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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 are not affected by any type of SMA.

Children with type 1 SMA show symptoms in the first 6 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

Spinal Muscular Atrophy UK has more information about type 1 SMA.

Children with type 2 SMA usually show symptoms when they're 7 to 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

Spinal Muscular Atrophy UK has more information about type 2 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

Spinal Muscular Atrophy UK has more information about type 3 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 does not normally cause problems with breathing or swallowing.

Spinal Muscular Atrophy UK has more information about type 4 SMA.

There are several other types of SMA, including:

• spinal muscular atrophy with respiratory distress (SMARD) – a type of SMA that's usually diagnosed during a baby's first year of life and can cause serious breathing problems
• Kennedy's disease, or spinobulbar muscular atrophy (SBMA) – a rare type of SMA that only affects men and usually starts in middle age; it does not usually affect life expectancy
• distal spinal muscular atrophy (DSMA) – a type of SMA that mainly affects the hands, feet, lower arms, and lower legs