By its very nature, intraoperative neurophysiological monitoring (IONM) must yield timely, accurate information in order to help reduce iatrogenic injury. Real-time, intraoperative monitoring provides immediate feedback so the surgeons can take action before permanent nerve injury occurs.
Among the benefits of IONM identified through research are a reduction in post-operative injuries1,2 a reduction in post-operative readmissions1 and lower opioid use1 within one year for spinal surgery patients who also received IONM compared to those who did not.
A range of complex surgeries including cardiac, spine, brain, vascular and orthopedic procedures can put the body’s neural circuits at risk. By delivering and measuring electrical impulses, trained clinicians can quickly identify changes that may require immediate attention by the surgical and anesthesia teams. Of course, electrodes are critical to the transmission of impulses to and from the patient. If the electrodes fail to deliver clear signal, effective IONM monitoring stops.
Given their importance, the following information provides factors to consider in electrode selection and application, along with potential solutions for common challenges that the neurodiagnostic team can encounter during IONM.
Select IONM modalities and associated electrode considerations
As the use of IONM has expanded, monitoring occurs in wide range of procedures. Among the most common pathway assessments are:
- Brainstem auditory evoked potentials (BAEP): auditorySomatosensory evoked potentials (SEP): lower limbs
- Somatosensory evoked potentials (SSEP): upper limbs
- Motor evoked potentials (MEP): spinal cord/spinal nerves
- Visual evoked potentials (VEP): visual
With any electrode in an IONM neurodiagnostic application, certain performance parameters apply: signal quality, signal-to-noise ratio and impedance, all of which affect the quality of monitoring. Other factors, like biocompatibility and patient comfort should be taken into account as well. Below are electrode considerations associated with two of the more common IONM procedures: SEP and MEP.
Somatosensory evoked potentials (SEP) — the most widely used IONM
SEPs measure sensation above and below the surgical area and are an important part of IONM. In spinal surgery, for example, electrodes are placed on potentially affected limbs, while other electrodes are secured to the skull to detect impulses from the monitored limbs. SEPs cannot always detect injuries to individual nerve roots, though. Inadequate technique, noise and non-optimized electrode selection and array configurations all can affect testing accuracy and patient outcomes.
Electrode considerations
Recording of SEPs
- Cervical and lumbar components: Placing needles electrodes to capture the cervical (P13) and Erb (N9) components can present risk, though rare, of hemorrhagic complications, particularly for patients who are anticoagulated.3 (Ares, 2017). Research indicates that surface electrodes can be a viable alternative to invasive subdermal electrodes.4 (Cheng 2016).
- Cortical components: According to the American Clinical Neurophysiology Society (ACNS): “Either standard disk EEG electrodes or sterile subdermal needle electrodes may be used. Disk EEG electrodes should be applied to the scalp with collodion and sealed with plastic tape or sheet to prevent drying and to protect them from blood or other fluids. If disk electrodes are used, impedance should be <5 Kohms. Subdermal needle electrodes can also be used but are more likely to be dislodged.”5 Corkscrew needles are also an excellent option for secure electrode placement.
Stimulation sites
Posterior stimulation zones, including the popliteal fossa and posterior tibial nerve, can be challenging to instrument with conventional stimulators, some of which are bulky or difficult to keep in place even with additional fastening. Flat, repositionable, pre-gelled surface electrodes can deliver the required electrical stimulation while remaining well-secured. ACSN guidelines indicate that at the posterior tibial or common peroneal nerve stimulation sites, either surface disk electrodes or subdermal needle electrodes may be placed.5
Motor evoked potentials (MEP)
The 1990s saw transcranial electric motor evoked potentials (MEPs) introduced as a method for monitoring spinal cord activity and for predicting postoperative motor problems. MEPs complement SEPs, which only access part of the spinal cord. MEPs record electrical signals from neural tissue or muscle following activation of central motor pathways, direct monitoring of which typically occurs via transcranial electrical stimulation (TES). TES involves high-intensity stimuli delivered to the scalp in order to stimulate the brain through the skull.
