Ipsilateral Motor Evoked Potentials Research Articles

Corticospinal activation of internal oblique muscles has a strong ipsilateral component and can be lateralised in man

Trunk muscles receive corticospinal innervation ipsilaterally and contralaterally and here we investigate the degree of ipsilateral innervation and any cortical asymmetry in pairs of trunk muscles and proximal and distal limb muscles. Transcranial magnetic stimulation (TMS) was applied to left and right motor cortices in turn and bilateral electromyographic (EMG) recordings were made from internal oblique (IO; lower abdominal), deltoid (D; shoulder) and first dorsal interosseus (1DI; hand) muscles during voluntary contraction in ten healthy subjects. We used a 7-cm figure-of-eight stimulating coil located 2 cm lateral and 2 cm anterior to the vertex over either cortex. Incidence of ipsilateral motor evoked potentials (MEPs) was 85% in IO, 40% in D and 35% in 1DI. Mean (+/- S.E.M.) ipsilateral MEP latencies were longer ( P<0.05; paired t-test) than contralateral MEP latencies (contralateral vs. ipsilateral; IO: 16.1+/-0.4 ms vs. 19.0+/-0.5 ms; D: 9.7+/-0.3 ms vs. 15.1+/-1.9 ms; 1DI: 18.3+/-0.6 ms vs. 23.3+/-1.4 ms), suggesting that ipsilateral MEPs were not a result of interhemispheric current spread. Where data were available, we calculated a ratio (ipsilateral MEP areas/contralateral MEP areas) for a given muscle (IO: n=16; D: n=8; 1DI: n=7 ratios). Mean values for these ratios were 0.70+/-0.20 (IO), 0.14+/-0.05 (D) and 0.08+/-0.02 (1DI), revealing stronger ipsilateral drive to IO. Comparisons of the sizes of these ratios revealed a bias towards one cortex or the other (four subjects right; three subjects left). The predominant cortex showed a mean ratio of 1.21+/-0.38 compared with 0.26+/-0.06 in the other cortex ( P<0.05). It appears that the corticospinal control of IO has a strong ipsilateral component relative to the limb muscles and also shows hemispheric asymmetry.

Organization of ipsilateral excitatory and inhibitory pathways in the human motor cortex.

Motor cortex stimulation has both excitatory and inhibitory effects on ipsilateral muscles. Excitatory effects can be assessed by ipsilateral motor-evoked potentials (iMEPs). Inhibitory effects include an interruption of ipsilateral voluntary muscle activity known as the silent period (iSP) and a reduction in corticospinal excitability evoked by conditioning stimulation of the contralateral motor cortex (interhemispheric inhibition, IHI). Both iSP and IHI may be mediated by transcallosal pathways. Their relationship to the contralateral corticospinal projection and whether iSP and IHI represent the same phenomenon remain unclear. The neuronal population activated by transcranial magnetic stimulation (TMS) is highly dependent on the direction of the induced current in the brain. We examined the relationship among iMEP, iSP, IHI, and the contralateral corticospinal system by examining the effects of different stimulus intensities and current directions. Surface electromyography (EMG) was recorded from both first dorsal interosseous (FDI) muscles. The iSP in the right FDI muscle was obtained by right motor cortex stimulation during voluntary muscle contraction. IHI was examined by conditioning stimulation of the right motor cortex followed by test stimulation of the left motor cortex at interstimulus intervals (ISIs) of 2-80 ms. The induced current directions tested in the right motor cortex were anterior medial (AM), posterior medial (PM), posterior lateral, and anterior lateral (AL). Contralateral MEPs (cMEPs) had the lowest threshold with the AM direction and the shortest latency with the PM direction. iMEPs were present in 8 of 10 subjects. Both iMEP and IHI did not show significant directional preference. iSP was observed in all subjects with the highest threshold for the AL direction and the longest duration for the AM direction. cMEP, iSP, and IHI all increased with stimulus intensity up to approximately 75% stimulator output. Target muscle activation decreased IHI at 8-ms ISI but had little effect on IHI at 40-ms ISI. iSP and IHI at 8-ms ISI did not correlate at any stimulus intensities and current directions tested, and factor analysis showed that they are explained by different factors. However, active IHI at 40-ms ISI was explained by the same factor as iSP. The different directional preference for cMEP compared with iMEP and IHI suggests that these ipsilateral effects are mediated by populations of cortical neurons that are different from those activating the corticospinal neurons. iSP and IHI do not represent the same phenomenon and should be considered complementary measures of ipsilateral inhibition.

