Evaluation of corticospinal excitability in cervical myelopathy, before and after surgery, with transcranial magnetic stimulation: a pilot study.

PURPOSE: A pilot study to examine the impact of cervical myelopathy on corticospinal excitability, using transcranial magnetic stimulation, and to investigate whether motor evoked potential (MEP) and silent period (SP) recruitment curve (RC) parameters can detect changes in corticospinal function pre- and post-surgery. METHODS: We studied six cervical myelopathy patients undergoing surgery and six healthy controls. Clinical and functional scores and neurophysiological parameters were examined prior to and 3 months following the surgery. RESULTS: MEP latencies for abductor pollicis brevis (APB) and tibialis anterior (TA) muscles and central motor conduction time were prolonged pre- and post-surgery; SP durations were differentially altered. There were significant differences in parameters of RCs for (1) MEP area in APB (max values, S50) and TA (slope) between controls and patients pre- and post-surgery and (2) SP duration in APB (max values) between patients pre-surgery and controls. CONCLUSIONS: The findings of this pilot study suggest an uncoupling of excitatory and inhibitory pathways, which persists at 3 months following cord decompression. RCs for MEP and SP at 3 months provide more information on the functional status of the cord and prompts for a longer term follow-up.

Trunk muscle responses following unpredictable loading of an abducted arm.

This study investigated if asymmetry exists in the responses of trunk muscles to a perturbation of the trunk induced by loading of an outstretched arm. Nineteen healthy right-handed subjects were recruited into the study. Electromyographic recordings were made from trunk muscles (erector spinae and rectus abdominus) and upper limb muscles (deltoid). A weight was dropped into a receptacle held out laterally by the subjects. The perturbation induced a rise in EMG activity in the deltoid at a latency which was not different between the dominant and non-dominant arms. It also induced a rise in EMG activity in the contralateral trunk muscles. Although not significantly different, there was a trend for the responses from the right trunk to be longer than those from the left trunk. Furthermore, there were higher levels of EMG activity in the trunk muscles opposite the dominant arm than those in the trunk muscles opposite the non-dominant arm. This study reveals a pattern of trunk muscle activation following movement in the arm induced by loading of an outstretched hand that is different on the side of the back opposite the dominant arm than on the side of the back opposite the non-dominant arm. These results may have implications in terms of mechanisms contributing to low back pain and further work is warranted to examine these responses in left-handed individuals.

Cortical control of erector spinae muscles during arm abduction in humans.

Abduction of one arm preferentially activates erector spinae muscles on the other side to stabilise the body. We hypothesise that the corticospinal drive to the arm abductors and the erector spinae may originate from the same hemisphere. In 18 subjects, transcranial magnetic stimulation (TMS) was applied using an angle double-cone coil placed symmetrically over the vertex. Motor evoked potentials (MEP) could not be evoked systematically seated at rest but could be evoked bilaterally in erector spinae muscles during unilateral arm abduction. TMS was applied at 110% and 120% motor threshold (MT) for the contralateral erector spinae muscle when an arm was abducted against resistance. The electromyographic (EMG) activity in the erector spinae at L4 vertebral level during contralateral arm abduction was significantly higher (P<0.05) than in the ipsilateral erector spinae. The mean (+/-S.E.M.) latencies of MEPs in the contralateral muscle to TMS at 120%MT (left 16.0+/-0.8 ms; right 17.0+/-0.8 ms) were significantly (P<0.05) longer than in the ipsilateral erector spinae (13.9+/-1.0 ms; 16.6+/-0.4 ms). In two of six subjects from the same group, it was possible to elicit MEPs by TMS applied selectively to one hemisphere using a figure-of-eight coil. MEPs ipsilateral to the TMS had longer latencies than contralateral MEPs. The study revealed an unexpectedly longer rather than shorter latency of the MEP recorded from the lumbar erector spinae muscles when co-activated during abduction of the opposite arm. A speculative explanation is that TMS might activate back muscles contralateral to arm abduction via an uncrossed, ipsilateral corticospinal tract that is slower conducting than the conventional crossed corticospinal tract. The study has implications for the design of measures to promote recovery and rehabilitation of motor function in disorders such as stroke and spinal cord injury.

Review of physiological motor outcome measures in spinal cord injury using transcranial magnetic stimulation and spinal reflexes.

