Natural history of denervation in SMA: relation to age, SMN2 copy number, and function

Ann Neurol. 2005 May;57(5):704-12. doi: 10.1002/ana.20473.


Denervation was assessed in 89 spinal muscular atrophy (SMA) 1, 2, and 3 subjects via motor unit number estimation (MUNE) and maximum compound motor action potential amplitude (CMAP) studies, and results correlated with SMN2 copy, age, and function. MUNE and maximum CMAP values were distinct among SMA subtypes (p < 0.05). Changes in MUNE and maximum CMAP values over time were dependent on age, SMA type, and SMN2 copy number. SMN2 copy number less than 3 correlated with lower MUNE and maximum CMAP values (p < 0.0001) and worse functional outcomes. As SMN2 copy number increases, so does functional status (p < 0.0001). Change in MUNE longitudinally over the time intervals examined in this study was not statistically significant for any SMA cohort. However, a decline in maximum CMAP over time was apparent in SMA2 subjects (p = 0.049). Age-dependent decline in MUNE and maximum CMAP was apparent in both SMA 1 (p < 0.0001) and SMA 2 (p < 0.0001) subjects, with age as an independent factor regardless of type. Maximum CMAP at the time of the initial assessment was most predictive of functional outcome (p < 0.0001). Prospective longitudinal studies in four prenatally diagnosed infants demonstrated significant progressive denervation in association with symptomatic onset or functional decline. These data highlight the potential value of such measures in increasing our understanding of pathophysiological factors involved in denervation in SMA.

Publication types

  • Clinical Trial
  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Activities of Daily Living
  • Adolescent
  • Adult
  • Aging / physiology*
  • Cell Count
  • Child
  • Child, Preschool
  • Conscious Sedation
  • Cyclic AMP Response Element-Binding Protein / genetics*
  • Cyclic AMP Response Element-Binding Protein / physiology
  • Evoked Potentials, Motor / physiology
  • Female
  • Humans
  • Infant
  • Infant, Newborn
  • Longitudinal Studies
  • Middle Aged
  • Motor Neurons / pathology
  • Muscle Denervation
  • Muscle, Skeletal / innervation
  • Muscle, Skeletal / pathology
  • Muscular Atrophy, Spinal / genetics*
  • Muscular Atrophy, Spinal / pathology*
  • Nerve Tissue Proteins / genetics*
  • Nerve Tissue Proteins / physiology
  • Neurologic Examination
  • Pregnancy
  • Prenatal Diagnosis
  • RNA-Binding Proteins / genetics*
  • RNA-Binding Proteins / physiology
  • Reproducibility of Results
  • SMN Complex Proteins
  • Survival of Motor Neuron 2 Protein
  • Ulnar Nerve / physiology


  • Cyclic AMP Response Element-Binding Protein
  • Nerve Tissue Proteins
  • RNA-Binding Proteins
  • SMN Complex Proteins
  • SMN2 protein, human
  • Survival of Motor Neuron 2 Protein