Ipsilateral motor-evoked potentials in patients with congenital and acquired mirror movements

We used transcranial magnetic stimulation (TMS) to investigate ipsilateral motor-evoked potentials (MEPs) in patients with congenital mirror movements together with adult stroke patients with acquired mirror movements on the unaffected hand. In patients with congenital mirror movements, there was an anomaly of corticospinal projections such as large inter-muscle differences in the ratio of the ipsilateral to contralateral corticospinal projections. An electrophysiological finding in stroke patients with acquired mirror movements was ipsilateral MEPs in the unaffected hand muscles after TMS to the damaged hemisphere. This could be explained by the presence of a small cerebral lesion with sparing of the uncrossed corticospinal projections from the damaged motor cortex.

Cortical Representation of the Orbicularis Oculi Muscle as Assessed by Transcranial Magnetic Stimulation (TMS)

To analyze characteristic features and details on motor-evoked potentials (MEPs) of the orbicularis oculi muscle resulting from cortical transcranial magnetic stimulation (TMS) in normal subjects as a basis for further investigations on motorcortical representation in patients presenting with facial nerve diseases. MEPs of the orbicularis oculi muscle resulting from focal cortical TMS with a figure-8-shaped coil were investigated in 17 healthy subjects with special regard to amplitude and onset latency as a function of the coil position on the head surface along the interaural line and in the anterior-posterior direction. The results were then compared with our data on lower-lip mimetic muscles and on the frontalis muscle obtained in previous studies. Bilateral reproducible responses could be observed at coil positions varying from 1 to 13 cm lateral to the vertex. During moderate muscle activation, maximum responses (mean amplitude 0.75 +/- 0.44 mV contralateral, 0.74 +/- 0.36 mV ipsilateral) were obtained at a mean stimulus position of 8.6 +/- 1.6 cm lateral and 2.0 +/- 2.2 cm anterior to the vertex for contralateral responses, and of 8.6 +/- 2.0 cm lateral and 2.8 +/- 2.4 cm anterior to the vertex for ipsilateral responses, respectively. Voluntary muscle activation by forced eye-closure was associated with a further increase in mean amplitudes. At rest, bilateral responses could be elicited in 15 subjects (88.2%). During moderate muscle activation, the shortest mean onset latencies were obtained at the optimum stimulus position on the interaural line, both for contralateral (10.2 +/- 1.3 ms) and ipsilateral (10.6 +/- 1.5 ms) MEPs. Comparing our data on the orbicularis oculi muscle with those obtained on lower-lip muscles and on the frontalis muscle, there was a considerable overlap of coil positions from which reproducible MEPs could be elicited in all three groups of mimetic muscles, but with the orbicularis oculi area being placed between forehead and lower-lip motorcortical areas. A statistically significant separation of the cortical representation areas of forehead, orbicularis oris, and lower-lip mimetic muscles is possible by focal cortical TMS reflecting a kind of somatotopic organization of the face-associated motorcortex. Compared with the results on lower-lip and forehead muscles, orbicularis oculi muscle responses show characteristics of both.

Task-dependent control of human masseter muscles from ipsilateral and contralateral motor cortex.

Corticotrigeminal projections to human masseter motoneuron pools were investigated with focal transcranial magnetic stimulation (TMS). Responses in left and right masseter muscles were quantified from the surface electromyogram (EMG) during different biting tasks. During bilateral biting, TMS elicited motor evoked potentials (MEPs) in both masseter muscles. On average, the MEP area in the masseter contralateral to the stimulus was 39% larger than in the ipsilateral muscle, despite comparable pre-stimulus EMG in both muscles. MEPs elicited while subjects attempted unilateral activation of one masseter muscle were compared with those obtained in the same muscle during a bilateral bite at an equivalent EMG level. MEPs in the masseter contralateral to the stimulated hemisphere were significantly smaller during unilateral compared with bilateral biting. There was no significant difference in the size of ipsilateral MEPs during ipsilateral and bilateral biting. We conclude that the corticotrigeminal projections to masseter are bilateral, with a stronger contralateral projection. The command for unilateral biting is associated with a reduced excitability of corticotrigeminal neurons in the contralateral, but not the ipsilateral motor cortex. We suggest that this may be accomplished by reduced activity of a population of corticotrigeminal neurons which branch to innervate both masseter motoneuron pools.

