Clinical presentation for SMA may differ according to the age of onset and severity, but hypotonia (floppy baby syndrome) and/or muscle weakness and atrophy are common signs or symptoms:2

  • Weakness is usually symmetrical
  • Weakness is more proximal than distal
  • Sensation is preserved
  • Tendon reflexes are absent or diminished
  • Weakness is greater in the legs than the arms
  • Severity of weakness generally correlates with the age of onset

Characteristics of spinal muscular atrophy (age at onset)

Type I (also known as Werdnig-Hoffmann disease) 

Age of onset


Highest motor milestone achieved


Life expectancy


Clinical characteristics

  • Hypotonia and impaired head control
  • “Frog leg” presentation
  • Weak cry
  • Weak cough
  • Swallowing, feeding, and handling of oral secretion are affected before 1 year of age
  • Atrophy and fasciculation of the tongue
  • Weakness and hypotonia in the limbs and trunk
  • Intercostal muscle weakness (note, the diaphragm is initially spared)
  • Paradoxical breathing
  • Bell-shaped trunk with chest wall collapse and abdominal protrusion





Type II (also known as Dubowitz disease) 

Age of onset


Highest motor milestone achieved

(“sitters”, although they may lose this ability by the mid-teenage years)

Life expectancy

70% still living
at age 25

Clinical characteristics

  • Bulbar weakness with swallowing difficulties that may lead to poor weight gain
  • Weak intercostal muscles
  • Diaphragmatic breathing
  • Difficulty coughing and clearing tracheal secretion
  • Fine tremors with extended fingers or when attempting hand grips
  • Kyphoscoliosis, or scoliosis requiring bracing or spinal surgery
  • Joint contractures






Type III (also known as Kugelberg-Welander disease) 

Age of onset


Highest motor milestone achieved

(“walkers”, although they may progressively lose this ability)

Life expectancy


Clinical characteristics

  • Scoliosis
  • Swallowing difficulty
  • Cough, and nocturnal hypoventilation
  • Muscle aching
  • Joint overuse symptoms








Type IV 

Age of onset



Highest motor milestone achieved


Life expectancy


Clinical characteristics

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










Time to diagnosis is critical – earlier diagnosis and intervention may help improve the outcomes for individuals with SMA.6

If you see symptoms of SMA in your patient, refer immediately for an urgent appointment with a paediatric neurologist.

Disease progression in SMA may be divided into 3 conceptual phases:
preclinical, subacute, and chronic7

Acquisition of gross motor milestones in typically developing children compared with individuals with SMA7

Adapted from Swoboda et al, 2007.7

Preclinical phase:

individuals with SMA may appear to develop normally7

  • Motor unit loss progresses, but collateral reinnervation may mask symptoms
  • May be rapid in infantile-onset (Type I) SMA
    • Severe denervation may occur by 6 months of age
  • In later-onset (Type II and Type III) SMA, the preclinical phase may last for months or years in mildly affected individuals

Subacute phase:

motor unit loss may reach a critical threshold7

  • Rapid motor-unit loss associated with significant reductions to the maximum compound muscle action potential (CMAP) amplitudes over a period of weeks to months
  • May be exacerbated by illness, nutritional compromise, or growth
  • Clinical symptoms may evolve:
    • Weakness and progressive paralysis in infantile-onset SMA
    • Loss of the ability to sit or roll may be seen in severe later-onset (Type II) SMA
    • Slowing of acquisition of expected gross motor milestones in less affected individuals with Types II or III SMA

Chronic phase:

motor unit loss may appear to plateau7

  • Functional motor abilities may remain stable for months or years
  • Motor skills that were previously lost, such as rolling, may return or the individual may slowly acquire some additional gross motor skills
  • Denervation progresses with age

Loss of function in later-onset SMA may have a major impact on quality of life

In a European survey of 822 individuals (8-73 years of age) with later-onset (Type II and Type III) SMA, a subgroup of patients identified their current functional abilities that if lost would affect their quality of life (QoL):8

Loss of functional ability
that would affect QoL

Patients currently able
to perform function

Using restroom alone


Washing oneself


Performing transfers on his/her own


Feeding self


Dressing self


Stabilisation of functional abilities may be important to individuals with later-onset SMA

In the same survey, individuals with later-onset SMA identified those functional abilities that they have retained and would most want to stabilise. These included:8

  • Feeding him/herself
  • Washing independently 
  • Using restroom independently 
  • Performing transfers independently
  • Using a keyboard
  • Turning in bed
  • Writing with a pen
  • Brushing his/her teeth
  • Dressing by him/herself
  • Brushing his/her hair

Participants were asked to choose the 3 functions, ranked 1 through 3, in decreasing order of priority they would most like to stabilise.


1. Prior TW, Finanger E. Spinal muscular atrophy. NCBI Bookshelf Web site. =printable. Updated December 22, 2016. Accessed October 2017. 2. Wang CH, et al. J Child Neurol. 2007;22:1027–49. 3. Darras BT, et al. Neuromuscular Disorders of Infancy, Childhood, and Adolescence: A Clinician’s Approach. 2nd ed. London, UK: Elsevier; 2015. 4. Lunn MR and Wang CH. Lancet. 2008;371:2120–33. 5. NIH National Center for Advancing Translational Sciences. Genetic and Rare Diseases Information Center. Spinal muscular atrophy type 2.  Accessed October 2017. 6. Rothwell E, et al. Am J Med Genet A. 2013;161A:679–86. 7. Swoboda KJ, et al. J Child Neurol. 2007;22:957–66. 8. Rouault F, et al. Neuromuscul Disord. 2017;27:428–38.