spinal muscular atrophy type I

Last edited 03/2020 and last reviewed 12/2021

Werdnig-Hoffman disease is an early infantile form of spinal muscular atrophy, with an autosomal recessive inheritance and is seen with an incidence of 1 in 20,000 births. The earliest feature may be decreased fetal movements during late pregnancy

  • infants with type 1 SMA, also known as Werdnig-Hoffman disease, present with hypotonia, poor head control and reduced or absent tendon reflexes prior to 6 months of age

SMA type 1

  • the mother of a Werdnig-Hoffman disease child may have noticed a lack of foetal movements

  • profound hypotonia can manifest as a "frogleg" posture when lying and poor to absent head control - weakness is maximal in the shoulder and hip girdle muscles

  • weakness in intercostal muscles, with relative sparing of the diaphragm, produces a bell-shaped chest and a pattern of paradoxical breathing sometimes referred to as "belly-breathing"
  • infants with type 1 SMA develop tongue and swallowing weakness and tongue fasciculations are often present

  • facial weakness does develop, although this is usually not manifest early in the course of the disease - as the tongue and pharyngeal muscles weaken, these infants are at risk of aspiration and failure to thrive. Infants with type 1 SMA usually develop respiratory failure prior to 2 years of life

  • reduced or absent tendon reflexes

  • occasionally there may be swelling of a limb; this is secondary to the inability of the infant to move it when lying on it


  • 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.