Cortical motor reorganization following early brain damage and hemispherectomy demonstrated by transcranial magnetic stimulation

Background: Transcranial magnetic stimulation (TMS) was utilized to study 7 patients who had undergone hemispherectomy for control of longstanding intractable epilepsy to assess cortical motor reorganization and MEP (motor evoked potentials) ipsilateral to the remaining hemisphere. Five patients were seizure-free post-operatively. All patients displayed a spastic hemiparesis, were able to walk, raise and bend the paretic arm and use the paretic hand to different degrees.Methods: We examined MEP threshold, site of maximal response to TMS, and latency to the biceps brachii, abductor pollicis brevis and the anterior tibial muscles using a 7 and 12 cm diameter round coil.Results: The following results were obtained: (1) No MEP could be induced to either side by use of the small coil. (2) When using the large coil MEP-threshold was elevated also when stimulating the non-affected hemisphere in comparison to normal controls. (3) Ipsilateral MEP were found to: be non-inducible in two patients, demonstrate largely increased latencies in 3 and be identical to contralateral MEP in two patients. (4) Ipsilateral MEP could be evoked more often in distal than proximal muscles. (5) The site of maximal ipsilateral MEP response was found approximately 2 cm anteriorly to that of the contralateral MEP to the non-affected side of the body (6) No correlation was found between the degree of hemiparesis and inducability of MEP.Conclusions: The pattern of reorganization of ipsilateral motor control following early brain damage and hemispherectomy appears to be extremely diverse and does not correlate with the degree of neurological impairment.

Functional recovery in hemiplegic cerebral palsy: ipsilateral electromyographic responses to focal transcranial magnetic stimulation.

The patterns of functional recovery after unilateral cerebral damage occurring in the prenatal to infantile periods were studied in nine patients with hemiplegic cerebral palsy. Motor evoked potentials (MEPs) recorded from the small hand muscles were investigated using focal transcranial magnetic stimulation (TMS). The MEPs findings could be separated into three subtypes based on the features of ipsilateral MEPs elicited by TMS over the unaffected motor cortex. Bilateral MEPs of similar latency were obtained in three patients. These patients each having a congenital lesion invariably exhibited mirror movements and severe hemiparesis. Meanwhile, ipsilateral MEPs with markedly prolonged latency were demonstrated in two other patients, who exhibited synergistic associated movements and severe hemiparesis caused by an acquired lesion. In the remaining four patients, who showed mild hemiparesis without such abnormal interlimb coordinations, there were no ipsilateral MEPs. Thus, we suggest that TMS is useful for confirming the electrophysiological findings relevant to functional recovery in hemiplegic cerebral palsy underlying such abnormal interlimb coordinations. Specifically, bilateral MEPs of similar latency were considered consistent with compensatory mirror movements originating from bilateral motor representation in the unaffected motor cortex.

Ipsilateral motor activation in patients with cerebral gliomas.

The aim of this study is to provide neurophysiologic evidence of ipsilateral hemispheric activation in patients affected by intracerebral gliomas via the use of transcranial magnetic stimulation. The mechanisms involved in such ipsilateral activation have yet to be established, but they may involve preexisting routes that either are suppressed or undetected in the normal brain. Ipsilateral pathways may act in reserve, activated by the impairment of contralateral control. This hypothesis is suggested by the fact that the considerable size of the tumors in our patients is not matched by a proportionate loss of motor performance in the limbs contralateral to the affected hemisphere. However, it remains possible that ipsilateral motor-evoked potentials (iMEPs) may reflect reorganizational changes without significant functional effects. The effects of such activation were investigated using both focal and nonfocal coils stimulating cortical motor areas, with MEPs recorded from both left and right thenar muscles. Fifteen healthy control subjects and seven patients were examined. iMEPs were generally absent in normal subjects, but in contrast they were obtained in the patients by stimulating the healthy hemisphere using both round and figure-of-eight coils. Distinct from contralateral MEPs, iMEPs are obtained with higher thresholds (range, 60 to 80% of stimulator output) and display longer latencies (20.9 msec versus 19.4 msec). Taken in conjunction with recent research using functional imaging brain exploration and a variety of clinical, anatomic, and neurophysiologic studies, our results reflect a growing awareness of ipsilateral motor control and its potential compensatory role when contralateral routes are damaged.

Ontogeny of ipsilateral corticospinal projections: a developmental study with transcranial magnetic stimulation.

Transcranial magnetic stimulation (TMS) has been used to describe the maturation of the corticospinal tract in children. Ipsilateral corticospinal connections have been demonstrated with TMS in patients with congenital mirror movements, in patients after hemispherectomy, and in children with hemiplegic cerebral palsy. The goal of the study was to find out whether corticospinal ipsilateral projections in children can be demonstrated during the first decade of life as part of normal ontogeny. For this purpose, we examined 50 normal children (age range, 3-11 years) with focal TMS over the left and right hemispheres to target muscles in proximal and distal parts of the upper extremity (first dorsal interosseus, biceps brachii, and brachioradialis). To lower the stimulation threshold, we stimulated under voluntary preinnervation. In two-thirds of the children we elicited ipsilateral motor evoked potentials (MEPs). This occurred more often in proximal than in distal muscles. The latency of the ipsilateral MEPs was about 12 to 14 msec longer than the usual contralateral response. From the age of 10, and in adults, ipsilateral MEPs could not be detected. Also considering lesion data from adult patients, the most likely explanation for the disappearance of ipsilateral corticospinal connections after the age of 10 years is an increasing transcallosal inhibitory influence during development. The presence of ipsilateral corticospinal connections appears to be a normal state in ontogeny.

