Change of stretch reflex threshold in spasticity: effect of botulinum toxin injections

Spasticity is a disorder of hypertonus associated with neurological diseases, characterized by a decrease in stretch reflex threshold. Stretch reflex threshold of wrist flexors has been recorded in subjects affected by forearm spasticity due to acute neurological lesions, occurred from one to sixty-one months before. In all the subjects a decreased stretch reflex threshold was recorded and a negative correlation between stretch reflex threshold and time of the disease resulted. In five subjects affected by mild spasticity the velocity stretch reflex threshold was tested one-three months after stroke and then six months later. In three cases a further decrease in stretch reflex threshold was recorded. Sixteen subjects affected by heavy forearm spasticity (quantified by Ashworth scale), were treated with Botulinum toxin injections to reduce spasticity. Fourteen of 16 subjects were responsive to the antispastic therapy: a decrease of at least 1 point in the Ashworth scale was detected after the treatment. In all the responsive cases an increase of stretch reflex threshold was recorded. The results confirm that the stretch reflex threshold is decreased in spastic muscles; it decreases progressively in time after the acute lesion. In addition, these results demonstrate that the decreased stretch reflex threshold can be reversed with Botulinum toxin injections. It is known that Botulinum toxin reduce the presynaptic release of Acetylcholine of neuromuscular synapses, but there are experimental evidences that it acts even on spindle's fibres, decreasing the sensitivity of intrafusal muscle fibres. This effect explains how Botulinum toxin increases the stretch reflex threshold in spastic muscles.