The Anatomie and Physiologie Bases of Motor-Evoked Potentials

Neurophysiological Examinations as Adjunctive Tool to Imaging Techniques in Spontaneous Intracerebral Hemorrhage: IRONHEART Study

Introduction: Intracerebral hemorrhage (ICH) is a devastating disease, which may lead to severe disability or even death. Although many factors may influence the outcome, neurophysiological examinations might also play a role in its course. Our aim was to examine whether the findings of electroencephalography (EEG) and transcranial magnetic stimulation (TMS) can predict the prognosis of these patients.

Methods: Between June 1 2017 and June 15 2021, 116 consecutive patients with ICH were enrolled prospectively in our observational study. Clinical examinations and non-Contrast computed tomography (NCCT) scan were done on admission for ICH; follow-up NCCT scans were taken at 14 ± 2 days and at 3 months ± 7 days after stroke onset. EEG and TMS examinations were also carried out.

Results: Of the patients in the study, 65.5% were male, and the mean age of the study population was 70 years. Most patients had a history of hypertension, 50.8% of whom had been untreated. In almost 20% of the cases, excessive hypertension was measured on admission, accompanied with >10 mmol/L blood glucose level, whereas their Glasgow Coma Scale was 12 on average. Presence of blood in the ventricles or subarachnoid space and high blood and perihematomal volumes meant poor prognosis. Pathological EEG was prognostic of a worse outcome. With TMS examination at 14 days, it might be possible to estimate outcome in a univariate model and the absence, or reduction of the amplitude of the motor evoked potentials was associated with poor prognosis.

Conclusion: Together with the clinical symptoms, the volume of bleeding, perihematomal edema (or their combined volume), and neurophysiological examinations like EEG and TMS play an important role in the neurological outcome of patients with ICH. This might affect the patients' rehabilitation plans in the future, since with the help of the examinations the subset of patients with potential for recovery could be identified.

Age, Height, and Sex on Motor Evoked Potentials: Translational Data From a Large Italian Cohort in a Clinical Environment

INTRODUCTION

Motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) are known to be susceptible to several sources of variability. However, conflicting evidences on individual characteristics in relatively small sample sizes have been reported. We investigated the effect of age, height, and sex on MEPs of the motor cortex and spinal roots in a large cohort.

METHODS

A total of 587 subjects clinically and neuroradiologically intact were included. MEPs were recorded during mild tonic contraction through a circular coil applied over the "hot spot" of the first dorsal interosseous and tibialis anterior muscles (TAs), bilaterally. Central motor conduction time (CMCT) was estimated as the difference between MEP cortical latency and the peripheral motor conduction time (PMCT) by cervical or lumbar magnetic stimulation. Peak-to-peak MEP amplitude to cortical stimulation and right-to-left difference of each parameter were also measured.

RESULTS

After Bonferroni correction, general linear (multiple) regression analysis showed that both MEP cortical latency and PMCT at four limbs positively correlated with age and height. At lower limbs, an independent effect of sex on the same measures was also observed (with females showing smaller values than males). CMCT correlated with both age (negatively) and height (positively) when analyzed by a single regression; however, with a multiple regression analysis this significance disappeared, due to the correction for the multicollinearity within the dataset.

CONCLUSION

Physical individual features need to be considered for a more accurate and meaningful MEPs interpretation. Both in clinical practice and in research setting, patients and controls should be matched for age, height, and sex.

A Pilot Clinical Study of Olfactory Mucosa Autograft for Chronic Complete Spinal Cord Injury

Recent studies of spinal cord axon regeneration have reported good long-term results using various types of tissue scaffolds. Olfactory tissue allows autologous transplantation and can easily be obtained by a simple biopsy that is performed through the external nares. We performed a clinical pilot study of olfactory mucosa autograft (OMA) for chronic complete spinal cord injury in eight patients according to the procedure outlined by Lima et al. Our results showed no serious adverse events and improvement in both the American Spinal Injury Association (ASIA) Impairment Scale (AIS) grade and ASIA motor score in five patients. The preoperative post-rehabilitation ASIA motor score improved from 50 in all cases to 52 in case 2, 60 in case 4, 52 in case 6, 55 in case 7, and 58 in case 8 at 96 weeks after OMA. The AIS improved from A to C in four cases and from B to C in one case. Motor evoked potentials (MEPs) were also seen in one patient, reflecting conductivity in the central nervous system, including the corticospinal tract. The MEPs induced with transcranial magnetic stimulation allow objective assessment of the integrity of the motor circuitry comprising both the corticospinal tract and the peripheral motor nerves.We show the feasibility of OMA for chronic complete spinal cord injury.

