Influence of the Montage of Stimulation Electrodes for Intraoperative Neuromonitoring During Orthopedic Spine Surgery

Stimulation parameters for motor evoked potentials during intraoperative spinal cord monitoring. A systematic review

OBJECTIVE

The aim of this systematic review was to find the optimal stimulation parameters for muscle recorded transcranial electrical stimulation motor evoked potential (mTc-MEP) and D-wave monitoring during spinal cord monitoring.

METHODS

A PRISMA systematic search in Medline and EMBASE and a QUADAS-2 quality evaluation was performed to identify studies that compared stimulation parameters consisting of stimulation location, number of pulses, pulse duration, interstimulus interval, double train (DTS) or recurrent train stimulation (RTS) and intertrain interval (ITI) for performing mTc-MEP and D-wave monitoring. Only studies that used total intravenous anaesthesia (TIVA) were included.

RESULTS

Ten studies that compared stimulation parameters for performing mTc-MEP monitoring (stimulation location n = 4, number of pulses n = 2, pulse duration n = 1, interstimulus interval n = 4, DTS n = 1, RTS n = 2, ITI n = 2) were included. No studies compared stimulation parameters (stimulation location and pulse duration) for performing D-wave monitoring.

CONCLUSIONS

Few studies examined the optimal stimulation parameters for monitoring mTc-MEPs and no studies were included for D-wave monitoring. There is a need for prospective research to investigate the optimal stimulation parameters for mTc-MEP with the use of TIVA and D-wave monitoring.

SIGNIFICANCE

For mTc-MEP monitoring, a table is provided in which the recommended stimulation parameters are stated.

Safety issues during surgical monitoring

While intra-operative neuro-physiologic assessment and monitoring improve the safety of patients, its use may also introduce new risks of injuries. This chapter looks at the electric safety of equipment and the potential hazards during the set-up of the monitoring. The physical and functional physiologic effects of electric shocks and stimulation currents, standards for safety limits, and conditions for tissue damage are described from basic physical principles. Considered are the electrode-tissue interface in relation to electrode dimensions and stimulation parameters as applied in various modalities of evoked sensory and motor potentials as to-date used in intra-operative monitoring, mapping of neuro-physiologic functions. A background is given on circumstances for electric tissue heating and heat drainage, thermal toxicity, protection against thermal injuries and side effects of unintended activation of neural and cardiac tissues, adverse effects of physiologic amplifiers from transcranial stimulation (TES) and excitotoxicity of direct cortical stimulation. Addressed are safety issues of TES and measures for prevention. Safety issues include bite and movement-induced injuries, seizures, and after discharges, interaction with implanted devices as cardiac pacemaker and deep brain stimulators. Further discussed are safety issues of equipment leakage currents, protection against electric shocks, and maintenance.

Extramuscular Recording of Spontaneous EMG Activity and Transcranial Electrical Elicited Motor Potentials in Horses: Characteristics of Different Subcutaneous and Surface Electrode Types and Practical Guidelines

Introduction

Adhesive surface electrodes are worthwhile to explore in detail as alternative to subcutaneous needle electrodes to assess myogenic evoked potentials (MEP) in human and horses. Extramuscular characteristics of both electrode types and different brands are compared in simultaneous recordings by also considering electrode impedances and background noise under not mechanically secured (not taped) and taped conditions.

Methods

In five ataxic and one non-ataxic horses, transcranial electrical MEPs, myographic activity, and noise were simultaneously recorded from subcutaneous needle (three brands) together with pre-gelled surface electrodes (five brands) on four extremities. In three horses, the impedances of four adjacent-placed surface-electrode pairs of different brands were measured and compared. The similarity between needle and surface EMGs was assessed by cross-correlation functions, pairwise comparison of motor latency times (MLT), and amplitudes. The influence of electrode noise and impedance on the signal quality was assessed by a failure rate (FR) function. Geometric means and impedance ranges under not taped and taped conditions were derived for each brand.

