5.3 Stimulators, Polarity, and Stimulation Pitfalls

The stimulator is the other half of Ohm’s law: it forces current through tissue so axons reach threshold. Domain II.D of the American Association of Electrodiagnostic Technologists (AAET) Registered Nerve Conduction Study Technologist (R.NCS.T.) outline asks you to know how the box delivers that current, which pole depolarizes, and which setup errors shrink, delay, or fake a response. This OpenExamPrep section is independent teaching on stimulators, polarity, Anodal Block, volume conduction, artifact, and supramaximal technique.

Constant current versus constant voltage

Two stimulator designs appear on laboratory equipment:

• A constant-current stimulator varies its output voltage as electrode-skin resistance changes so that the current you selected (for example 25 mA) stays the same — until the device reaches its compliance voltage and can no longer compensate (Section 5.1). Because nerve depolarization depends on current density in tissue, constant current is the usual NCS choice.
• A constant-voltage stimulator holds voltage. Current then follows Ohm’s law: I = V / R. If paste dries and R rises, current falls and the stimulus becomes submaximal without a knob change. If R falls (sweat, extra gel), current rises and you may overstimulate.

If amplitudes swing when you reseat a cathode, ask which mode you are in and what the impedance did. Do not first assume the patient “tried less.” Watch the displayed milliamperes on a constant-voltage unit; they are the true stimulus, not the voltage setting.

Intensity, duration, and threshold

You have two main stimulus parameters:

• Intensity (mA in constant-current mode, or V in constant-voltage mode) — how strong each pulse is.
• Duration (pulse width), commonly about 0.05–0.2 milliseconds (50–200 microseconds) as a teaching range for routine NCS, with about 0.1 ms a frequent starting width. Longer pulses recruit with less intensity but enlarge artifact and can increase discomfort.

Rheobase and chronaxie are strength-duration ideas: a very short pulse needs more current; a longer pulse can use less. For daily NCS you rarely plot a full strength-duration curve, but you use the idea when a nerve is deep: lengthen duration slightly rather than endlessly cranking intensity into co-stimulation territory.

Intensity without a plateau is not a complete stimulus description. The same 30 mA may be supramaximal for a superficial sensory nerve and inadequate for a deep motor nerve, or excessive for a neighbor nerve at the wrist.

Polarity: cathode and anode

A bipolar stimulator has two poles:

• Cathode: negative pole. Conventionally black. The cathode depolarizes nearby axons (makes the outside of the membrane less positive, moving the nerve toward threshold). The recorded impulse starts under the cathode.
• Anode: positive pole. Conventionally red. The anode hyperpolarizes nearby axons (makes firing less likely).

For orthodromic motor studies (stimulus on nerve, recording over muscle in the natural direction of motor axons), the depolarizing cathode should face the recording site. At a distal stimulation site (wrist, ankle), that orientation is cathode distal, anode proximal. The cathode sits closer to the muscle; the anode sits further up the limb. If you reverse the banana plugs, you reverse the poles even if the handheld probe still “looks” pointed the right way. Color coding only helps if black is actually plugged into the cathode jack.

Latency is measured from the stimulus trigger, but the nerve fires under the cathode. Moving the cathode 2–3 cm farther from the muscle lengthens latency by the conduction time of that extra distance, even if intensity is unchanged.

Anodal Block

Anodal Block is the handbook name for a classic polarity error. If the anode lies between the cathode and the recording electrodes, the impulse born under the cathode must pass under the hyperpolarizing anode to reach the muscle. Hyperpolarization can block propagation. The CMAP shrinks, becomes inconsistent, or vanishes at intensities that should have been adequate. The fix is not “more milliamps first”; it is restoring cathode-toward-the-muscle orientation and then repeating a supramaximal series.

Reversed polarity therefore contaminates both amplitude (block, incomplete recruitment) and latency (cathode no longer at the intended site). A small, late CMAP after a plug swap is a polarity problem until you have ruled it out. Write Anodal Block the way handbooks do when a stem uses that name; do not invent a softer synonym on the exam if the question already used the term.

Supramaximal stimulation versus submaximal and overstimulation

Increase intensity until the CMAP or SNAP amplitude plateaus, then add a modest increment (many laboratories teach about 20% above the intensity that first produces that plateau). That is supramaximal stimulation: all axons in the nerve that can be recruited from that site are firing, so amplitude reflects the nerve, not the knob.

• Submaximal stimulation leaves axons silent. Amplitude is underestimated. Latency may also be wrong if the remaining fibers are not the fastest ones. Never report a small CMAP as axonal loss until you have proved the stimulus was supramaximal. A “repeatable” small response can still be submaximal if you stopped climbing too early.
• Overstimulation does not usefully enlarge a plateaued CMAP. Extra current volume-conducts to neighboring nerves and muscles. Median wrist stimulation that is too strong co-stimulates ulnar fibers; fibular (peroneal) stimulation at the knee can spread to tibial fibers. A bonus bump on the tracing may be the wrong nerve. Back down to the true plateau and confirm cathode position on the target nerve.

Supramaximal is a physiologic end-point (plateau plus a small extra), not a fixed milliampere number that works for every limb and every edema.

Stimulus artifact

Stimulus artifact is the electrical spike of the pulse appearing at the recording electrodes. It is worse with high impedance, long pulse duration, high intensity, poor cathode orientation, widely spread recording leads, and stimulator-to-recorder cable proximity. Artifact can overlay SNAP onset, especially in short distal segments (for example palmar or plantar studies).

Reduce it with the Ohm’s law hygiene from Section 5.1, correct polarity, shorter duration when recruitment still reaches plateau, rotating the stimulator slightly, and physically separating stimulator cables from recording cables. Remember from Section 5.2: averaging will not cancel time-locked artifact. An averaged early spike that is too soon to be a SNAP is still artifact.

Volume conduction

The body is a volume conductor: current spreads through extracellular fluid, not only along the nerve you aimed at. Consequences for the technologist:

• A far-field potential generated at a distance can appear under your recording electrodes and look like a SNAP or CMAP component.
• Co-stimulated muscle can inject a volume-conducted CMAP into a sensory channel.
• An initial positivity may be volume conduction from a nearby generator rather than a finding you should measure as the nerve of interest.

Ask whether the putative response has the latency, morphology, and montage behavior of the nerve under study. If moving the stimulator off the nerve kills a true SNAP but a far-field bump remains, you were looking at volume conduction. If a “sensory” potential is millivolts tall, suspect a volume-conducted muscle response until proven otherwise.

Volume conduction is also how overstimulation reaches the neighbor nerve. The current does not stay in a neat cylinder under the cathode; it spreads. That is useful (you can still stimulate a slightly deep nerve) and dangerous (you can stimulate two nerves at once).

Pitfalls table

Mastering stimulation is not theatrical high milliamperes. It is delivering a known current, at a known pole, to the intended nerve, without recruiting the neighbor, without blocking at the anode, and without mistaking a far-field bump for physiology.