Motor evoked potential and voluntary EMG activity after olfactory mucosal autograft transplantation in a case of chronic, complete spinal cord injury: case report

The efficacy of olfactory mucosal autografts (OMAs) for chronic spinal cord injury (SCI) has been reported, but there is no report documenting electrophysiological conductivity via the emergence of motor evoked potentials (MEPs). We report the case of a 39-year-old man with chronic, complete SCI at T8, who exhibited MEPs after OMA transplantation, and, with intensive rehabilitation, was ultimately able to ambulate with short leg braces and Lofstrand crutches. The initial injury occurred in a motor vehicle accident in November 1999 and resulted in a complete loss of sensorimotor function below T8. OMA transplantation to the injury site was performed in March 2010 in combination with intensive pre- and postoperative rehabilitation. The patient exhibited voluntary electromyograph (EMG) activity and MEPs at 96 and 144 weeks after transplantation and he was was ambulatory with short leg braces and Lofstrand crutches at 144 weeks after transplantation. We were able to elicit MEPs after OMA with intensive rehabilitation. To our knowledge, this is the first report of recovery of electrophysiological conductivity in the spinal cord after any type of treatment for chronic, complete SCI.

Top-down suppression of incompatible motor activations during response selection under conflict

Top-down control is critical to select goal-directed actions in changeable environments, particularly when several options compete for selection. This control system is thought to involve a mechanism that suppresses activation of unwanted response representations. We tested this hypothesis, in humans, by measuring motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) in a left finger muscle during motor preparation in an adapted Eriksen flanker task. Subjects reported, by a left or right button-press, the orientation of a left- or right-facing central arrow, flanked by two distractor arrows on each side. Central and peripheral arrows either pointed in the same (congruent trial) or in the opposite direction (incongruent trial). Top-down control was manipulated by changing the probability of congruent and incongruent trials in a given block. In the "mostly incongruent" (MI) blocks, 80% of trials were incongruent, producing a context in which subjects strongly anticipated that they would have to face conflict. In the "mostly congruent" (MC) blocks, 80% of trials were congruent and thus subjects barely anticipated conflict in that context. Thus, we assume that top-down control was stronger in the MI than in the MC condition. Accordingly, subjects displayed a lower error rate and shorter reaction times for the incongruent trials in the MI context than for similar trials in the MC context. More interestingly, we found that top-down control specifically reduced activation of the incompatible motor representation during response selection under high conflict. That is, when the central arrow specified a right hand response, left (non-selected) MEPs became smaller in the MI than in the MC condition, but only for incongruent trials, and this measure was positively correlated with performance. In contrast, MEPs elicited in the non-selected hand during congruent trials, or during all trials in which the left hand was selected, tended to increase more after the imperative signal in the MI than the MC condition. Another important observation was that, overall, MEPs were already strongly suppressed at the onset of the imperative signal and that this effect was particularly pronounced in the MI context. Hence, suppression of motor excitability seems to be a key component of conflict resolution.

