6.1 Latency, Amplitude, Duration, Area, and Rise Time

Why waveform parameters are the language of the tracing

Nerve conduction studies do not hand you a diagnosis as a single number. They hand you waveforms. Independent OpenExamPrep study of outline item III.A.2 Waveform Evaluation starts with the marks a technologist places on those traces: latency, amplitude, duration, area, and rise time. Each mark answers a different physiologic question. Latency asks how long the fastest conducting elements took to start a recognizable potential. Amplitude and area ask how much electrically active tissue contributed. Duration asks whether those contributions arrived together or straggled. Rise time asks whether you are even looking at a near-field generator under the active electrode.

If you mix up the marks, you mix up myelin, axon count, and technical quality. A delayed compound muscle action potential (CMAP) onset is not the same statement as a small CMAP. A small sensory nerve action potential (SNAP) is not the same statement as a SNAP whose onset you cannot find. This section teaches how to measure parameters 1–2 and 4–6 on the waveform-evaluation list (latency, amplitude, duration, area, and rise time) and what each one is actually reflecting. Conduction velocity (parameter 3) is the next section.

Latency: onset versus peak

Latency is time from the stimulus artifact to a defined point on the response, reported in milliseconds (ms).

Onset latency is marked where the potential leaves baseline. For a motor CMAP recorded with the active electrode (G1) over the motor point and the reference (G2) over the tendon, the physiologic onset is the initial negative deflection. In the conventional negative-up display used in most electrodiagnostic laboratories, that is the first upgoing takeoff. That takeoff is produced when muscle fibers under G1 depolarize. Onset latency of the CMAP therefore estimates arrival of the fastest motor axons still conducting, plus the time to cross the neuromuscular junction (NMJ) and start muscle-fiber action potentials. It is a distal time measurement: myelin integrity, internodal function, axonal continuity of the fastest fibers, NMJ delay, and the standardized distal distance all sit inside it.

If G1 is not over the motor point, the CMAP often begins with an initial positive (downgoing) dip. That positivity is a volume-conducted approaching field, not the motor-point depolarization. Teaching practice is to reposition G1 onto the motor point until the CMAP starts with a crisp negative takeoff, then mark that takeoff. Marking the positive dip as onset shortens latency artifactually and compares the tracing to the wrong physiologic event.

Peak latency is marked at the negative peak (or, in some sensory conventions, at a specified peak). Peak latency is later than onset latency. It does not isolate the single fastest axon; it is influenced by the shape of the compound waveform.

Motor studies almost universally use onset latency for distal motor latency and for conduction velocity (velocity uses two onsets, taught in the next section). Sensory studies are mixed. Many laboratories report SNAP peak latency for distal sensory latency because a 5–20 microvolt SNAP sits near the noise floor. The onset takeoff of a small SNAP is noisy: 60 Hz, stimulus artifact, and a wandering baseline make two readers mark different onsets on the same trace. The peak is a reproducible extremum. Some montages still use SNAP onset latency when the goal is conduction velocity of the fastest sensory fibers, especially when both stimulation sites share one recording site and the onsets are clear. Teach both conventions, and teach the reason: onset is physiologically the fastest-fiber mark, but it is the noisier mark on small SNAPs.

Whatever you mark, the laboratory's reference data must have been collected the same way. A peak-latency SNAP cannot be judged against an onset-latency normal table.

Amplitude: baseline-to-peak versus peak-to-peak

Amplitude is the size of the potential, in millivolts (mV) for CMAPs and microvolts (µV) for SNAPs.

Baseline-to-peak amplitude measures from the pre-stimulus baseline to the peak of the initial negative phase. For motor studies this negative-peak CMAP amplitude is the usual teaching default. It reflects how many muscle fibers depolarized in relative synchrony under G1, which in turn depends on conducting motor axons, NMJs, and muscle. It is the everyday proxy for axon (and motor-unit) census, with the caveat that desynchronization can shrink a peak without deleting that many axons.

Peak-to-peak amplitude measures from the negative peak to the following positive peak. It is larger. Some laboratories, and many SNAP protocols, use it. It includes more of the biphasic shape and can be steadier when the baseline is slightly sloped, but it also includes the positive phase, which is more sensitive to reference electrode position and to phase cancellation.

SNAP amplitude may be reported either way. Antidromic digital SNAPs are usually larger than orthodromic palmar or digital studies because the recording electrodes sit over a different generator geometry. Compare amplitude only against norms gathered with the same montage and the same peak convention. Amplitude is not interchangeable with latency: a nerve can be slow (myelin or long distal pathway) with a still-generous amplitude, or low in amplitude (axon loss, poor contact, excess subcutaneous tissue, mis-aimed G1) with a normal latency.

Supramaximal stimulation is assumed. A submaximal CMAP is a small CMAP for technical reasons. That is an instrumentation issue, not a waveform-parameter definition, but it is the most common way amplitude lies.

Duration, area, and the desynchronization problem

Duration is how long the waveform lasts, in milliseconds. For a CMAP, teaching practice marks duration from onset of the negative phase to the return of that negative phase to baseline (some laboratories continue to a later baseline crossing; again, match the lab). Duration lengthens when contributing muscle-fiber action potentials arrive at different times. That temporal dispersion is the electrical signature of desynchronization: unequal slowing among axons, as in demyelinating neuropathies, or a long, irregular distal pathway.

Area is the integral under a defined portion of the curve, commonly negative-peak area (mV·ms for CMAPs, µV·ms for SNAPs). Area tracks how much electrical activity occurred during that phase, not only how tall the peak was. When axons remain present but fire out of step, phase cancellation chops the peak amplitude while area falls less. When axons (or muscle fibers) are truly lost, or when conduction block removes them from the recorded volley, both amplitude and area fall. Duration, amplitude, and area together are therefore the technologist's first screen for small because dispersed versus small because missing. Full rules for conduction block versus temporal dispersion belong in a later lesion-pattern chapter; the measurement idea starts here.

SNAP duration is shorter than CMAP duration because a SNAP is nerve, not muscle, and the generator is a tighter volley. Marking SNAP duration on a noisy trace is often less reliable than marking SNAP amplitude; still, a markedly broad, low SNAP is a quality and physiology clue, not just an aesthetic one.

Rise time as a quality metric

Rise time is the time from onset (or from a defined fraction of the peak) to the peak of the same phase. A near-field potential recorded over its generator — G1 on the motor point, or ring electrodes over a digital nerve — has a steep, fast rise. A far-field or volume-conducted potential recorded at a distance from the generator has a slow, rounded rise. Slow rise is a warning that you may be measuring a hump conducted through tissue from another nerve or muscle, an electrode that is not on the generator, or a badly desynchronized volley.

Do not treat a slow far-field bump as a trustworthy SNAP latency or amplitude. Rise time will not diagnose a disease by itself; it tells you whether the waveform is measurable. Amplifier filters also change rise time (high-frequency filtering rounds peaks), which is why filter settings must match the settings used for the laboratory's norms — an instrumentation crossover, not a reason to ignore rise time.

What each parameter is reflecting

Worked measurement scenario

A median motor study is recorded from abductor pollicis brevis (APB). G1 is over the APB motor point; G2 is over the thumb tendon. After a supramaximal wrist stimulus the CMAP leaves baseline with a sharp negative takeoff at 3.6 ms. That number is onset latency. The negative peak stands 7.4 mV above baseline, so baseline-to-peak amplitude is 7.4 mV. The following positivity reaches 4.1 mV below baseline; peak-to-peak amplitude is 11.5 mV. This laboratory's motor reference data were collected baseline-to-peak, so 7.4 mV is the reported amplitude. The negative phase returns to baseline at 9.8 ms; duration is 9.8 − 3.6 = 6.2 ms. The instrument reports negative-peak area 24 mV·ms. Rise time from onset to the negative peak is steep (about 1.5 ms on this sweep), consistent with a near-field CMAP.

The technologist then slides G1 two centimeters off the motor point. An initial positive dip appears at 3.2 ms. That earlier positivity is not the CMAP onset to report. G1 is moved back until the tracing starts negative again.

An antidromic median SNAP to digit II is 22 µV baseline-to-peak and 36 µV peak-to-peak. Two readers argue whether onset is 2.4 ms or 2.8 ms because the takeoff is buried in noise; both agree the negative peak is at 3.1 ms. The laboratory's sensory table uses peak latency and peak-to-peak amplitude, so the reported pair is 3.1 ms and 36 µV. A separate, rounded 8 µV potential with a slow rise is discarded as a likely volume-conducted pickup rather than forced into the sensory table.

If you can mark onset versus peak, choose an amplitude convention that matches the reference data, read duration and area as dispersion versus loss, and reject slow-rise far-field traces, you are evaluating waveforms the way outline III.A.2.a intends — as independent OpenExamPrep measurement skill, not as a claim of official endorsement by the certifying body.