Reduced spinal motoneuronal excitability during wakefulness in hyperkinetic children: A replication and extension

Abstract

This investigation is a follow-up of our recent report ( Pivik & Mercier, 1981) of reduced or absent secondary facilitation (100–300 msec inter-stimulus intervals) in the recovery function of the monosynaptic H-reflex in hyperkinetic (Hk) children during wakefulness (W). Subjects were 7 Hk and 6 control children, all males, 8–12 years old, unmedicated, free from organicity, and of average or above average intelligence. Hk diagnosis was based on scores of ≥ 15 on Conners Parent and Teacher Hyperactivity Index and the presence of DSM-III criteria of attention/deficit disorder with hyperactivity. Control children scored negatively on this index. Muscle action potentials elicited by pericutaneous tibial nerve stimulation were recorded from gastrocnemius-soleus muscles. Above threshold equal intensity stimuli were delivered in pairs at intra-pair intervals ranging from 40–2000 msec. Ratios of test (H 2) to conditioning (H 1) reflexes were averaged (5/interval) to generate a recovery function. Data were analyzed using repeated measures analysis of variance. Recovery functions were determined during relaxed W under two conditions, i.e., with and without the Jendraissik maneuver — a procedure thought to facilitate motoneuronal excitability via gamma (γ) efferent activation. Under both conditions reflex amplitude was depressed in Hk children relative to controls in the intervals spanning the time of secondary facilitation (C > Hk, 100 msec: p < .01; C > Hk, 300 msec: p < .05). Non-significant reflex amplitude increases were present in control subjects in Jendraissik relative to non-Jendraissik data, but results were inconsistent in Hk children. The present findings replicate our initial report of reduced facilitation during W in Hk children and argue for the use of this measure as objective index identifying a subclass of hyperkinetic children. The absence of reflex facilitation following the Jendraissik maneuver could be interpreted as a failure of γ-efferent activation in Hk children. However, since the role of this procedure in reflex facilitation remains controversial, the contribution of γ-efferent activity to the observed reduced motoneuronal excitability remains to be determined.