This article reviews methods that have been developed as part of a clinical initiative on improving outcome measures for motor function assessment in subjects with spinal cord injury (SCI). Physiological motor outcome measures originally developed for limbs-transcranial magnetic stimulation (TMS) of the motor cortex to elicit motor-evoked potentials (MEPs) and mechanical stimulation to elicit spinal reflexes-have been extended to muscles of the trunk. The impetus for this development is the lack of a motor component in the American Spinal Injury Association clinical assessment for the thoracic myotomes. The application of TMS to the assessment of limb muscles is reviewed, followed by consideration of its application to the assessment of paravertebral and intercostal muscles. Spinal reflex testing of paravertebral muscles is also described. The principal markers for the thoracic SCI motor level that have emerged from this clinical initiative are (1) the threshold of MEPs in paravertebral muscles in response to TMS of the motor cortex, (2) the facilitation pattern and latency of MEPs in intercostal muscles during voluntary expiratory effort, and (3) the absence of long-latency reflex responses and the exaggeration of short-latency reflex responses in paravertebral muscles.

An investigation of leg and trunk strength and reaction times of hard-style martial arts practitioners.

The purpose of this study was to investigate trunk and knee strength in practitioners of hard-style martial arts. An additional objective was to examine reaction times in these participants by measuring simple reaction times (SRT), choice reaction times (CRT) and movement times (MT). Thirteen high-level martial artists and twelve sedentary participants were tested under isokinetic and isometric conditions on an isokinetic dynamometer. Response and movement times were also measured in response to simple and choice auditory cues. Results indicated that the martial arts group generated a greater body-weight adjusted peak torque with both legs at all speeds during isokinetic extension and flexion, and in isometric extension but not flexion. In isokinetic and isometric trunk flexion and extension, martial artists tended to have higher peak torques than controls, but they were not significantly different (p > 0.05). During the SRT and CRT tasks the martial artists were no quicker in lifting their hand off a button in response to the stimulus [reaction time (RT)] but were significantly faster in moving to press another button [movement time (MT)]. In conclusion, the results reveal that training in a martial art increases the strength of both the flexors and extensors of the leg. Furthermore, they have faster movement times to auditory stimuli. These results are consistent with the physical aspects of the martial arts. Key PointsMartial artists undertaking hard-style martial arts have greater strength in their knee flexor and extensor muscles as tested under isokinetic testing. Under isometric testing conditions they have stronger knee extensors only.The trunk musculature is generally higher under both conditions of testing in the martial artists, although not significantly.The total reaction times of the martial artists to an auditory stimulus were significantly faster than the control participants. When analysed further it was revealed that the decrease in reaction time was due to the movement time component of the total reaction time.The training involved for the practice of the hard-style martial arts increases the strength of muscles involved in kicking. This increased strength is not seen in the trunk muscles. Furthermore, martial artists have a faster response time; the cause of which appears to be only the faster movement time.

Corticospinal excitability in patients with chronic low back pain.

OBJECTIVE: This study was designed to investigate corticospinal excitability of lumbar muscles using transcranial magnetic stimulation (TMS) in patients with chronic low back pain and correlate this with self-rated measures of disability and pain. METHODS: Twenty-four patients with chronic low back pain and 11 healthy control subjects were used in this study. TMS was delivered through an angled double-cone coil, with its cross-over on the vertex and a posterior-to-anterior current flow in the brain. Electromyographic (EMG) recordings were made from erector spinae (ES) muscles at the fourth lumbar level. Motor cortical excitability was assessed using motor threshold (MTh) for motor evoked potentials (MEPs) and threshold for silent period (SP) during facilitation of the back muscles. Latency, duration, and area of MEPs and SPs were also measured. RESULTS: The latency, duration, and size of MEPs and SPs did not differ between the left and right ES muscles in either the patients or the control subjects and also did not differ between the patients and the control subjects. However, there was a significantly higher MTh and threshold for the SP in the patients as compared with the control subjects; the full significance of this requires further investigation. Interestingly, there was a positive correlation between the self-rated measure of disability (the Oswestry Disability Index score) and both the MTh and the threshold for the SP in the patients. There was also a positive correlation between the self-rated index of back pain and the threshold for the SP in the patients. This finding of an association between clinical and neurophysiologic measures reinforces the need for further research to establish the clinical relevance of these rises in MTh and SP threshold. CONCLUSIONS: In summary, this study has revealed that corticospinal excitability, driving ES muscles close to the site of pain, is lowered in patients with chronic low back pain.

The ability of physiological stimuli to generate the sympathetic skin response in human chronic spinal cord injury.

PURPOSE: Sympathetic sudomotor function in chronic spinal cord injury (SCI) has been evaluated to determine if recording the sympathetic skin response (SSR) provides evidence of integrity of the spinal component of the sympathetic pathways. METHODS: Thirty subjects with chronic SCI and 15 healthy normal subjects were studied. The SSR was elicited using two physiological (auditory and inspiratory gasp) stimuli. In addition, electrical (median and peroneal nerve) stimulation was also performed. Recordings were made from palmar and plantar sites. RESULTS: Palmar and plantar SSRs could be readily elicited in all control subjects by all stimuli. In the majority of SCI subjects, the presence or absence of the SSR was related to the American Spinal Injury Association (ASIA) impairment scale, which incorporates only motor and sensory assessments. The exceptions indicated preserved (or damaged) sympathetic spinal cord pathways. CONCLUSIONS: We conclude that the SSR, using either physiological or electrical stimuli, may be a reliable, non-invasive method of determining integrity of sympathetic cholinergic pathways in SCI, with potential for monitoring the effects of intervention and spinal repair.

