P 182. Normal lateral inhibition mechanism during sensory-motor plasticity in dystonia

Objective To explore if sensory information is processed and integrated during sensory-motor plasticity phenomena by using lateral inhibition mechanisms in normal humans and in patients with dystonia. Background Several evidence suggest that lateral inhibition is a system within sensory-motor cortex operating during the acquisition of new motor tasks in order to select the appropriate muscle sequence to be stored within the final motor engram. This mechanism is thought to be lost in dystonia and this should explain the development of redundant motor memories which could culminate in overflow phenomena and overt dystonia. Methods We have used transcranial magnetic stimulation to explore lateral inhibition during sensory-motor plasticity in 12 dystonic patients (7 focal hand dystonia, 5 cranial dystonia) and in 8 healthy subjects. In particular we looked at motor evoked potential (MEP) facilitation, in the abductor pollicis brevis (APB) and abductor digiti minimi (ADM), obtained after 5 Hz repetitive paired associative stimulation after median (PAS M), ulnar nerve stimulation (PAS U) and median + ulnar nerve (PAS MU) stimulation. In this way we evaluated the ratio MU/M + Ux100 (lateral inhibition index). Moreover, we evaluated the lateral inhibition index (Li index). This parameter is easily obtained by using the following formula: ratio MU/(M + U) x 100. MU is the MEP facilitation measured after PAS with simultaneous stimulation of median and ulnar and M + U is the amount of MEP facilitation, after PAS, induced from stimulation of the individual nerves. Results Our data confirmed that patients with dystonia had two main abnormalities: first the amount of facilitation was larger than normal subjects; second and more important the spatial specificity was lost. A three-factorial ANOVA demonstrated a significant time × group × conditioning interaction ( F = 7.9; p = 0.005). Lateral inhibition index was similar (about 50%) in healthy subjects and dystonic patients. Conclusions These data suggest that lateral inhibition is normal in dystonia during sensory-motor plasticity. Another mechanism could contribute to the formation of motor memories with redundant information, which could culminate in overt dystonia.