[Electrophysiological assessment of reflex pathways involved in spasticity]

Various spinal reflexes can be involved in spasticity. Monosynaptic Ia excitation of motoneurons is the most widely studied electrophysiological phenomenon and can be explored by recording the H-reflex, particularly in the soleus muscle. Using conditioning stimuli, the H-reflex technique can be used to study most of the many spinal mechanisms of motor control such as disynaptic, presynaptic, or reciprocal inhibition, post-activation synaptic depression, or facilitation by type I or II muscle afferents. The results obtained in spastic subjects show that the myotactic stretch reflex is exaggerated, but that all the other spinal reflexes are modified. The most recently reported results show an exaggeration of heteronymous facilitation in group II afferents could be one of the main mechanisms leading to spasticity. At the present time, results obtained for the various spinal pathways remain to be compared before concluding about the precise pathophysiology of spasticity, particularly concerning its origin in humans. Despite their complexity, electrophysiological reflex studies offer essential information to reach this goal.