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.

Corticospinal excitability in patients with unilateral sciatica.

Patients suffering from sciatica may develop altered patterns of corticospinal drive to muscles of the leg. Electromyographic recordings were made bilaterally from tibialis anterior and lateral gastrocnemius muscles during transcranial magnetic stimulation of the motor cortex from nine patients with unilateral sciatica and seven healthy controls. The mean thresholds for eliciting motor evoked potentials (MEPs) and the silent period were higher in the patients than in the control subjects. In addition, there was a positive correlation between the patients' self rated score of disability (the Oswestry Disability Index) and their threshold for MEPs and the silent period. This study suggests that the chronic pain experienced by the patients leads to a reduction in corticospinal drive to the legs. These changes might help to relax muscles close to the perceived site of pain and so alleviate symptoms. Investigation of these patients following remedial treatment will allow us to establish if the changes are transient in nature.

Stability of corticospinal excitability and grip force in intrinsic hand muscles in man over a 24-h period.

The maximum voluntary muscle force can vary throughout the day; typically being low in the morning and high in the evening. The nature of this possible variation has been investigated with respect to corticospinal excitability. Six healthy subjects were studied. Maximum voluntary contraction (MVC) in the thenar muscles was measured. In addition, we monitored several indices of corticospinal excitability using electromyographic (EMG) recording and transcranial magnetic stimulation (TMS) of the motor cortex. Motor evoked potentials (MEPs) were recorded while relaxed and at 10% MVC when the silent period was assessed as an index of corticospinal inhibition. Readings were taken every 3 h for 24 h. MVC of the thenar muscles did not change significantly over the 24 h. The mean areas, latencies and durations of MEPs did not show significant changes over the 24-h test period with the muscle relaxed or contracted; however, MEP area did vary between sessions at all stimulus intensities suggesting non-time-of-day-dependent changes in corticospinal excitability. Furthermore, the extent and duration of the silent period seen after the MEP in the contracted muscle did not change significantly over the 24 h of the experiment at any stimulus intensity. These results provide evidence that the MVC force of the thenar muscles and their responses to TMS are stable throughout the course of the day and suggest that, in hand muscles, corticospinal excitability may not be subject to circadian variation.

Segmental recording of cortical motor evoked potentials from thoracic paravertebral myotomes in complete spinal cord injury.

STUDY DESIGN: A study of thoracic paravertebral muscle motor-evoked potentials using transcranial magnetic stimulation in spinal cord injury patients and control participants. OBJECTIVES: To develop a method to study the level and density of corticospinal lesions in thoracic spinal cord injury. SUMMARY OF BACKGROUND DATA: Cervical and lumbar spinal cord injury, unlike thoracic spinal cord injury, can be quantified by recording muscle motor-evoked potentials from limb muscles. For thoracic spinal cord injury, the use of paravertebral muscles is limited by complex innervation patterns and the greater difficulty in obtaining muscle motor-evoked potentials. METHODS: In 10 patients with complete midthoracic spinal cord injury (T4-T7) and 10 age-matched control participants, muscle motor-evoked potentials were recorded from all thoracic paravertebral muscles using transcranial magnetic stimulation with a double-cone stimulating coil over the vertex. RESULTS: In control participants, muscle motor-evoked potential responses evoked in all myotomes had progressively increasing latency in a rostrocaudal direction. Threshold was comparable in all segments. The duration of muscle motor-evoked potentials was unrelated to the spinal level. In spinal cord injury, responses were elicited in all segments above a lesion and in a varying range of segments below the lesion. In comparison with control participants, threshold was lower above and higher below the lesion (P < 0.001) in patients with spinal cord injury. Latency was longer than normal both above and below the lesion (P < 0.001). Duration was not significantly different from that in control participants at any level. CONCLUSIONS: Paravertebral muscle motor-evoked potentials can be elicited below the level of a complete spinal cord injury. Possible reasons for this include the multisegmental innervation of these muscles and the long muscle fiber conduction. Stretch reflex activation elicited by contraction of muscles above the lesion is thought to be an unlikely mechanism because of the latency of the response. Although the presence or absence of muscle motor-evoked potentials does not appear to be a sensitive indicator of the level of thoracic spinal cord injury lesion, analysis of muscle motor-evoked potentials reveals abnormal patterns that may assist in defining lesions. Finally, lower threshold above the lesion suggests corticospinal hyperexcitability of this pathway as a result of central plasticity after spinal cord injury.