Electrode considerations: Needle or corkscrew electrodes are most often used for TES, though surface electrodes can also be used. Low impedances—which correlate with a larger contact area between electrode and tissue—can help prevent tissue injury from the high stimulus voltage and current levels used in TES. Sometimes up to 200V is needed to elicit a reliable motor limb response during TES.
Stimulation sites: As an alternative to conventional bipolar configurations, e.g., linked quadripolar (LQP-TceMEP), corkscrew electrode arrays may help decrease burn risk by facilitating a robust response with less stimulus intensity. Corkscrew electrodes have lower impedances than needle or EEG cup electrodes6 and are also less likely to become dislodged.7
Common IONM challenges and solutions
As the use of IONM has broadened, the products and techniques used to support it have advanced as well. Some of the more common challenges that neurodiagnostic technicians face their day-to-day practice are noted below, along with potential solutions.
| Challenge | Potential Solution | Rationale |
| Decrease signal noise; improve signal-to-noise ratio | Electrodes in a twisted wire configuration |
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| Needle electrodes detach from patient | Subdermal needles with a bend |
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| Needle electrodes placed on the head detach | Corkscrew electrodes |
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| Cumbersome lead/electrode arrays in the OR | Ribboned or twisted wire leads |
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| Accidental needle sticks in the OR | Pre-gelled adhesive surface electrodes |
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| Potential cross-contamination risk | Single-patient use surface electrodes and stimulation and grounding products |
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Effective preventative IONM services are cost- and time-efficient when compared to the risks and costs of serious neurological complications. As IONM continues to grow, robust studies about its utility and efficacy will contribute to greater confidence in IONM as an accepted practice in modern surgical care. In the meantime, neurodiagnostic professionals will continue to rely on the best available techniques and equipment to yield meaningful results for patients in their care.
LifeSync Neuro is a leading provider of neurodiagnostic and neuromonitoring products that deliver patient comfort, excellent signal quality and accurate test results. For more information, visit the LifeSync Neuro website or call 1-800-328-5544 to request more information or a product catalog.
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- References
1. Ney JP, Kessler DP. Neurophysiological monitoring during cervical spine surgeries: longitudinal costs and outcomes. Clin Neurophysiol.2018;129:2245-2251.
2. Cheah J, Zhang, AL, Tay B. Intraoperative Use of Neuromonitoring in Multilevel Thoracolumbar Spine Instrumentation and the Effects on Postoperative Neurological Injuries. ClinicalSpine Surgery. 2017:30(7) :321-327.
3. Ares WJ, et al. Interdisciplinary Neurosurgery: Advanced Techniques and Case Management. 2017:10: 4-7.
4. Cheng HL, Thirumala PD, Crammond DJ, Habeych ME, Balzer J. Comparison of Subdermal Needle and Surface Adhesive Stimulating Electrodes for Somatosensory Evoked Potential Monitoring during Anterior Cervical Discectomy and Fusion. Neurdiagn J. 2016;56(3):186-200.
5. ACNS Guideline 11B: Recommended standard for intraoperative monitoring of somatosensory evoked potentials. Accessed at: https://www.acns.org/pdf/guidelines/Guideline-11B.pdf
6. MacDonald DB. Intraoperative motor evoked potential monitoring: Overview and update. J Clin Monit Comput. 2006; 20:347-77.
7. Legatt A, Emerson R, Epstein C, MacDonald D, Deletis V, Bravo R, Lopez J. ACNS Guideline: Transcranial Electrical Stimulation Motor Evoked Potential (TES-MEP) Monitoring. J Clin Neurophysiol. 2016; 33:42-50.
8. Usakli AB. Improvement of EEG Signal Acquisition: An Electrical Aspect for State of the Art of Front End. Comput Intell Neurosci.2010. Accessed at: https://www.hindawi.com/journals/cin/2010/630649/