Optimising the detection of upper motor neuron function dysfunction in amyotrophic lateral sclerosis?a transcranial magnetic stimulation study

Evidence of upper motor neuron (UMN) dysfunction is essential in making the diagnosis of amyotrophic lateral sclerosis (ALS). Central motor conduction (CMC) abnormalities detected using transcranial magnetic stimulation (TMS) are presumed to reflect UMN dysfunction. CMC is, however, often normal in patients with classical sporadic ALS. The aim of the study was to determine whether the utility of the CMC measure in ALS could be enhanced. We measured CMC to four pairs of muscles (abductor digiti minimi (ADM), biceps, vastus medialis (VM) and abductor hallucis (AH) in 20 controls and 25 ALS patients. The commonest abnormality detected in the ALS patients was an absent MEP, found in 11 patients (44 %) and in 25 of 200 muscles examined. Studying a minimum of three muscles increased the probability of detecting UMN dysfunction. Weakness in the muscle as well as selecting a distal rather than a proximal muscle was significantly associated with an abnormal CMC. Interside differences in CMC were significantly more pronounced in the patient group. In 30% of patients a significant interside difference in AH CMC time was the sole abnormality, suggesting mild UMN dysfunction on the side with the longer CMC.

A novel central motor conduction abnormality in D90A-homozygous patients with amyotrophic lateral sclerosis

Patients with amyotrophic lateral sclerosis (ALS) who are homozygous for the D90A SOD1 mutation have been noted to have central motor abnormalities distinct from those of patients with idiopathic ALS. We stimulated the motor cortex of ten patients homozygous for the D90A SOD1 mutation, using transcranial magnetic stimulation (TMS), and recorded the response evoked in the right first dorsal interosseous muscle when the muscle was at rest and when voluntarily active. A subgroup of patients had two distinct evoked responses when the cortex was stimulated at high intensity with the muscle at rest. When the muscle was modestly contracted, the first of these responses disappeared, whereas the second response was facilitated. Both fast and slow components of the corticospinal tract were usually intact and excited by TMS in these patients. We propose that there is an abnormality of intracortical or intraspinal inhibition in a subgroup of D90A SOD1 ALS patients, which suppresses fast-conducted activity when the muscle is active. Apart from further defining the phenotype of familial ALS, these findings may have importance in understanding the pathogenesis of central motor abnormalities in these patients.

Corticomotoneuronal connections in primary lateral sclerosis (PLS)

BACKGROUND

The relationship between primary lateral sclerosis (PLS) and amyotrophic lateral sclerosis (ALS) is uncertain. The slow progression and dominant upper motor neuron features of PLS are associated with a high threshold to cortical magnetic stimulation and sometimes slow central motor conduction. In ALS the cortical threshold may be reduced early in the disease and central conduction is usually normal. Corticomotoneuronal function appears to be impaired differently in PLS and ALS.

SUBJECTS AND METHODS

We assessed corticomotoneuronal function by analyzing the primary peak in the peristimulus time histograms (PSTHs) in 12 PLS and 12 ALS patients. Surface recorded motor evoked potentials (MEPs) and central motor conduction time (CMCT) were determined. PSTHs were constructed from 4-5 different, voluntarily recruited motor units in each patient and the onset latency, number of excess bins, duration and synchrony of the primary peak were measured.

RESULTS

The mean cortical threshold of single motor units in PLS was 73.6%, significantly higher than in ALS (60.3%; p < 2.2 x 10(-5)). Profoundly delayed primary peaks occurred in both PLS and ALS. Onset latency and desynchronization of the primary peak were similar in PLS and ALS, but the duration of the primary peak was significantly longer in PLS (p < 0.04).

CONCLUSIONS

Desynchronized primary peaks indicate dysfunction or demise of corticomotoneurones. Higher threshold and longer duration of the primary peak in PLS probably reflect different excitability and greater loss of corticomotoneuronal connections than in ALS.

Correlation between severity of clinical signs and motor evoked potentials after transcranial magnetic stimulation in large-breed dogs with cervical spinal cord disease

OBJECTIVE

To evaluate use of transcranial magnetic motor evoked potentials for assessment of the functional integrity of the cervical spinal cord in large-breed dogs with cervical spinal cord disease.

DESIGN

Randomized, controlled, masked study.

ANIMALS

10 healthy large-breed control dogs and 25 large-breed dogs with cervical spinal cord diseases.

PROCEDURE

Affected dogs were allocated to 3 groups on the basis of neurologic status: signs of neck pain alone, ambulatory with ataxia in all limbs, or nonambulatory. Transcranial magnetic stimulation was performed on each dog with the same standard technique. Motor evoked potentials (MEP) were recorded from electrodes inserted in the tibialis cranialis muscle. Following the procedure, each dog was anesthetized and cervical radiography, CSF analysis, and cervical myelography were performed. The MEP latencies and amplitudes were correlated with neurologic status of the dogs after correction for neuronal path length.

