SMA is divided into four types based on the age of onset and functional ability. Individuals with SMA therefore experience a different range and/or severity of symptoms. Severity can also range within each type, and many individuals (up to 25%) cannot be classified into a precise type.1

 

Characteristics of SMA

Type I (also known as Werdnig-Hoffmann disease) 

Age of onset

0-6 MONTHS
(infantile-onset)

Highest motor milestone achieved

UNABLE TO SIT
(“non-sitters”)

Life expectancy

≤2 YEARS

Clinical characteristics

  • Poor head control
  • Weak cough
  • Weak cry
  • Progressive weakness of muscles used to chew and swallow
  • Poor muscle tone
  • 'Frog-legs' posture when lying 
  • Severe muscle weakness on both sides of body
  • Progressive weakness of muscles that help in breathing (intercostal muscles)

 

Type II (also known as Dubowitz disease) 

Age of onset

7-18 MONTHS
(intermediate)

Highest motor milestone achieved

ABLE TO SIT INDEPENDENTLY
(“sitters”)

Life expectancy

>2 YEARS
70% still living
at age 25

Clinical characteristics

  • Muscle weakness 
  • Swallowing, coughing, and breathing problems may occur but are typically less common
  • Muscle aching and joint stiffness symptoms
  • Children may develop spinal problems such as scoliosis (curvature of the spine), which may require bracing or surgery

 

 

Type III (also known as Kugelberg -Welander disease) 

Age of onset

18+ MONTHS
(juvenile-onset)

Highest motor milestone achieved

ABLE TO WALK INDEPENDENTLY
(“walkers”, although they may progressively lose this ability)

Life expectancy

NORMAL

Clinical characteristics

  • Scoliosis (curvature of the spine)
  • Swallowing difficulty
  • Muscles in the legs are generally more severely affected than the arms
  • Muscle aching
  • Joint overuse symptoms

 

 

Type IV  

Age of onset

LATE ADOLESCENCE/
ADULTHOOD
(adult-onset)

Highest motor milestone achieved

ALL

Life expectancy

NORMAL

Clinical characteristics

  • Physical symptoms are similar to juvenile-onset SMA, with the gradual onset of weakness, tremors, and muscle twitching first noted in late teens or early adulthood

 

 

 

 

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For children with SMA, the disease is often first suspected by a parent, who may notice that their child is not meeting certain developmental milestones

Parents may observe that their child is not reaching typical physical milestones for their age, including:

  • ability to hold their head up
  • ability to roll over
  • ability to sit up independently. 

Swallowing or feeding may also become difficult, and children may lose the ability to swallow safely without choking or inhaling food into their lungs (aspiration).

While all babies develop at their own pace, the World Health Organization (WHO) offers the following general motor milestone guidelines as part of the Multicentre Growth Reference Study (MGRS):

Motor milestones in the MGRS

Adapted from the WHO Multicentre Growth Reference Study.

Milestone checklist by age

Adapted from the Centers for Disease Control and Prevention Milestone Checklist.

Doctor's stethascope icon - green

It is important that parents share their observations of potential motor delays with their doctor as this may lead to an earlier diagnosis and improved outcomes for children with SMA.

REFERENCE

1 . Kolb S and Kissel J. Neurol Clin . 2015;33:831–46.