Spinal and cortical inhibition in Huntington's chorea

In this article we studied spinal and cortical inhibitory mechanisms in pat ients with Huntington's disease. To evaluate spinal cord inhibitory circuit ries, we assessed reciprocal inhibition between antagonist forearm muscles and the recovery cycle of the I-I reflex in the flexor carpi radialis. Pati ents showed a significant decrease in the presynaptic phase of reciprocal i nhibition reaching a minimum at the conditioning-test interval of 20 msec a nd an abnormal facilitation of the test H reflex at the conditioning test i nterval of 40 to 60 msec. Throughout its time course (10-200 msec), the H r eflex recovery cycle showed a more prominent facilitation in patients than in control subjects. To assess whether the observed pathophysiological abno rmalities might have arisen from an abnormal motor cortical excitability, w e examined the recovery cycle of the motor potentials evoked by paired tran scranial magnetic stimuli. We found that the inhibitory mechanisms controll ing motor cortical excitability were normal. An interpretation of the spina l cord abnormalities is that the intrinsically normal but deafferentated mo tor cortex in Huntington's disease partly loses its inhibitory control, thu s disinhibiting spinal cord circuitry. Our findings from paired transcrania l magnetic stimulation suggest that cortical motor areas are not hyperexcit able in Huntington's disease. Hence, the postulated thalamocortical overact ivity in experimental models of Huntington's disease needs to be reappraise d.