7.2 Upper & Lower Extremity SSEP Technique

7.2 Upper & Lower Extremity SSEP Technique

Quick Answer: Stimulate median or ulnar nerve at the wrist for upper-extremity SSEPs and posterior tibial nerve (PTN) at the ankle for lower-extremity SSEPs; deliver constant-current pulses at non-line-locked rates with adequate intensity; record multi-station montages; average enough sweeps to resolve peaks within an epoch long enough for N20 or P37.

Technique is where pathway knowledge becomes usable data. Poor stimulation, wrong montage, or inadequate averaging produces “alerts” that are really technical failures — and missed alerts that are really invisible signals. This section focuses on OR-ready upper- and lower-extremity SSEP setup for CNIM.

Stimulation Sites

Upper extremity: median and ulnar

Median nerve stimulation at the wrist (cathode proximal, anode distal — follow your lab’s polarity convention for orthodromic proximal conduction) is the workhorse upper-extremity SSEP. Median fibers course through the brachial plexus and into the lateral cord / middle and upper trunk contributions relevant to many cervical and plexus risk cases.

Ulnar nerve stimulation at the wrist is often added or substituted when:

• Median neuropathy or wrist pathology limits median responses
• Ulnar-innervated structures or medial cord / lower trunk are specifically at risk
• You want a second peripheral pathway for redundancy during cervical or brachial plexus surgery

Place stimulating electrodes securely (surface pads or subdermal needles per protocol). Confirm a visible or palpable twitch when neuromuscular blockade allows, or confirm a robust peripheral recorded potential when twitch is blocked.

Lower extremity: posterior tibial nerve (PTN)

Posterior tibial nerve stimulation at the medial ankle (typically behind the medial malleolus) is the standard lower-extremity SSEP site. PTN afferents travel via the tibial/sciatic pathway into the lumbosacral plexus and ascend in fasciculus gracilis. Alternative or adjunct sites (peroneal nerve at the fibular head, femoral nerve) may be used for specific surgical risks or when PTN responses are absent, but PTN remains the default CNIM expectation.

Always document left vs right stimulation separately. Unilateral surgical risk requires unilateral interpretation — never average left and right into one “global” trace for alert decisions.

Stimulus Parameters: Rate, Intensity, Duration

Pulse width and intensity

Typical SSEP pulses are brief rectangular constant-current stimuli (commonly on the order of ~0.2–0.3 ms pulse width in many labs — follow institutional protocol). Intensity should be supramaximal for the peripheral response within safe limits: strong enough that small intensity drifts do not change amplitude, but not so high that they cause burns, excessive artifact, or unnecessary patient movement under light anesthesia.

Practical cues:

• Increase intensity until the peripheral potential (Erb’s / popliteal) plateaus, then add a modest margin if protocol allows
• Recheck intensity after repositioning, fluid on the stim site, or electrode replacement
• Constant-current stimulators compensate better for impedance change than constant-voltage in most OR settings — know which your machine uses

Rate selection

Faster rates update the surgeon sooner but can reduce amplitude (especially cortical synaptic components) and increase patient movement. Slower rates improve morphology but delay feedback. Common intraoperative SSEP rates cluster around ~2–5 Hz, with non-integer rates (for example, 2.7, 3.1, 4.7 Hz) preferred to avoid phase-locking with 60 Hz line noise.

Avoid rates that are exact subharmonics of line frequency. If 60 Hz invades the average, changing rate is often more effective than endlessly narrowing filters.

Left/right interleaving

Many systems alternate left and right stimulation so both limbs update within one monitoring cycle. Ensure the software correctly labels sides and that surgical alerts name the correct limb.

Recording Montages

A defensible SSEP montage includes stations, not only cortex:

Upper extremity example (conceptual)

1. Erb’s – reference — peripheral N9
2. Cervical spine – reference — N13
3. Scalp far-field / subcortical derivation — P14 emphasis (montage-dependent)
4. Contralateral centroparietal – Fz (or Cp–Fz) — cortical N20/P25

Lower extremity example (conceptual)

1. Popliteal fossa – reference — peripheral check
2. Lumbar / thoracic spine channel when used by the lab — spinal station
3. Midline centroparietal (CPz) – Fz — cortical P37/N45
4. Additional scalp channels as needed for redundancy

Use consistent 10–20 placements. After any electrode replacement, re-establish baselines before applying percentage criteria. Differential amplifier inputs (G1–G2) must match the documented montage; swapped inputs invert polarity and confuse N/P labeling.

Averaging, Epochs, Filters, and SNR

SSEPs are microvolt-to-submicrovolt signals buried in EEG/EMG/OR noise. Signal averaging of time-locked sweeps improves SNR approximately with √N. Rough working ranges often start near 100–300+ averages depending on noise and patient factors; neuropathic or obese patients may need more. Communicate the time cost of heavy averaging during critical surgical steps.

Epoch / analysis window:

• Upper extremity: commonly ~50 ms to capture N20 and following peaks
• Lower extremity: commonly ~75–100+ ms to capture P37

If the window truncates the peak, amplitude criteria become meaningless.

Filters: Use lab-standard bandpass settings that preserve SSEP morphology. Over-filtering can round peaks and shift apparent latency. Do not “fix” 60 Hz by destroying the physiologic bandwidth; fix grounding, rate, and cables first.

Artifact rejection: Reject sweeps contaminated by cautery, movement, or huge EMG. Continuous Bovie may force a pause — announce that averages are frozen rather than silently reporting stale data.

Artifact Control Specific to SSEPs

• Ground the stimulated limb (per lab electrical-safety protocol) to reduce stimulus artifact that will not average away
• Route stim and recording cables to minimize loop area and separation from electrocautery return paths when possible
• Keep impedances balanced and low; document checks after positioning and prep
• Distinguish stimulus artifact from true early near-field potentials — do not measure “N9” on artifact

Anesthesia and Technique Interactions

Technique cannot overcome every anesthetic effect, but you can optimize:

• Prefer TIVA or minimized volatiles when cortical SSEPs are critical (coordinate with anesthesia — covered in depth in Chapter 6)
• Expect cortical amplitude to be more fragile than subcortical/peripheral stations under volatiles
• Maintain stable MAP and temperature; technical “perfection” will not restore signals lost to profound hypotension or hypothermia

Pre-Incision Baseline Workflow

1. Confirm stim sites, polarity, and limb identity
2. Verify impedances and montage map
3. Obtain clear peripheral potentials
4. Collect stable cortical (and subcortical) baselines after anesthesia has settled
5. Store marked baselines and communicate quality to the team
6. Only then apply alert percentage rules

Skipping a true baseline — or baselining during unstable induction — creates false alerts later. Technique discipline is patient safety, not bureaucracy.

Common Technical Failure Modes

Mastering UE/LE SSEP technique gives the surgical team timely, localized, trustworthy information — the practical heart of intraoperative SSEP monitoring on the CNIM exam.