors)

Weakness progresses very gradually, first affecting the thighs and hips and then the upper arms and shoulders. Life expectancy is normal and the muscles for swallowing and breathing are rarely affected. People with SMA type 4 may walk with a waddling gait but only a small number of patients eventually require wheelchair assistance. Despite a set pattern of weakness, each person is different in the extent to which they are affected. SMA does not affect intelligence.

The symptoms of SMA type 4 are similar to that seen in SMA type 3 but the motor weakness is less severe. There is also often a lot of overlap between the different types of SMA. You can read more about SMA type 3 here.


SMA is a genetic condition caused by changes to a gene called ‘survival motor neuron 1’ (SMN1) which is located on chromosome number 5. For an individual to have SMA, they need to inherit two altered SMN1 genes – one from their mother and one from their father). This is what is called an ‘autosomal recessive’ inheritance pattern.


Genetic counseling is recommended for people with a family history of spinal muscular atrophy who want to have children.


The health care provider will take a careful history and perform a brain/nervous system (neurologic) examination to find out if there is:

  • A family history of neuromuscular disease
  • Floppy (flaccid) muscles
  • No deep tendon reflexes
  • Twitches (muscle fasciculation) of the tongue muscle


  • Aldolase
  • Erythrocyte sedimentation rate (ESR)
  • CPK levels
  • DNA testing to confirm diagnosis
  • Electromyography (EMG)
  • Lactate/pyruvate
  • MRI of the spine
  • Muscle biopsy
  • Nerve conduction
  • Serum amino acids
  • Thyroid-stimulating hormone (TSH)


People with SMA type I rarely live longer than 2 - 3 years because of respiratory problems and infections. Survival time with type II is longer, but the disease kills most of those who are affected while they are still children.

Children with type III disease may survive into early adulthood. However, people with all forms of the disease have weakness and debility that gets worse over time.


There is no treatment for the weakness caused by the disease. Supportive care is important. Attention must be paid to the respiratory system because affected people have trouble protecting themselves from choking. Breathing complications are common.

Physical therapy is important to prevent contractions of muscles and tendons and abnormal curvature of the spine (scoliosis). Bracing may be needed.