Chapter 126. Principles of Peripheral Nerve Stimulation (PNS)

Caroline Buhay, MD

Ability to stimulate peripheral nerve or plexus depends on:

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Electrical Parameters of Peripheral Nerve Stimulation

Variable

Clinical significance

Electrical impedance

Varies with tissue composition
Most stimulators have constant current output generators that automatically compensate for impedance changes

Electrode to nerve distance

Ability to stimulate nerve at low current flow (<0.5 mA) indicates close proximity to
nerve
Target muscle twitch still present at <0.2 mA probably indicates intraneural tip position
Increased current flow and pulse duration increase ability to stimulate nerve at a greater distance from stimulating electrode

Current flow (amperage)

Presence of appropriate muscle twitch at current 0.23–0.5 mA generally results in safe, reliable block

Position of electrodes

Cathode (negative electrode) is attached to insulated needle and anode (positive electrode) is attached to patient skin via an EKG electrode
Inverting polarities requires four times current to achieve similar response

Insulated needle—insulated needle shaft ensures that current dispersion is concentrated at needle tip, allowing specificity of needle tip location
Nerve stimulator device

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Basic Features of Nerve Stimulators

Feature

Function

Settings

Constant current output

Automatically compensates for changes in impedance of tissues, needles, connecting wires, and grounding electrodes during needle placement to ensure consistent delivery of set current

Present in most modern stimulators

Current meter

Displays the current being delivered
Make sure it displays in 0.01 mA increments for better accuracy

PNS: 0.2–1.5 mA
TES: 5 mA

Pulse width/duration

Short duration (0.05–0.1 ms) targets A-α motor fibers and avoids stimulation of pain fibers at longer duration
Pulse duration required to depolarize pain fibers: A-δ (0.17 ms) and C fibers (0.4 ms)

PNS: 0.1 ms
Diabetic patients with neuropathy may require a longer pulse duration (0.3 or 1ms) to achieve target twitch
TES: 0.2–0.3 ms to achieve motor response at greater distance from nerve

Stimulating frequency

Determines how quickly pulses are delivered to elicit twitch (pulses/s)
Older models with 1 Hz (1 pulse/s) frequency require slow needle manipulation to avoid missing target twitch

Usual setting: 2 Hz allows for faster manipulation of needle tip

Identify landmarks, insertion site, direction, target muscle(s) to elicit response:
- Due to anatomical variance or large body habitus, the exact course of a nerve can be difficult to identify. In these cases consider doing a surface mapping of the nerve/plexus by transcutaneous electrical stimulation (TES, see below)
Ensure cathode (negative electrode, black) is attached to insulated needle and anode (positive electrode, red) is attached to patient
Set starting current to about 1.2–1.4 mA. Much higher current can result in patient discomfort
Insert needle and advance while observing target muscle group
Once target muscle twitch is elicited, slowly dial down current while adjusting needle position to optimize muscle response
If twitch disappears at ≥0.5 mA, turn current up until twitch is again present
Goal: response present at current 0.2–0.4 mA
Hold needle securely to ensure ...

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