Elsevier

Clinical Neurophysiology

Volume 111, Issue 8, 1 August 2000, Pages 1340-1345
Clinical Neurophysiology

Remote effects of cortical dysgenesis on the primary motor cortex: evidence from the silent period following transcranial magnetic stimulation

https://doi.org/10.1016/S1388-2457(00)00330-8Get rights and content

Abstract

Objective: In cortical dysgenesis (CD), animal studies suggested abnormal cortico-cortical connections. Cerebral areas projecting to the primary motor cortex (M1) modulate the cortical silent period (CSP) following transcranial magnetic stimulation (TMS). Therefore, we used the CSP to investigate remote effects of CD on the M1.

Methods: A detailed investigation, including single-pulse TMS and electrical nerve stimulation, was performed in 3 consecutive adults with focal CD located outside the M1 and in 18 controls. Two patients with unilateral CD were epileptic and treated with anti-epileptic drugs. One patient with focal CD on both sides had no history of seizures. Neurological examination was normal in all patients. Recordings were made from both first dorsal interosseous muscles.

Results: In CD patients, the CSP was significantly lengthened contralaterally to the affected hemispheres. In treated patients with unilateral CD, the interside difference of the CSP duration was also significantly increased. In contrast, excitability threshold, peripheral and corticospinal motor conduction studies, and peripheral as well as ipsilateral silent periods were not significantly modified.

Conclusions: Our findings indicate that focal CD outside the M1 may produce CSP modifications, which are likely due to changes of afferent control.

Introduction

Cortical dysgenesis (CD) is a heterogeneous disorder of cerebral cortex development frequently associated with epilepsy (Raymond et al., 1995). In patients who underwent epilepsy surgery for intractable seizures, neuropathological studies and intraoperative as well as in vitro electrophysiological investigations have shown striking cytoarchitectonic changes and intrinsic epileptogenicity of dysgenic cortex, respectively (Palmini et al., 1995, Preul et al., 1997). Moreover, studies in animal models suggested altered connections between dysgenic areas and other cortical regions (Colacitti et al., 1998).

Transcranial magnetic stimulation (TMS) provides a non-invasive evaluation of distinct excitatory and inhibitory functions of the human cerebral cortex (for recent review, see Rossini and Rossi (1998)). In particular, the TMS-induced post-excitatory suppression of the voluntary EMG activity mainly depends on interneuronal inhibition within the contralateral primary motor cortex (M1) (Fuhr et al., 1991, Cantello et al., 1992, Inghilleri et al., 1993, Roick et al., 1993). These inhibitory circuits are modulated by various cortical and subcortical areas projecting to the M1 (von Giesen et al., 1994, Classen et al., 1997). Recently, interictal modifications of this cortical silent period (CSP) have been described in patients with cryptogenic motor cortex epilepsy (Cincotta et al., 1998).

In reference to CD patients, limited information about motor evoked potentials (MEPs) is available (Di Capua et al., 1993, Maegaki et al., 1995). On the contrary, to our knowledge, data addressing the CSP have not been previously reported. We present 3 consecutive adult patients with focal CD located outside the M1.

Section snippets

Subjects

Patient 1, a 24-year-old man, had had 4 complex partial seizures consisting of vertiginous sensation followed by loss of consciousness since the age of 17 years. MRI showed right parietal/temporal heterotopia and parietal gyral simplification associated with a retrocerebellar arachnoid cyst (Fig. 1). He became seizure-free after introduction of carbamazepine (CBZ) at 600 mg/day. The present neurophysiological studies were performed at least 13 months after the last seizure.

Patient 2, a

Results

Neurophysiological findings are given in Table 1. In all 3 patients, ET, as well as peripheral and corticospinal motor conduction studies, were within normal limits on both sides; however, the ET was shifted toward the upper limit of the normal range in patients 1 and 2. In patients 1 and 2, the CSP in the left FDI was significantly prolonged at both stimulus intensities. In patient 1, the CSP in the right FDI was normal at both stimulus intensities, although a shift toward the higher normal

Discussion

The principal finding of the present study is that the CSP was significantly prolonged in each patient with focal CD located outside the M1. Patient 1 was treated with CBZ and patient 2 was on CBZ and VGB. In normals, a single CBZ dose lengthens the CSP duration by up to 30 ms, whereas VGB is ineffective (Ziemann et al., 1996, Mavroudakis et al., 1997). Moreover, both patients 1 and 2 showed a bilateral shift of the ET toward the higher normal values, probably resulting from CBZ treatment (

Acknowledgements

We are grateful to Dr M. Hallett and Dr N.J. Patronas for helpful advice and criticism and to Ms A. Minnis for skilful language revision.

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