Pathophysiological Mechanisms of Spasticity at the Spinal Cord Level

Abstract

For a clinician, the term “spasticity” may have a broad meaning, sometimes synonymous with upper motor neuron syndrome. However, it is well known that disability resulting from a lesion of the descending motor pathways depends on distinct positive (hyper-reflexia, muscle hypertonia, spasms) and negative (paresis or palsy, loss of dexterity) signs which may evolve independently. For experts only, spasticity is just one of these positive signs. It was defined by Lance in 1980 as follows: “Spasticity is a motor disorder characterized by a velocity-dependent increase in tonic stretch reflexes (‘muscle tone’) with exaggerated tendon jerks resulting from hyperexcitability of the stretch reflex as one component of the upper motorneurone syndrome”. In the 1980s, this definition enjoyed a large popularity. However, it has also been criticized on the foundation that a clinical sign is described in physiological terms. Moreover, Lance attributes spasticity only to hyperexcitability of the stretch reflex. While there is little doubt that brisk reflexes depend on that factor, the mechanisms of the tonic stretch reflex are less clear and still debated.