Electroencephalography and clinical Neurophysiology, 85 (1992) 425-428

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© 1992 Elsevier Scientific Publishers Ireland, Ltd. 0924-980X/92/$05.00

E L M O C O 92579

Short c o m m u n i c a t i o n

Central motor conduction in relation to contra- and ipsilateral activation Machiel J. Zwarts Martini Hospital, Dept. of Clinical Neurophysiology, 9700 RM Groningen (The Netherlands) (Accepted for publication: 11 August 1992)

Summary Differences of central motor conduction between slight activation of the target muscle (ipsilateral: IL) and strong contraction of the contralateral (CL) muscle following magnetic motor cortex stimulation were studied in 18 controls; responses were recorded at abductor pollicis brevis (APB) and tibial anterior (TA). For A P B a clearly faster response was obtained with IL activation (mean: 1.7 msec). The amplitude increased only slightly. The reverse was found for T A muscle: amplitude nearly doubled with IL activation, but the latency did not change. In both facilitation procedures a clear correlation was found between left and right normalised amplitude (cortex a m p l i t u d e / M wave) for APB and TA, but not for the T A response with IL activation. This points to a different mechanism of e n h a n c e m e n t for the ipsilateral activation of T A muscle. It is argued that a rise in excitability of spinal motoneurones is largely responsible for the increase in amplitude. In clinical practice contralateral activation as a method of facilitation seems appropriate in most cases: lack of E M G contamination of the baseline makes it easier to read the latency onset. Only in cases of a low T A response can 1L activation give a better response. Key words: Central motor conduction time; Magnetic stimulation; Facilitation; Cortical stimulation; Motor evoked potentials

Transcranial magnetic stimulation has revolutionised the clinical neurophysiological investigation of central motor disorders. In most studies preactivation of the target muscle has been used to improve the motor evoked response (Hess et al. 1986; Ingrain et al. 1988; Claus 1990; Eisen and Shtybel 1990). It is well known that preactivation of the target muscle (hereafter termed ipsilateral (IL) activation) results in a higher and faster response. Alternatively, activation of the contralateral (CL) muscle can be tried. This has obvious advantages such as: (1) No contamination of the baseline with E M G activity, thus resulting in a more certain reading of latency and response duration. (2) In case of paralysis of one limb, contralateral activation is still possible, and normal values can still be applied. The Use of contralateral activation was also suggested by Murray (1991) in cases of severe hemiparesis or inability to maintain a sustained contraction. A possible disadvantage of contralateral activation is the absence of a response due to insufficient facilitation. Furthermore, the silent period can be studied only with a strong activation of the target muscle (Uozumi et al. 1991). In any case, it seems important to investigate the differences between these two m e t h o d s of facilitation in a systematic way; most studies have focused on the differences between the relaxed and activated states of the target muscle. Only the study of Hess et al. (1987) has given attention to the differences between ipsi- and contralateral activation, concerning just hand muscles in 5 controls, however. In this study the results of both ways of facilitation were compared in 18 controls for both hand and leg muscles.

Correspondence to: Dr. M.J. Zwarts, Martini Hospital, Dept. of Clinical Neurophysiology, Van Swietenlaan 4, PO Box 30033, 9700 R M Groningen (The Netherlands).

Methods and materials Transcranial magnetic stimulation was performed after obtaining informed consent on 18 controls (all hospital employees) free from neurological disease. Their m e a n age was 32.1 years (range 19-44). All persons underwent the same protocol. The legs were warmed in a bath, so that the skin temperature was 30-35°C. The motor responses of the abductor pollicis brevis (APB) and tibial anterior (TA) muscles were recorded with silver cup electrodes placed in a belly-tendon position. For that purpose the skin was prepared by gentle abrasion and application of conductive paste. Next, the median and peroneal nerves were stimulated electrically at the wrist and knee, respectively. A Cadwell MES-10 stimulator was used with a standard round stimulator (inside diam.: 7.5 cm). The output is a damped oscillatory magnetic field (maximal 2 Tesla). As a consequence of the nature of the oscillatory output, the direction of the coil is not important (Claus 1990). M e a s u r e m e n t s were made with a Nomad E M G apparatus (Tracor Inc., Middleton, Wl, U.S.A.). The sweep time was 50 msec (for APB) to 100 msec (for TA). Magnetic stimulation was done in the midline over the spinal enlargements, while the place of stimulation was found by trial and error. The upper edge of the stimulator was usually at the vertebral level of the appropriate d e r m a t o m e (C8), whereas lumbar stimulation was done at the vertebral level L4. Transcranial stimulation was done on the vertex for the hand and consecutively for the leg muscles at Fz. Stimulation was performed at maximal output. Cortical stimulation was done 3 times with slight preactivation of the target muscles (ipsilateral activation). This was controlled using the E M G baseline activity (about 5 - 1 0 % of the maximal activity) and 3 times with strong activation of the contralateral (homologous) muscle and relaxation of the target muscle.

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M.J. Z W A R T S

For all stimulus sites, latencies were m e a s u r e d at high gains using the first deflection of the baseline. With cortical stimulation, the shortest onset latency and largest peak-to-peak amplitude was used. T h e relationships between variables were studied using linear regression analysis. Differences between the two facilitation procedures were evaluated with Student's paired t test.

Results

T h e results are summarised in Table I. Results on the left and right were treated as one, since no differences were found between them. The central conduction time (CCT) for APB and T A muscle was calculated by subtracting the spinal latency from the cortical latency. For the hand, C C T with ipsilateral activation (CCT-IL) was clearly shorter (1.7 msec) than with contralateral activation (CCTCL). Latencies for T A were very similar, and no significant difference was found between the 2 facilitation procedures (see Table I). The a m o u n t of E M G activity generated by cortical stimulation was expressed as a ratio by dividing the amplitude of the cortical response by the peripheral response ( M E P / M wave). For APB, only a slight increase was found between the two ways of facilitation. For the leg, ipsilateral activation almost doubled the amplitude, and it became even greater than the amplitude of the hand. Typical exam-

A:

pies of responses for APB and T A muscle are shown in Fig. IA and B, respectively. A rather high correlation was found between left and right cortical evoked normalised amplitudes. This was true for both methods of activation of the hand and for contralateral activation of TA, but not for ipsilateral activation (Fig. 2). No correlation was found between hand and leg amplitudes (Fig. 2C). The results of the regression analysis were for IL activation of APB: right M E P / M wave = 0 . 6 3 x l e f t M E P / M wave+0.1 (r = 0.55, P < 0.02); for CL activation: right M E P / M wave = 0.55 x le0 M E P / M wave + 0.11 (r = 0 . 6 , P

Central motor conduction in relation to contra- and ipsilateral activation.

Differences of central motor conduction between slight activation of the target muscle (ipsilateral: IL) and strong contraction of the contralateral (...
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