Results

High coherencies between EMGs of needle-surface pairs degraded to 0.7 at moderate and disappeared at strong noise. MLTs showed sub-millisecond simultaneous differences while sequential variations were several milliseconds. Subcutaneous MEP amplitudes were somewhat lower than epidermal. The impedances of subcutaneous needle electrodes were below 900 Ω and FR = 0. For four brands, the FR for surface electrodes was between 0 and 80% and declined to below 25% after taping. A remaining brand (27G DSN2260 Medtronic) revealed impedances over 100 kΩ and FR = 100% under not taped and taped conditions.

Conclusion

Subcutaneous needle and surface electrodes yield highly coherent EMGs and TES-MEP signals. When taped and allowing sufficient settling time, adhesive surface-electrode signals may approach the signal quality of subcutaneous needle electrodes but still depend on unpredictable conditions of the skin. The study provides a new valuable practical guidance for selection of extramuscular EMG electrodes. This study on horses shares common principles for the choice of adhesive surface or sc needle electrodes in human applications such as in intraoperative neurophysiological monitoring of motor functions of the brain and spinal cord.

State-of-the-Art Diagnostic Methods to Diagnose Equine Spinal Disorders, With Special Reference to Transcranial Magnetic Stimulation and Transcranial Electrical Stimulation

Spinal cord disorders are a common problem in equine medicine. However, finding the site of the lesion is challenging for veterinarians because of a lack of sensitive diagnostic methods that can assess neuronal functional integrity in horses. Although medical imaging is frequently applied to help diagnose corticospinal disorders, this approach does not reveal functional information. For the latter, transcranial magnetic stimulation (TMS) and more recently transcranial electrical stimulation (TES) can be useful. These are brain stimulation techniques that create either magnetic or electrical fields passing through the motor cortex, inducing muscular responses, which can be recorded either intramuscularly or extramuscularly by needle or surface electrodes. This permits the evaluation of the functional integrity of the spinal motor tracts and the nerve conduction pathways. The interest in TES in human medicine emerged these last years because unlike TMS, TES tends to bypass the motor cortex of the brain and predominantly relies on direct activation of corticospinal and extrapyramidal axons. Results from human medicine have indicated that TMS and TES recordings are mildly if not at all affected by sedation. Therefore, this technique can be reliably used in human patients under either sedation or full anesthesia to assess functional integrity of the corticospinal and adjunct motor tracts. This opens important new avenues in equine medicine.

Kyphosis - A risk factor for positioning brachial plexopathy during spinal surgeries

OBJECTIVE

The aim of this study was to evaluate the differences in transcranial electric motor-evoked potentials - TceMEP on upper limbs and the incidences of postoperative brachial plexopathy between patients with kyphotic and scoliotic trunk shapes.

METHODS

In the period of January 2011-January 2017, 61 consecutive patients (mean age: 18.4 years ± 4.4 years (range: 10-32)) with pediatric spinal deformity underwent surgery in our Department. Eight of them had a kyphotic trunk deformity (Scheuermann kyphosis, neurofibromatosis, posterior thoracic hemivertebra), and the rest of the 53 patients had a scoliotic trunk deformity (mostly adolescent idiopathic scoliosis - AIS, lateral hemivertebra). The TceMEP recordings in all four limbs were analyzed every 30 min, or upon the surgeon's command. Upper limb TceMEP recordings were used as a control of systemic and anesthetic related changes, and as the indicator of positioning brachial plexopathy.

RESULTS

Four out of 8 patients (50.0%) from the kyphotic group experienced noteworthy decreases in TceMEP amplitude (≥65%) in one or both arms, and only 2 out of 53 patients (3.8%) from the scoliotic group, confirming significant statistical difference (Chi-square 16.75, p < 0.05). Two out of 8 patients with decreases in TceMEP amplitude suffered from transitory postoperative brachial plexopathy, and both of them were from the kyphotic group.

CONCLUSION

It seems that kyphotic trunks have a higher risk for positioning-related brachial plexopathy, probably due to distribution of trunk's weight onto only four points (two iliac bones and two shoulders), compared to the scoliotic trunks that have wider weight-bearing areas. We emphasize the importance of proper patient positioning and close intraoperative neuro-monitoring of all four limbs in more than one channel per limb.

LEVEL OF EVIDENCE

Level IV Therapeutic Study.