spinal muscular atrophy type III

Last edited 03/2020

SMA type 3

  • juvenile spinal muscular atrophy is an uncommon autosomal recessive disorder of the lower motor neurones
  • onset occurs between 2 and 17 years. It is characterised by wasting and weakness of proximal limb muscles. The deep tendon reflexes are decreased or absent. Fasciculations of the tongue may be occur
    • children and adults with type 3 SMA, also referred to as Kugelberg-Welander disease, are able to walk unassisted at some point during their lifetime
    • present with progressive proximal weakness of the legs more than the arms
    • leg weakness may necessitate the need of a wheelchair at some point

  • unlike, type 2, these individuals are mostly spared the comorbities of scoliosis and have little or no respiratory muscle weakness

  • cognition and life expectancy are not altered in this group


  • term spinal muscular atrophy (SMA) refers to a group of genetic disorders all characterized by degeneration of anterior horn cells and resultant muscle atrophy and weakness

    • most common SMA, accounting for over 95% of cases, is an autosomal recessive disorder that results from a homozygous deletion or mutation in the 5q13 survival of motor neuron (SMN1) gene
  • clinical severity of SMA correlates inversely with SMN2 gene copy number and varies from an extreme weakness and paraplegia of infancy to a mild proximal weakness of adulthood


  • Sugarman EA, et al. Pan-ethnic carrier screening and prenatal diagnosis for spinal muscular atrophy: clinical laboratory analysis of >72,400 specimens. Eur J Hum Genet. 2012; 20(1):27–32.
  • Kolb JS, Tissel SpinalJT. Muscular Atrophy. Neurol Clin. 2015 November ; 33(4): 831–846.