5.3 Site Preparation & Electrode Application

5.3 Site Preparation & Electrode Application

Quick Answer: Prep skin with integrity in mind, place electrodes on reproducible landmarks (10-20 and modality sites), secure them against OR movement, and adapt immediately when the incision or sterile field blocks a standard site—without ever contaminating the sterile field.

Application technique converts a good equipment plan into usable baselines. Sloppy prep, wrong landmarks, loose leads, or sterile-field breaches create avoidable signal loss and patient-safety events. This section ties skin care, placement systems, mechanical security, and OR sterile discipline into one workflow.

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Skin Integrity and Preparation

Before any abrasion or needle insertion, inspect planned sites:

• Broken skin, rashes, burns, prior surgical scars, and radiation changes
• Active infection (choose alternate sites; do not seed infection with needles)
• Anticoagulation or bleeding risk (gentle technique; communicate with anesthesia/surgeon)
• Known allergies to adhesives, latex, iodine, or collodion-related products

Prep sequence (typical clean sites): Gently clean away oils and debris → apply abrasive prep per product instructions to lower impedance → wipe excess → place electrode → secure. Do not grind fragile skin until it bleeds "for better impedance"; micro-abrasion is enough. On the face and near the eyes, control needle angle and depth carefully.

If skin is too compromised for needles at a preferred muscle, select an alternate muscle innervated by the same nerve when the monitoring question allows, and document the substitution. Never place needles through obviously infected tissue.

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Landmarks: 10-20 and Modality-Specific Placement

International 10-20 system

Scalp EEG and many EP recording sites derive from the International 10-20 system, which proportions electrode positions to nasion–inion and preauricular distances. Intraoperative EEG for carotid or other cerebrovascular cases commonly uses a reduced montage built from 10-20 positions; measure rather than guess so serial recordings remain comparable.

Key orientation points you will use repeatedly:

• Nasion / inion and left/right preauricular points define the measurement grid
• Midline sagittal sites (for example, Cz, Fz, Pz) and parasagittal sites (C3/C4) for SSEP cortical recordings
• Ear or mastoid references as specified by your montage

For upper-extremity SSEPs, cortical recordings often use C3' / C4' (slightly posterior to C3/C4) referenced appropriately; lower-extremity cortical sites often use Cz' or similar lab-standard variants. Follow your laboratory's written montages—consistency beats improvisation.

Peripheral and muscle sites

Mark sites before prepping when hair or body habitus obscures landmarks. After Mayfield pin placement or head holder application, re-verify that scalp electrodes were not displaced or shorted against metal.

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Secure Application and Strain Relief

An electrode that falls out under the drape looks like sudden signal loss. Secure intentionally:

1. Anchor the electrode at the skin (proper depth for needles; adequate adhesive/collodion for surface).
2. Strain-relieve the lead with tape or wrap several centimeters away so a tug hits the tape, not the electrode.
3. Bundle by region (left arm, right arm, legs, scalp) and label laterality.
4. Clear the surgical side and pin sites; keep loops out of the knife path and retractor footprint.
5. Recheck after positioning — log-roll, Wilson frame, table flexion, and shoulder taping commonly pull leads.

After final positioning and before incision, repeat a quick impedance or live-signal glance on critical channels. Pre-incision baselines are only meaningful if electrodes still sit where you placed them.

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Modify Placement as Required

Surgery rarely grants perfect textbook sites. Modify deliberately:

• Incision overlaps a standard site: Move to the nearest valid alternate and document (for example, shift a scalp electrode away from the planned craniotomy flap while preserving interpretive value).
• Sterile field will include a limb: Place and secure that limb's electrodes before prep/drape, or coordinate with scrub for sterile placement if policy requires electrodes inside the field.
• Obesity, edema, or contractures: Adjust approach angle; consider alternate nerves/muscles discussed in the monitoring plan.
• Implanted devices or hardware: Keep stim and recording sites away from generators and leads per team guidance.
• Pediatric or small adults: Scale spacing; avoid overcrowding that shorts electrodes together.

Announce modifications to the surgeon when they affect interpretation ("facial EMG moved to mentalis because of prior scar"). Undocumented improvisation creates postoperative chart confusion and weakens alerts.

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Sterile-Field Awareness

The sterile field is a hard boundary.

Do:

• Place as many electrodes as possible before skin prep when they will remain outside the field.
• Hand sterile stimulator probes only through the scrub tech using sterile technique.
• Ask before reaching near the field; wait for permission if you must adjust a lead at the margin.
• Use sterile sleeves/covers for cables that must enter the field.

Do not:

• Reach over an open sterile tray or draped incision with nonsterile arms or cables.
• Drop nonsterile tape, markers, or needle caps onto the field.
• Yank a lead that disappears under a drape—call for a controlled pause and sterile adjustment.
• Assume "I only touched the edge" is acceptable; edges are still sterile until the team says otherwise.

If contamination occurs, say so immediately so the scrub team can correct it. Silence to avoid embarrassment is a patient-safety failure.

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Application Workflow Summary

Inspect skin & allergies → Measure/mark landmarks → Prep → Apply electrodes
        → Secure & label → Route cables → Position patient → Recheck contact
        → Impedance/live check → Pre-incision baselines → Protect field during prep/drape

Site preparation and electrode application are where planning becomes physiology on the screen. Treat skin respectfully, measure landmarks, lock down mechanics, adapt to the operation, and honor sterility—then your baselines reflect the nervous system rather than setup error.