Practical instrumentation and common sources of error.

A thorough comprehension of electrodiagnostic equipment is essential to consistently obtain accurate and reproducible data. Unreliable waveform latencies or morphologies may result from inappropriate filter settings, sensitivity comparisons, sweep speeds, interelectrode separation, cathode/anode reversals and stimulus artifact. A low frequency filter with too high a frequency limit may decrease amplitude, shorten peak latency, decrease the negative spike duration, add a phase and increase total waveform duration. A high frequency filter with too low a cut-off may decrease amplitude and prolong onset and peak latencies. Increasing the amplifier's sensitivity may shorten the onset latency of a response. Sweep speeds that are too slow may omit phases, turns or entire potentials when using digital equipment. If the interelectrode separation is inadequate, waveform morphology and amplitude can be altered. Reversing cathode and anode placement affects latency and velocity determinations. Stimulus artifact may obscure a response and its reduction must be understood. Comparing latencies and amplitudes at different instrument settings is never appropriate and can lead to serious errors and misdiagnoses. A naive approach to instrumentation, therefore, is indefensible.