RESULTS

Mean MEP latencies and amplitudes were significantly different between control dogs and dogs in each of the 3 neurologic categories, but were not significantly different among dogs in the 3 neurologic categories. A linear association was evident between MEP latencies and amplitudes and severity of neurologic deficits; the more severe the neurologic deficits, the more prolonged the latencies and the more decreased the amplitudes.

CONCLUSIONS AND CLINICAL RELEVANCE

Transcranial magnetic MEP are useful to assess severity of cervical spinal cord disease in large-breed dogs. Impairment of the functional integrity of the cervical spinal cord was found even in dogs with neck pain alone.

Effects of a photic input on the human cortico-motoneuron connection

OBJECTIVES

Disease manifestations such as photic cortical reflex myoclonus or myoclonus due to intermittent light stimulation rely on a pathologic interaction between non-structured visual inputs and the corticospinal system. We wanted to assess the normal interaction, if any, between a prior photic input and the output of the cortico-motoneuron connection.

METHODS

In 9 consenting healthy subjects we quantified the changes exerted by a sudden, unexpected bright light flash on (i) the motor potentials (MEPs) evoked in the right first dorsal interosseous muscle (FDI) by transcranial magnetic or electrical stimulation (TMS/TES) of the primary motor cortex, (ii) the FDI F-waves and (iii) the soleus H-wave. Separately, we measured the simple reaction times to the flash itself. All determinations were repeated twice with an interval of 2-24 months.

RESULTS

When the flash preceded TMS by 55-70 ms, the MEP size was reduced, while at interstimulus intervals (ISIs) of 90-130 ms it was enlarged. Statistical significance (P<0.05) emerged at ISIs of 55, 70, 100, 105 and 120 ms. Conversely, the MEP latency was prolonged at ISIs of 55-70 ms and shortened at ISIs of 90-130 ms (P<0.05 at ISIs of 55, 110 and 130 ms). Electrical MEPs were enhanced at an ISI of 120 ms. The F-wave size showed a non-significant trend of enhancement at ISIs of 90-130 ms. The soleus H-wave showed significant enlargement at ISIs of 90-130 ms (P<0.05 at ISIs of 100 and 105 ms). The minimum reaction time was on average 120 ms.

CONCLUSIONS

An unexpected photic input, to which no reaction is planned, can cause an early inhibition of the responses to TMS. We think its origin lies within the primary motor cortex, since it is not associated with changes in spinal excitability or electrical MEPs. A later facilitation persists using TES and has a temporal relationship with an enlargement of the soleus H-wave. Thus, it likely results from activation of descending (possibly reticulospinal) fibers that excite the spinal motor nucleus.

Prognostic value of motor evoked potential obtained by transcranial magnetic brain stimulation in motor function recovery in patients with acute ischemic stroke

BACKGROUND AND PURPOSE

The early prognostic application of transcranial magnetic brain stimulation (TMS) for assessing motor and functional recovery in ischemic stroke patients has yielded contradictory results. We performed a prospective study of patients with acute ischemic stroke and motor deficit to evaluate the early prognostic value of TMS in motor and functional recovery.

METHODS

Fifty patients with different degrees of hemiparesis were studied in the first week after ischemic stroke and evaluated by clinical scales (Medical Research Council Scale, Canadian Neurological Scale, Barthel Index), with clinical follow-up over 6 months. TMS (Magstim 200) was performed at the same time, recording the motor evoked potential (MEP) in the thenar eminence muscles, with facilitation by voluntary contraction.

RESULTS

Of the total group of 50 patients, MEP was absent in 20 and present in 30 (17 with normal and 13 with delayed central conduction time [CCT]). The patients with MEP showed better motor and functional recovery than those without. The MEP provided information on patient recovery, regardless of the initial strength and/or Barthel values. The degree of recovery was better in those patients with normal CCT than in those with delayed CCT.

