In the analysis of each motor, sensory or mixed response recorded, some physical parameters are taken into account, such as:
- LATENCY is the time between the onset of electrical stimulus and the onset of the nerve response recorded (wave). The latency is called “onset latency” when it is measured using marker 1 – the onset of the negative phase, and the term “peak latency” is used when latency is measured using marker 2 – the peak of the negative phase (Fig. 1). In general, the onset latency is preferred but peak latency can be useful to compare two responses obtained by the same nerve or by several nerves, as it happens in the comparative techniques (median-ulnar, median-radial, etc.). The sensory latency represents the conduction through the largest and fastest cutaneous sensory fibers (onset latency) and the set of cutaneous sensory fibers of the large and small caliber (peak latency). The distal motor latency (MDL) represents the conduction time from the site of stimulation to the neuromuscular junction (NMJ) and muscle depolarization. Latency is measured in millisecond (ms).
- AMPLITUDE is the difference between two points of a wave recorded. The “negative peak amplitude” is measured from the onset of the nerve response (marker 1) to the negative peak (marker 2) of the wave recorded, and “peak-to-peak amplitude” is measured from the negative peak (marker 2) to the positive peak (marker 3 – for the sensory responses, marker 4 – for the motor responses). The amplitude is measured in microvolt (µV) and millivolt (mV), for sensory and motor responses, respectively (Fig. 1).
- DURATION is the time interval between the onset of the nerve response (marker 1) and its return to the baseline (marker 4 – for the sensory responses, marker 5 – for the motor responses). The duration of both sensory and motor responses (waves) is measured from the initial deflection of the negative phase of the wave from the isoelectric line to the return of the positive phase of the response to the isoelectric line (Fig. 1). The duration is measured in millisecond (ms).
- NERVE CONDUCTION VELOCITY is the speed of propagation of nerve impulses along a nerve or nerve trunk fiber. The nerve conduction velocity (NCV) is measured in meter per second (m/s), and it is calculated by measuring the distance (mm) between two stimulation sites and dividing by the difference in latency (ms) from the more proximal stimulus and the latency (ms) of the distal stimulus, as follows:
Sensory nerve conduction velocity (SNCV) is calculated using a single site of stimulation and measuring the distance between the point of stimulation and recording electrodes. Motor nerve conduction velocity (MNCV) is calculated dividing the distance between two stimulation points by the difference between latencies, and it does not include the neuromuscular junction or the muscle.
Fig. 1 Antidromic sensory nerve action potential (SNAP) recorded to the digit III – left,
and compound muscle action potential (CMAP) recorded from the Biceps Brachii (BB) muscle – right.
SNAP: marker 1 – onset latency (2.70 ms); marker 2 – peak latency (3.75 ms).
CMAP: marker 1– onset latency (4.85 ms).
SNAP: markers 1–2 – negative-peak amplitude (17.6 μV); markers 2–3 – peak-to-peak amplitude (25.7 μV).
CMAP: markers 1–2 – negative-peak amplitude (13.4 mV); markers 2–4 – peak-to-peak amplitude (21.6 mV).