Magnetic transcranial stimulation: clinical interest of the silent period in acute and chronic stages of stroke

Abstract

There is little information on the silent period during facilitation of the target muscle at the acute stage of stroke and the ultimate clinical status. We studied 69 subjects with transcranial magnetic stimulation: 20 matched controls and 49 hemiparetic patients investigated 7 and 90 days after the stroke (D₇, D₉₀). We measured the silent period duration (SPD) in the first dorsal interosseous muscle at 10 and 100% of maximal voluntary isometric contraction (VIC). The SPD index (the ratio of SPD at VIC 100% by SPD at VIC 10%) at D₇ was matched with the clinical outcome at D₉₀. Two patterns of responses could be determined at D₇. In the normal subjects and in 27 out of 32 patients who eventually recovered satisfactory function at D₉₀, the SPDs were stable during facilitation (SPD index 100%). On the contrary, in 10 out of the 17 patients with a poor functional outcome, the mean SPD decreased when VIC was increased (SPD index 80%); besides, their muscle tone was significantly increased at D₉₀. Similar patterns were still present in the patients at D₉₀: the mean SPD indexes were not significantly different from D₇. We conclude that in the early stage of stroke, a low SPD index appears to be correlated with the eventual occurrence of spasticity.

Introduction

Magnetic transcranial stimulation during voluntary target muscle contraction elicits a motor evoked potential followed by a pause of the muscle activity (Calancie et al., 1987). This `EMG silence' following the motor potential event has been called `silent period' (Rossini, 1990) and is presumed to be of inhibitory origin (Calancie et al., 1987; Hölmgren et al., 1990; Fuhr et al., 1991; Wasserman et al., 1991; Cantello et al., 1992; Uozumi et al., 1992).

However, its usefulness in clinical practice is limited by its variability or by the diversity of procedural factors such as intensity of the brain stimulation and/or of target muscle contraction. Comparison of results from different studies with neurological diseases is thus uneasy.

In normal subjects, the silent period duration (SPD) can vary from 126 to 300 ms (Fuhr et al., 1991; Wasserman et al., 1991; Cantello et al., 1992; Uozumi et al., 1992; Inghilleri et al., 1993; Haug and Kukowski, 1994), increasing with the intensity of the magnetic brain stimulation (Hölmgren et al., 1990; Uozumi et al., 1992).

Effect of voluntary contraction was diversely reported. Haug et al. (1992)and Kukowski and Haug (1992)observed no variation of the SPD during increasing contraction while Wilson et al. (1993)reported a decrease of the SPD when tonic muscle contraction increased. In clinical conditions SPD was found to be shorter in patients with Parkinson's disease (Cantello et al., 1991) or with upper motor neuron disorders (Uozumi et al., 1991; Uozumi et al., 1992), and altered in lower motor neuron disorders (Triggs et al., 1992) or in patients with focal dystonia (Mavroudakis et al., 1995).

As there is little information on the clinical value of the SPD in stroke, we analysed the results obtained in 49 patients investigated with magnetic brain stimulation at suprathreshold intensity and two levels of voluntary isometric contraction (VIC) of the target muscle. We then derived a SPD index and its clinical interest was studied.