CONCLUSIONS

MEP obtained by TMS in patients with hemiparesis after acute ischemic stroke is useful as an early prognostic indicator of motor and functional recovery. This technique would allow the early identification of those patients who will have a good recovery, particularly among those with severe initial paresis.

The use of a circumferential cathode improves amplitude of intraoperative electrical transcranial myogenic motor evoked responses

Measurement of motor evoked responses to transcranial electrical stimulation (tc-MER) is a technique for intraoperative monitoring of motor pathways. Since most anesthetics significantly reduce motoneuronal excitability, optimal stimulation paradigms should be sought. We compared the efficiency of stimulus delivery using two different configurations of the cathode component of the stimulating electrode pair (circumferential: Fz, F3, F4, A1, and A2 versus a single cathode at Fz). The anode was positioned at Cz with both cathode configurations. Fourteen neurologically normal patients undergoing spinal surgery were anesthetized with sufentanil-N2O-ketamine. Partial neuromuscular blockade (single twitch height 25%) was maintained with vecuronium. Compound action potentials to transcranial stimulation with both cathode configurations were recorded from the tibialis anterior muscle. All recordings were completed before spinal manipulation. The median amplitude response using the Fz cathode configuration was 256 microV (10th-90th percentiles: 50-641 microV). With the circumferential cathode configuration, tc-MER amplitude increased to 281 (87-1479) microV (P < 0.01). There was no significant difference in onset latency between electrode configurations. The observed tc-MER amplitude augmentation with the use of a circumferential cathode might allow tc-MER monitoring in those patients who do not have sufficiently reproducible responses when a single cathode is used. A possible explanation is that the circumferential cathode alters the direction of the electrical currents in the cortex, resulting in more efficient depolarization of cortical motor neurons.

Measurement of motor conduction in the thoracolumbar cord. A possible predictor of surgical outcome in cervical spondylotic myelopathy

STUDY DESIGN

A prospective motor-evoked potential study with measurement of spinal cord motor conduction velocity in the thoracolumbar cord was performed before and after decompression surgery in 30 patients with cervical spondylotic myelopathy.

OBJECTIVES

To evaluate the neurofunctional integrity of the spinal motor pathways in cervical spondylotic myelopathy in patients compared with age-matched control subjects; to assess any changes after posterior surgical decompression; and to correlate such changes with functional outcomes so that the predictability of preoperative motor-evoked potentials could be determined.

SUMMARY OF BACKGROUND DATA

Previous studies evaluating neurologic function and predictability of surgical results in cervical spondylotic myelopathy patients always depended on the morphologic changes of the cord and spinal structures. The recently developed motor-evoked potential study and noninvasive measurement of spinal cord motor conduction velocity may provided an objective method to evaluate physiologic motor function in cervical spondylotic myelopathy patients.

METHODS

Spinal cord motor conduction velocity in the thoracolumbar cord was measured using percutaneous magnetic stimulation over the motor cortices and F-wave studies in median and peroneal nerves. Motor function of cervical spondylotic myelopathy patients was graded according to evaluation of signs of cord involvement, ambulation, and degree of dependence in activities of daily living. Evaluation was performed at 6 months, 1 year, and 2 years after decompression surgery.

RESULTS

Motor functional improvement accompanied by increased spinal cord motor conduction velocity occurred in Grade I patients with a mild neurologic dysfunction but not in Grade II or III patients with a moderate-to serve neurologic deficit. Neurologic improvement does not appear to occur until 6 months after surgery.

CONCLUSIONS

Measurement of spinal cord motor conduction velocity may provide an objective and quantitative approach to assessing the motor functional integrity of the spinal cord and serving as a predictor in evaluating surgical outcome in patients with cervical spondylotic myelopathy.

