18.2 Technical Factors and Troubleshooting Abnormal Studies

18.2 Technical Factors and Troubleshooting Abnormal Studies

Quick Answer: Unexpected NCS numbers are technical until proven physiologic. Electrode size, G1/G2 and stimulator polarity, reference distance, motor-point placement, impedance, measuring errors (slack tape, wrong landmarks, ulnar elbow position), limb position, and antidromic versus orthodromic sensory method can each create fake axon loss, fake slowing, or fake block. Antidromic digital SNAPs are larger but carry volume-conducted motor artifact. Work a systematic checklist before anyone treats a tracing as disease.

Electrode size

Recording electrodes are not interchangeable ornaments. Recording-surface area changes spatial averaging. With the active electrode centered on the same motor point, larger electrodes generally lower peak CMAP amplitude while reducing sensitivity to small placement shifts; they also have less spatial selectivity and can mix thenar activity with neighboring ulnar-innervated muscle (crosstalk). A smaller electrode can record a higher peak but is more position-sensitive. If it sits slightly off the motor point, amplitude falls and an initial positivity appears even though the nerve is healthy.

Sensory recording follows the same need for standardization: published surface-recording studies generally find higher SNAP peaks with smaller recording electrodes, while electrode pressure, interelectrode distance, and nerve-to-electrode distance also change amplitude. A larger pickup averages a wider field and may lower the peak while admitting more volume-conducted activity. Pediatric versus adult sizes, disc versus bar, and improvised montages all change numbers. Lab reference values assume a stated electrode size. Mid-study size changes, or left-versus-right size mismatch, invalidate side-to-side comparison. If a CMAP is unexpectedly small, ask whether G1 is a tiny disc parked on the edge of the muscle rather than the belly.

Electrode polarity: G1/G2 and stimulator cathode/anode

The differential amplifier displays G1 minus G2. In standard motor setup, G1 (active) sits on the motor point and G2 (reference) sits on the tendon. Depolarization under G1 produces the usual negative (upgoing) CMAP. Swap G1 and G2 and the waveform inverts. Cursors then land on the wrong peak, latency looks bizarre, and a perfectly normal nerve is marked as unobtainable or positive-dip only.

The stimulator has a cathode (negative, depolarizing) and an anode (positive, hyperpolarizing). The cathode is the stimulation site that matters. For routine motor NCS at the wrist, the cathode is closest to the recording electrodes (distal cathode, proximal anode). Measure distance from the cathode, never from the anode. If the probe is reversed, two errors stack: the effective depolarization point moves, so latency changes, and anodal block can appear when the anode sits between the cathode and the muscle, hyperpolarizing the nerve and reducing or delaying the propagating impulse. A sudden need for huge intensity with a strangely delayed CMAP is a polarity check, not proof of demyelination.

Mark cathode orientation on every run. Handheld probes rotate. A flipped probe is one of the cheapest ways to manufacture a new “lesion.” Color conventions vary by vendor, so believe cathode/anode labels, not folklore about which wire is always black.

Distance of the reference electrode

Because the amplifier subtracts G2 from G1, G2 is not electrically silent by magic. If G2 sits too close to G1 over active muscle, both electrodes see similar CMAPs and in-phase cancellation shrinks amplitude. Belly-tendon montages put G2 on a relatively inactive tendon so G1 dominates. For SNAPs, many laboratories teach about 3–4 cm G1–G2 separation. Shorter separation usually reduces SNAP amplitude. Longer separation can enlarge the SNAP but also increases artifact and far-field contributions, including EKG on lower-limb recordings.

A tiny SNAP after someone clustered two discs almost on top of each other is a montage problem. So is a noisy tibial study with G2 halfway up the calf. Match the lab’s stated G1–G2 distance before you believe axon loss.

Electrode placement and the off-point initial positivity

The motor point is the surface region over the end-plate zone, usually the muscle belly. On-point recording: sharp negative takeoff, maximal amplitude, simple shape. Off-point recording: a volume-conducted initial positivity (positive dip) as the impulse approaches G1, then a smaller negative peak, often with a longer rise time. That positivity is your cue to move G1, not to invent a new disease. If you mark latency at the late negative peak instead of a proper takeoff, distal latency looks long.

Sensory G1 belongs over the nerve. Off-nerve digital recording quietly kills SNAP amplitude. Ground placement typically between stimulator and recording electrodes reduces stimulus artifact. Re-check G1 after the patient moves. A disc that slid from APB onto the palmar crease will not match yesterday’s CMAP.

Electrode impedance

The electrode–skin interface is part of the circuit. High impedance produces baseline noise, 60 Hz (power-line) interference, enormous stimulus artifact that buries onsets, and unstable triggering. Dried paste, oily skin, loose tape, cracked leads, and a ground that is barely touching all raise impedance.

Prep: clean, light abrasion when policy allows, fresh conductive gel, firm contact. Common laboratory teaching aims for impedances on the order of about 5 kΩ or less, with G1 and G2 matched so common-mode rejection can cancel noise. That 5 kΩ figure is common lab teaching, not a number this guide claims as a published AAET numeric law. Mismatched impedances (one disc prepped, one disc sitting on dry keratin) are especially noisy. If the tracing looks like a seismograph, fix the interface before you raise stimulus to painful levels.