Mechanisms of central motor reorganization in pediatric hemiplegic patients.

Twenty hemiplegic patients were studied with transcranial magnetic stimulation (TMS). Motor evoked potentials (MEPs) of the biceps brachii (BB) and the abductor pollicis brevis muscles (APB) were recorded on both sides simultaneously. TMS was carried out with an 8-shaped coil over different scalp positions in the intact hemisphere. Bilateral MEPs of BB were elicited in patients with later childhood lesions as well as early lesion, but those of APB were only elicited in the latter (up to 2 years). In patients with prenatal or birth lesion on BB and in all patients on APB, cortical maps of MEP amplitude of paretic and non-paretic sides showed similar distributions. There were no remarkable differences in mean latency between both sides, and correlation coefficients of MEP amplitude between both sides were high in these patients. In patients with postnatal lesion on BB, MEP maps of both sides showed different distributions, ipsilateral MEP latencies were delayed and correlation coefficients were low. We suspect that ipsilateral MEPs after early lesion derive from axonal sprouting both in the proximal and the distal muscles. After postnatal lesion, other mechanisms of ipsilateral motor projection take place in the proximal muscles, but not in the distal ones.

Sternocleidomastoid muscle responses to transcranial magnetic stimulation in patients with cervical dystonia

Ten cervical dystonia (CD) patients, with involuntary head rotation to one side and contralateral sternocleidomastoid muscle (SCM) hypertrophy, were investigated with transcranial magnetic stimulation, and the results were compared to those of 10 healthy subjects. Monopolar needle electrodes with isolated shafts were used for bilateral electromyographic recordings in the SCMs of the motor evoked potentials (MEPs) elicited by the magnetic stimulator. The latencies of ipsilateral SCM MEPs were shorter in the CD patients than in the control subjects ( P<0.001). The latencies of SCM activity suppression by TMS were longer in the CD patients than in the control group when stimuli were given on the contralateral side ( P<0.05). Both the clinically dystonic and the contralateral SCM of the CD patients exhibited significantly abnormal latencies of the ipsilateral SCM MEPs ( P<0.01) and of the SCM suppression ( P<0.05). Three CD patients also had consistent activity in the SCM counteracting the direction of head rotation during the suppression experiments. The latencies of the suppression of this abnormal activation were shorter ( P<0.05), than the latencies of the suppression in the SCM during normal voluntary activation by these CD patients (i.e. rotation of the head in the contrary direction). The results suggest bilaterally enhanced motoneuronal excitability and disturbed inhibitory regulation in patients with CD.

Cerebral plasticity after stroke as revealed by ipsilateral responses to magnetic stimulation.

The present study aims to provide neurophysiological evidence of ipsilateral activation during motor recovery in patients after stroke. The effects of cortical reorganization were investigated using magnetic brain stimulation in order to record motor evoked potentials (MEPs) both from contralateral and ipsilateral hands. Ten healthy subjects and 13 patients were examined. The patients had suffered their first hemispheric stroke and consequent motor deficit. While ipsilateral responses (iMEPs) were absent in normal subjects, they were obtained from both ipsilateral and contralateral hands in patients. The ipsilateral MEP differed from contralateral MEP in the following respects: (1) elicitation during contraction; (2) a shorter latency; (3) a lower amplitude. The presence of optimal iMEPs (lower excitability threshold, larger amplitude) in recovered hands points to a role for the undamaged hemisphere, and in particular to the involvement of secondary motor areas. We suggest that iMEPs in general may be a marker of brain plasticity, and that the specific type described here appear in the context of fast autochthonous motor recovery.

Effects of transcranial magnetic stimulation on ipsilateral muscles

We studied the effects of transcranial magnetic stimulation of the motor cortex on ipsilateral upper extremity muscles in six normal men. Stimulation had inhibitory and excitatory effects on the muscles during voluntary activation. Transient inhibition, an ipsilateral silent period (ISP), occurred in all muscles tested, often without any preceding excitatory response. Motor evoked potentials (MEPs) occurred ipsilaterally in the proximal muscles of some subjects. Ipsilateral MEPs and ISPs were delayed relative to the MEPs evoked by the same stimulus in the corresponding contralateral muscles. The excitability of the alpha motoneuron pool, assessed during the period of the ISP by eliciting H-reflexes, showed no change, suggesting that ipsilateral inhibition acts at a level above the alpha motoneuron. Connections from motor cortex to ipsilateral muscles could be via the corpus callosum and contralateral hemisphere or via purely ipsilateral pathways.