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Spinal muscular atrophy

Types of spinal muscular atrophy

There are several different types of spinal muscular atrophy (SMA), which start at different ages.

They can all cause weakness and difficulty moving, but how bad these problems are varies.

Intelligence and learning ability aren't affected by any type of SMA.

This page covers:

Type 1 SMA (young babies)

Type 2 SMA (older babies and toddlers)

Type 3 SMA (children and young adults)

Type 4 SMA (adults)

Other types of SMA

Type 1 SMA

Children with type 1 SMA show symptoms in the first six months of life.

Babies with the condition:

  • have very weak and floppy arms and legs (hypotonia)
  • have problems moving, eating, breathing, and swallowing
  • are unable to raise their head or sit without support

Most babies with type 1 SMA die during the first few years of life, usually as a result of serious breathing difficulties.

SMA Support UK has more information about type 1 SMA.

Type 2 SMA

Children with type 2 SMA usually show symptoms when they're 7-18 months old. The symptoms are less severe than type 1.

Children with the condition may:

  • be able to sit up without help, but not stand or walk
  • have weak arms or legs
  • have shaking (tremors) in their fingers and hands
  • later develop problems with their joints, such as an unusually curved spine (scoliosis)
  • have weak breathing muscles and difficulty coughing – this can put them at risk of chest infections

Type 2 SMA can shorten life expectancy, but most children with it survive into adulthood and can have long, fulfilling lives.

SMA Support UK has more information about type 2 SMA.

Type 3 SMA

People with type 3 SMA usually develop symptoms after 18 months of age, but this is very variable and sometimes it may not appear until late childhood or early adulthood.

People with the condition:

  • are able to stand and walk without help, although they may find walking or getting up from a sitting position difficult
  • may have balance problems, difficulty running or climbing steps, and a slight shaking in their fingers
  • may find walking gets gradually harder over time, and they may eventually lose the ability to walk when they're older

Type 3 SMA doesn't usually affect life expectancy.

SMA Support UK has more information about type 3 SMA.

Type 4 SMA

Type 4 SMA, also called adult-onset SMA, usually begins in early adulthood.

Someone with the condition may have:

  • weakness in the hands and feet
  • difficulty walking 
  • shaking and twitching muscles

Type 4 SMA gets slowly worse over time, but doesn't normally cause problems with breathing or swallowing. It doesn't affect life expectancy.

SMA Support UK has more information about type 4 SMA.

Other types of SMA

There are several other types of SMA, including: