NMJ Disorders: RNS & Clinical Testing

Technique: where the test is won or lost

RNS is exquisitely sensitive to method, and most “abnormal” studies in inexperienced hands are technical artefact. Four disciplines are non-negotiable. Immobilization: any movement of the recording limb between stimuli shifts electrode geometry and fabricates a pseudodecrement; the joint is splinted and the muscle held still. Supramaximal stimulation: the stimulus must exceed the threshold for every axon on every pulse, or amplitude changes reflect a drifting stimulus rather than junctional physiology. Warming:cool muscle paradoxically masks a true decrement by augmenting acetylcholinesterase efficiency and quantal content, so a limb temperature above roughly 32 °C must be maintained. A standard sequence:because junctional reserve is exhausted and replenished by activity, the protocol — rest, a baseline train, a defined period of exercise, then repeat trains at fixed intervals — is fixed precisely so that facilitation and exhaustion can be read against a stable baseline.

Low-rate stimulation and the myasthenic decrement

At low stimulation rates — conventionally 3 Hz — the readily releasable pool of acetylcholine quanta normally runs down over the first four to five stimuli before mobilization replenishes it. In a healthy junction the safety factor absorbs this run-down and every fibre still fires, so the CMAP is unchanged. In myasthenia the receptor population is depleted by antibody and complement, the safety factor is narrow, and the physiologic run-down drops a fraction of endplates below threshold; those fibres drop out and the CMAP decrements by more than 10%. The decrement is characteristically U-shaped, maximal at the fourth or fifth response and recovering slightly afterward as mobilization catches up.

The dynamic manoeuvres confirm the mechanism. A brief (10–30 s) maximal voluntary contraction floods the terminal with calcium and transiently augments release, repairing the decrement —post-exercise facilitation. Sustained exercise instead exhausts the terminal, and trains obtained two to four minutes later show a deepened decrement — post-exercise exhaustion. The presence of both facilitation and exhaustion authenticates a junctional decrement and separates it from technical drift.

Calculating percent decrement

Decrement is computed from amplitude (or negative-peak area) as the fractional fall from the first response to the trough, typically the fourth or fifth: decrement (%) = (A1 − A4) / A1 × 100. The trough rather than the last response is used because the curve recovers slightly after the nadir. A reproducible decrement exceeding 10% that repairs after exercise is the diagnostic threshold; smaller or non-reproducible drops are treated as technical until proven otherwise.

High-rate stimulation and the LEMS increment

Lambert–Eaton syndrome is presynaptic: antibodies against voltage-gated P/Q-type calcium channels reduce the calcium influx that triggers quantal release, so even the first stimulus releases too few quanta and the resting CMAP is low. When stimulation is rapid (20–50 Hz) or follows a brief maximal contraction, calcium accumulates faster than it is cleared, release climbs, previously sub-threshold fibres are recruited, and the CMAP increments dramatically — frequently >60–100%. Because sustained high-frequency trains are painful, the preferred equivalent is to compare the resting CMAP with the amplitude immediately after ten seconds of maximal voluntary exercise. A low baseline CMAP that more than doubles after exercise is the signature of presynaptic failure and should prompt evaluation for small-cell lung carcinoma.

The complementary role of single-fibre EMG

RNS is specific but insensitive, particularly in ocular and mild generalized myasthenia where it is frequently normal. Single-fibre EMG is the most sensitive electrodiagnostic test of neuromuscular transmission: by recording potential pairs from the same motor unit it quantifies jitter — the variability in interpotential interval that reflects the timing instability of endplate transmission — and detects blocking when transmission fails outright. Jitter is abnormal in the great majority of myasthenics even when RNS and antibodies are unrevealing, making SFEMG the test of choice to confirm a junctional disorder when the clinical suspicion is high and RNS is normal. The two techniques are complementary: RNS demonstrates the population-level decrement that explains fatigable weakness, while SFEMG exposes the single-junction instability that underlies it.