Magnetic motor evoked potentials during methohexital anesthesia in the dog

Magnetic motor evoked potentials (MMEPs) were recorded from the right cranial tibial muscle after magnetic stimulation of the left motor cortex in six dogs sedated with oxymorphone. Anesthesia was induced with an intravenous bolus of 5.5 mg/kg of methohexital and maintained with a methohexital infusion. The dogs inspired 100% oxygen during anesthesia. Blood pressure, heart rate, respiratory rate, esophageal temperature, and end-tidal carbon dioxide tension were recorded. The depth of anesthesia was increased until the amplitude of the MMEP was less than 5% of the control value, and the dogs were then allowed to recover. Every 5 minutes during anesthesia, a blood sample was taken for methohexital assay and at the same time, four replicate MMEPs were recorded. Plasma methohexital levels were significantly (P < 0.05) correlated with heart rate (p = 0.38) and end-tidal carbon dioxide tension (p = 0.49) and negatively correlated with respiratory rate (p = 0.74). There was no significant correlation between blood pressure and methohexital levels. The dogs regained consciousness at a plasma methohexital level of 10.4 +/- 3.8 micrograms/ml (mean +/- SD). The amplitude of the MMEP decreased significantly with increasing methohexital levels. In four dogs, the relationship was reasonably linear. The MMEP disappeared at a plasma methohexital level of 23 +/- 6.6 micrograms/ml. The latency of onset of the MMEP increased significantly from its control value of 14.7 +/- 1.0 ms to 17.5 +/- 1.3 ms at the highest methohexital levels at which MMEPs were recordable. This study demonstrated that MMEPs can be reliably recorded under methohexital anesthesia.

Magnetic motor evoked potentials for assessing spinal cord integrity in dogs with intervertebral disc disease

Magnetically elicited transcranial motor evoked potentials (MEPs) were studied in 37 dogs with type 1 intervertebral disc (IVD) disease. The waveforms were recorded from both cranial tibial muscles before and after surgery. The latencies and amplitudes obtained were compared to those of a control population (n = 14). MEPs were recordable in all dogs with mild or no neurologic deficits, but they were recordable in only 50% of ambulatory dogs that were severely ataxic. MEPs could not be elicited from nonambulatory dogs. There was a significant attenuation of the amplitudes in all clinical cases, even if the dogs demonstrated back pain alone. Significantly prolonged latencies were associated with neurologic deficits. MEPs were not good predictors of neurologic recovery. The responses obtained from the side where the disc material was found were not different from those recorded from the opposite side. MEPs were very sensitive to lesions of the spinal cord, as indicated by the significant changes in the waves in patients with mild or no neurologic deficits and in the loss of response in dogs that still demonstrated purposeful movement. Neurologic exams provided more accurate diagnoses and prognoses than did MEPs in dogs with IVD disease.

Age-related changes of motor evoked potentials in healthy humans: non-invasive evaluation of central and peripheral motor tracts excitability and conductivity

A comparative analysis of the corticospinal tract nervous propagation and excitability threshold was carried out in young (25 subjects, age range 16-35 years) and in elderly (40 subjects, 51-86 years) populations of healthy volunteers. Motor evoked potentials (MEPs) were recorded from the hand and foot muscles following transcranial magnetic stimulation (TCS) during complete relaxation and active contraction of the target muscles. Threshold intensities corresponded to the stimulator's output eliciting liminal MEPs in about 50% of stimuli during relaxation. It was found that threshold values of magnetic TCS were significantly higher in the elderly (44 +/- 6.4% vs 39 +/- 3.5% for the hand; 66 +/- 10.1% vs 56 +/- 6.7% for the foot; P < 0.001) than in the young subjects. Moreover, this index progressively increased with age (P < 0.001), whilst the propagation time along the central motor tracts did not parallel such an age-related trend.

Motor-evoked responses in primary lateral sclerosis

Primary lateral sclerosis (PLS) may be distinguished on the basis of clinical and pathological features from amyotrophic lateral sclerosis (ALS). The former is featured by a much longer clinical course, exclusively upper motor neuron findings, losses of precentral pyramidal neurons, and preservation of anterior horn cells. Electrophysiological studies of 7 PLS cases have shown normal peripheral motor conduction, absent or very delayed motor-evoked potentials, the occasional late development of denervation activity in distal muscles, and normal somatosensory-evoked potentials.

