Excitability of spinal inhibitory circuits in patients with spasticity

The excitability of Ia inhibition and D1 inhibition after stimulation of the common peroneal nerve to the soleus motoneuron pool was investigated in 37 spastic patients at rest and onset of voluntary ankle dorsiflexion. Ia inhibition was determined as the short-latency depression of the soleus H-reflex and D1 inhibition as the long-latency depression. There was no significant difference in Ia inhibition between the paraplegic and control groups, however Ia inhibition in the hemiplegic group was significantly decreased. D1 inhibition was reduced in the paraplegic and hemiplegic groups compared with controls. Although inhibition of the soleus H-reflex appeared at the onset of voluntary dorsiflexion in control subjects, it was not observed in the patients. Although the excitability of the Ia inhibitory pathway at rest in the patients did not differ from that in control subjects, facilitation of the Ia inhibitory pathway at the onset of movement was decreased in the patients. Ia inhibition and D1 inhibition were evaluated in two paraplegic patients who were treated with local anesthesia and surgery, respectively. The excitability of both inhibitory pathways at rest was unchanged despite improvement of reciprocal movement in one patient, and was enhanced despite reduction in muscle strength in the other patient. The excitability of spinal inhibitory pathways at rest was not always reflected by motor function in spastic patients.