Spinal Muscular Atrophy (SMA)

Spinal Muscular Atrophy (SMA) is a rare genetic disorder affecting the motor neurons of the spinal cord and causing muscle weakness, damage, wasting (atrophy) and eventual loss of function. Children with SMA may have difficulty crawling, sitting, walking, and moving their limbs and neck. Their ability to breathe and swallow may also be affected.

People with SMA have a mutation (change) in their Survival Motor Neuron (SMN) gene. Parents of children with SMA have no neurological problems themselves and are generally not aware that they carry the SMA gene until they have a child with SMA. 

SMA is a very complex disease, affecting each child differently. With the recent development of disease modifying treatments the way we describe the different types of SMA is changing. We used to refer to the three types as SMA types 1, 2 and 3 (as described below), but these terms are now becoming less useful. We now think more in terms of motor milestone achievements and use the classification of non-sitter (unable to sit without support), sitter and walker.

Traditional Classification: Types of SMA

Traditionally the  three common types of SMA, were classified based on the age of the child when they first have symptoms.

SMA Type 1

Also referred to as infantile-onset, this is the most severe form of SMA. Symptoms may be present at birth or develop within the first few weeks or months of life. They include severe weakness or “floppiness”, poor head control, weak cry and cough, and difficulty with swallowing and feeding.

SMA Type 2

Children with SMA type 2 demonstrate symptoms of weakness later than children with type 1. They usually reach the developmental milestone of sitting when placed, but cannot get to a sitting position by themselves. Older children with SMA type 2 generally have delayed motor milestones, a weak cough, hand tremors, contractures and scoliosis (curved spine).

SMA Type 3

People with SMA type 3 develop symptoms sometime between the age of 18 months to teens or adulthood. It is a much milder form of SMA; mobility is affected, but progression is much slower. Patients have variable muscle weakness, cramps and fatigue. Some people with SMA type 3 can walk independently; others require assisted mobility devices. SMA type 3 is sometimes classified as 3a (onset of symptoms before 3 years of age) and 3b (onset of symptoms after 3 years of age).

Current Classification: Motor Milestone Achievement

The advent of gene and genetic therapies to successfully treat SMA has changed the natural history and severity of weakness seen in the condition. Classification of SMA severity now focuses on motor milestones achieved rather than symptom onset. We recognise that this is not a perfect system for grouping patients as there will be significant variation is skills within these groups, but it is an overall more usual way of grouping patients than the traditional types.


Patients with infantile-onset SMA commonly show symptoms of SMA around two to four months of age and will not independently develop the ability to sit without any support. This group of patients is the most common, accounting for around 60% of all newly diagnosed patients. This group of patients are also the most severely affected, being prone to difficulties feeding and gaining weight and being more susceptible to breathing problems such as chest infections and weakness of the breathing muscles.  


Patients with traditional SMA type 2, diagnosed a little later as an older infant or toddler often achieve the ability to sit independently without support for more than five seconds. These patients are referred to as sitters. This group may also include those with infantile-onset SMA who have received disease modifying treatments and allied health therapy input and have made developmental gains to achieve independent sitting. It is important to be aware that not all children treated with disease modifying treatments achieve independent sitting. See the disease modifying treatments section for more details.


Patients in this group either have achieved this milestone through use of disease modifying treatments and allied health therapy or have later onset of symptoms after they have attained walking.

Early signs

How SMA affects a child depends on when the disorder first causes symptoms. Early signs of SMA include muscle weakness, poor muscle tone and floppiness, with the legs usually weaker than arms. Babies who are affected may have feeding or breathing difficulties and a weak cry.

When SMA signs are present early on, children may miss developmental milestones, such as rolling over, crawling, sitting without support, and walking.


SMA symptoms are progressive, causing increasing weakness in the muscles of the limbs and chest, making breathing progressively more difficult. Weakness of the breathing muscles is the most serious complication of all forms of SMA, especially in children.

It is important to note that SMA does not affect all parts of the body. Intelligence is unaffected and children with SMA attend mainstream schools with physical assistance.


In the past, the treatment of SMA has been based on the management of its complications; muscle weakness, feeding problems and breathing difficulties, joint contractures and scoliosis.

More recently, several novel therapies have been shown to improve strength and motor function in children with SMA type 1.  Nusinersen (Spinraza) has now been licensed in Australia for treatment of children with Type 1, Type 2 and Type 3a SMA.  Nusinersen works by improving function of the SMN gene. In some cases progression of the disease is halted by treatment with nusinersen; in other children motor function may actually improve. 

Couples with a family history of SMA are at increased risk of having children with SMA. Prenatal testing can clarify this risk. Talk to your obstetrician, neurologist or geneticist about this.  

Other therapies

  • Physiotherapy can help prevent muscle contractures, but cannot restore lost muscle strength in SMA.
  • Children with some forms of SMA may have difficulty swallowing and need a feeding tube.
  • Chest infections can be life-threatening in children with SMA and need quick treatment with antibiotics and chest physiotherapy.
  • Scoliosis (curvature of the spine) is common in children with SMA because of weakness in the muscles around the vertebral column. Sometimes surgery can help.

Getting support

SMA is a difficult condition for families to live with, but your multidisciplinary care team has a lot of experience in helping families and individuals cope with diseases like SMA, and can help you.

Support groups and counselling can be a great source of comfort and information, and are available to all families. 

Further reading

Below is a list of topics that have been compiled by Australian neuromuscular healthcare professionals to teach you more about SMA. Each topic has multiple pages and resources for you to read through. Click on each heading to see a brief overview of each topic and a link to learn more. If you have any questions about the information presented below, please speak with your doctor or your hospital's neuromuscular nurse. 

Key points to remember

  • Spinal muscular atrophy (SMA) is a progressive disease of the motor nerves.
  • Supportive treatment improves the health of children with SMA.
  • New therapies may have an even greater impact on muscle weakness, breathing, and feeding problems in children and adults with SMA.

The internet

While the internet can be a source of good information, it can often be a source of false hope, inaccuracies, and misleading information. Always rely on your doctor for expert and professional advice.

Zac's Play Day

Zac's Play Day is a picture book for children about SMA. Click on the image above to read a PDF of the book. We thank Biogen for creating and sharing this resource.


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