Elsevier

Clinical Neurophysiology

Volume 113, Issue 12, December 2002, Pages 2006-2012
Clinical Neurophysiology

Cortico-motor excitability of the lower limb motor representation: a comparative study in Parkinson's disease and healthy controls

https://doi.org/10.1016/S1388-2457(02)00301-2Get rights and content

Abstract

Objective: To compare indices of cortico-motor excitability derived from transcranial magnetic stimulation (TMS) of the lower limb motor representation in patients with Parkinson's diseases (PD) and healthy controls.

Methods: The cortico-motor excitability of the lower limb motor area was studied both at rest (motor threshold, amplitude of motor evoked potentials (MEPs)) and during active contraction of the quadriceps (Quad) muscle (MEPs facilitation and silent period) in 10 PD patients (11 legs) and 11 healthy controls using single pulse TMS.

Results: At rest, the motor threshold was found to be significantly lower and the amplitude of MEPs larger in patients than in controls. During active knee contraction, patients produced lower levels of MEP facilitation with respect to baseline values and the silent period was lengthened in comparison to controls.

Conclusions: The present results provide further evidence from the lower limb motor area that enhanced cortico-spinal excitability is an important feature in the pathophysiology of PD.

Introduction

Recent developments in the pathophysiology of Parkinson's disease (PD) with transcranial magnetic stimulation (TMS) point out to alterations in cortico-spinal excitability as an important feature in this disorder (Cantello et al., 2002, Rothwell, 1999). While reports on changes in the motor threshold have been inconsistent in PD, other indices of cortico-motoneuronal excitability are clearly affected (reviewed in Cantello et al., 2002). For instance, some authors have reported an increase in the size of motor evoked potentials (MEPs) at rest and a reduced ability to facilitate MEPs with active contraction in PD patients (Chen et al., 2001, Valls-Sole et al., 1994); both indicative of an enhanced cortico-spinal excitability. Further evidence of alterations in cortico-spinal excitability in PD has come from studies looking at motor cortical inhibitory phenomena. For example, the shortening of the silent period evoked in small hand muscles by single pulse TMS during active contraction is thought to reflect impaired GABA-ergic inhibition at the cortical level (Cantello et al., 1991, Priori et al., 1994). Similarly, the attenuation of MEPs induced by paired-pulse TMS, which reflect intra-cortical inhibition, is also reduced in PD (Ridding et al., 1995). These findings, taken together, have been interpreted as evidence that the basal ganglia defect in PD can lead to imbalance in favor of disinhibition or hyperexcitability of the cortico-motoneuronal system (Cantello et al., 2002, Rothwell, 1999).

The majority of TMS studies, including those referred to above, have focused on the upper extremity, with the intrinsic hand muscles being the most frequent target. In fact, in a very recent review of TMS applications in PD, Cantello et al. (2002) noted the paucity of information regarding the lower extremity. This contrasts with the wealth of information from electrophysiological studies showing abnormalities in the lower limb interneuronal spinal circuits (e.g. decreased disynaptic reciprocal inhibition and decreased Ib inhibition) in PD (see Valls-Sole (2000) for a recent review). While it has been shown that upper and lower limb motor representations share similar mechanisms in terms of intra-cortical facilitation and inhibition in paired-pulse TMS paradigms (Chen et al., 1998), the two representations differ qualitatively in terms of descending projections. For example, the activation of distal hand muscles is achieved primarily via direct cortico-motoneuronal projections (Maier et al., 1998), whereas recent evidence suggests that cortical activation in thigh muscles is achieved mainly via an indirect route through lumbar pre-motoneurones (Marchand-Pauvert et al., 1999). In the present study, we selected the quadriceps muscle to explore the excitability of the lower limb motor representation using TMS. The goal was to investigate possible changes in the lower limb motor representation by comparing indices of excitability measured at rest and during voluntary activation in patients with PD and healthy controls.

Section snippets

Participants

Participants consisted of 10 patients with PD (mean age±SD, 60.2±7.5 years) and 11 age-matched healthy controls (56.8±7.2 years). In most cases (7/10), healthy controls consisted of the spouse of the patient in the PD group. As a group, the PD patients consisted of mildly to moderately affected individuals with a median duration of disease of 3.5 years (see Table 1 for clinical details). At the time of assessment, all patients were on their regular medication schedule. As well, the main motor

Relaxed motor threshold and MEPs at rest

The mean values of excitability indices derived in each group are summarized in Table 2. In patients, the lower and upper threshold limits determined at rest corresponded, on average, to 50 and 60% of the stimulator output, respectively (Table 2). In controls, the same limits spread in the 60–70% range. Accordingly, the mean motor threshold was found to be significantly lower in patients than in controls (Table 2). Patients also tended to show larger MEPs at rest, but the difference did not

Discussion

In the present study, we compared cortico-motor indices of excitability derived from TMS of the lower limb motor representation in patients with PD and age-matched controls. Although PD patients were mildly affected and tested while ON-medication, several differences emerged when compared to controls: (1) at rest, patients showed lower motor thresholds in the Quad and larger MEPs amplitude; (2) during active contraction , they produced lower levels of MEPs facilitation with respect to baseline

Acknowledgments

The authors wish to thank the Canadian Society for Parkinson (section Ouatouais) for their help with the recruitment of patients and all participants for their time. The authors also thank H. Manning and H. Lacroix for their assistance with data collection and data analysis. We also thank W. Jelley for her help in revising the manuscript in English.

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