Measuring errors: slack, landmarks, and the ulnar elbow

Conduction velocity is distance divided by time (proximal latency minus distal latency). Time can be marked to a tenth of a millisecond. Distance is a tape measure on curved anatomy. A 1 cm error on a 10 cm segment is a 10% velocity error.

Slack and path. The tape must follow the true nerve course, taut enough to represent length, without cutting a chord that shortens a curve. A tape that shortcuts the medial epicondyle or the fibular head underestimates distance and makes CV look slow. A tape that bows off the skin and records extra centimeters overestimates distance and makes CV look fast. Slack in everyday lab speech covers both failures to control path length. Re-measure every time a velocity looks heroic or tragic.

Wrong landmarks. Measure cathode to cathode for segment velocity, not anode to G2, not a skin pen mark that slid, and not the machine’s default “10 cm” when your cathode is at 8 cm. Distal distance for latency comparison must match the lab’s method (often a stated 7 cm or 8 cm to G1).

Ulnar elbow position is the celebrity error. Across the elbow, measure and stimulate with the elbow flexed, commonly about 70–90° per lab protocol. In full extension, the ulnar nerve is redundant in the groove; surface distance underestimates true nerve length, so across-elbow CV looks pathologically slow in a normal arm. Keep the same angle for both stimulation and measurement. Watch for ulnar subluxation in flexion: the nerve may hop anterior to the medial epicondyle, so your cathode is no longer on the nerve. Many labs also teach an across-elbow span of at least about 10 cm so a 5 mm tape error is not a 20% velocity swing.

Limb position

Position is part of the protocol, not a comfort afterthought. Wrist flexion versus extension changes median and ulnar distal geometry. Pronation/supination rotates the nerves under the cathode. Shoulder abduction and head position change Erb’s-point distance. Slight knee flexion often lets the common fibular sit more accessibly at the fibular head; a tightly extended knee can hide it. Foot plantarflexion versus dorsiflexion changes fibular and tibial recording geometry.

The silent error is measuring in one position and stimulating in another. If you flexed the elbow to stimulate, you flex the elbow to measure. If the patient sits up between sites, re-check distance. Document the position used when the lab’s reference values assume it.

Temperature is not a substitute for position, but it still sneaks into troubleshooting: a cold limb slows conduction and usually enlarges SNAPs. Warm to the lab’s stated skin-temperature target before you believe demyelination. Follow your lab’s thermometer protocol; do not import a number from memory as if it were a national statute.

Antidromic versus orthodromic sensory studies

Antidromic sensory NCS stimulates proximal (wrist) and records distal (digits) — opposite physiologic sensory direction. SNAPs are typically larger and easier because digital electrodes sit close to the nerve with little overlying tissue. The tax is volume-conducted motor artifact. Once stimulus intensity recruits motor axons, lumbrical, thenar, or interosseous CMAPs can overlay the SNAP as a later, often larger bump. The sensory potential can look polyphasic, inflated, or “unmeasurable” because someone marked the motor lump. Use intensity that is supramaximal for the SNAP, not automatically motor-supramaximal. Identify the SNAP by latency, a lower-intensity series, and (when needed) comparison with an orthodromic run.

Orthodromic sensory NCS stimulates the digits (ring electrodes) and records at the wrist. Direction matches physiology. SNAPs are smaller, often needing averaging, but motor contamination is much less because you are not dumping current into the palm motor branches. Do not mix antidromic amplitudes with orthodromic reference values. If an antidromic digit SNAP is unexpectedly huge and delayed, suspect a motor artifact rather than a miraculous sensory axon.

Troubleshooting unexpected low amplitude, slowing, and block

Work the machine before you work the diagnosis. The physician interprets remaining abnormalities after technical exclusion. The technologist’s job is to not hand over avoidable fiction.

Unexpected low amplitude. Confirm the cathode is on the nerve (slide medially and laterally). Confirm supramaximal intensity and adequate pulse duration. Confirm G1 is on the correct muscle and on the motor point (kill initial positivity). Check leads, paste, impedance, G1–G2 distance, and polarity. Consider anomalous innervation (Riche-Cannieu for thenar, accessory fibular for EDB, MGA for ulnar site comparisons). Only then is a small potential ready to be treated as axon loss.

Unexpected slowing. Warm the limb. Remeasure distance. Fix ulnar elbow flexion. Recheck onset cursors (artifact versus true takeoff; peak versus onset if the protocol specifies one). Confirm you stimulated the intended nerve. Remaining slowing can be demyelination — after those checks.

Unexpected block (proximal CMAP much smaller than distal, or the reverse pattern that fakes block). First repeat proximal stimulation: fibular head, across elbow, popliteal fossa, and Erb’s point are famous submaximal sites. Press, relocate, raise intensity, and watch the twitch. Exclude anomalous innervation: accessory fibular when EDB is smaller at the anterior ankle than at the fibular head; MGA when the ulnar wrist CMAP exceeds the below-elbow response and median-elbow stimulation accounts for the difference. Exclude distal co-stimulation that inflated the distal CMAP. True conduction block is a physiologic conclusion after this list, and it should sit at a clinically plausible focal site.

Artifact source table