The effect of graded spinal cord injury on the extrapyramidal and pyramidal motor evoked potentials of the rat

This study investigated the differential effects of graded spinal cord injury on the rat extrapyramidal motor evoked potential (exp-MEP) and pyramidal motor evoked potential (pyr-MEP) and the prognostic value of these effects in predicting postinjury motor performance in the rat model. In 20 rats subjected to graded spinal injury (10-100 g-cm), there was a differential injury threshold for ablation of exp-MEP and pyr-MEP. All peaks of the pyr-MEP were extinguished in the animals subjected to impact forces of 50 g-cm and above (n = 12). In contrast, the exp-MEP was completely abolished in only two animals at injuries of 80 g-cm or above. A residual exp-MEP response persisted in the remaining 18 animals. Motor performance was monitored in 16 additional animals for up to 1 week after spinal injury. The pyr-MEP was abolished in 100% of the rats subjected to a 50-g cm injury (n = 7), whereas the exp-MEP persisted up to the highest impact forces (80 g-cm). Hind leg paralysis was present in the five rats where the pyr-MEP was extinguished but with persistence of the exp-MEP. An 80% reduction in the amplitude of the pyr-MEP in four animals resulted in mild ataxia with motor improvement at the end of a week. An increase or a 70% loss in pyr-MEP peak amplitude resulted in no clinical motor deficits (n = 5).(ABSTRACT TRUNCATED AT 250 WORDS)

Age-dependent decline in motor evoked potential (MEP) amplitude: with a comment on changes in Parkinson's disease

Peak-to-peak measurement of the maximum amplitude motor evoked potential (MAXMEP) elicited by 20 consecutive transcranial magnetic stimuli recorded from the contracting thenar and hypothenar muscles measured 9.8 +/- 2.0 mV and 7.25 +/- 2.9 mV respectively (P less than 0.01). The ratio of MAXMEP/CMAP measured 92.6 +/- 25.8% and 54.8 +/- 12.3% respectively (P less than 0.001). Repeat studies showed good individual reproducibility. Amplitudes declined linearly with age (r = -0.836 for thenar MAXMEP P less than 0.001). It is argued that MAXMEP related to age is more meaningful than the MEP/CMAP wave ratio and is proportional to the number of fast conducting cortical motor neurons excited. In 7/18 patients with Parkinson's disease (PD) MAXMEP was increased; in 2 other patients MAXMEP was decreased for their age.

AAEM minimonograph #35: Clinical experience with transcranial magnetic stimulation

We elicited motor evoked potentials (MEPs) using transcortical magnetic stimulation in 150 control subjects aged 14 to 85 years and 275 patients with a variety of diseases. There were no significant side effects. Cortex-to-target muscle latencies measured 20.2 +/- 1.6 ms (thenar), 14.2 +/- 1.7 ms (extensor digitorum communis), 9.4 +/- 1.7 ms (biceps), and 27.2 +/- 2.9 ms (tibialis anterior). Central motor delay between the cortex and the C-7 and L-5 measured 6.7 +/- 1.2 ms and 13.1 +/- 3.8 ms, respectively. Mean spinal cord motor conduction velocity measured 65.4 m/s. MEP amplitude expressed as a percentage of the maximum M wave was never less than 20% of the M wave. A value of less than 10% is considered abnormal. MEP latency increases linearly with age and central motor delay is longer in older subjects. Compound muscle action potentials and absolute MEP amplitudes decreased linearly with age. In multiple sclerosis (MS), MEP latency and central delay were often very prolonged. The MEP was more sensitive than the SEP in MS. In amyotrophic lateral sclerosis, MEP latencies were only modestly prolonged; the characteristic abnormality was reduced amplitude. When pseudobulbar features predominated MEPs were often absent. The MEP was of normal latency in Parkinson's disease, but age-related amplitude was often increased. MEP latency and amplitude were normal in Huntington's disease. Abnormal MEPs persisted several months after stroke despite good functional recovery. The MEP could be used to advantage to demonstrate proximal conduction slowing and block in demyelinating neuropathies. In plexopathy, ability to elicit an MEP several days after onset of paresis was good evidence of neuronal continuity in motor fibers.