Instrumentation: Fundamental Concepts and Pitfalls

The electrodiagnostic instrument is foundational to the acquisition of electrophysiologic data and its subsequent interpretation and diagnostic implications. The initiation of data acquisition occurs at the three recording electrodes which consist of: E-1 (the noninverting amplifier port), E-2 (the inverting amplifier port), and E-0 (the so-called ground electrode). All three electrodes are essential in forming the recording montage. They must have similar, high-quality impedance and be properly connected to the amplifier. Their recording surfaces and the tissues from which they record must have low impedance and be properly prepared and applied at the recording site to ensure an accurate representation of the electrophysiologic signal. Both surface and needle electrodes can be used, contingent on the technique and data required (i.e., nerve conduction studies or needle electromyography, respectively). These biologic signals are small, ranging from millivolts to microvolts, depending on the generator source (e.g., muscle and nerve). Therefore, they must be amplified and subsequently filtered. Artifacts, distorted signal amplification, or inappropriate filtration will result in waveform distortion leading to erroneous interpretation (i.e., false positives or negatives). The electrodiagnostic medicine consultant (EMC) has other tools to deal with this, such as averaging. The signal is then digitized and displayed visually and acoustically through an analog-to-digital converter/loudspeaker requiring appropriate amplification and time scale to avoid any signal distortion. The EMC can then visually as well as auditorily analyze the signal of interest and store or print out the data for further interpretation. Current technology in instrumentation permits greater precision and accuracy in data analysis. Failure at any one of the above sequential processing steps can lead to data misinterpretation. It is incumbent upon the EMC to be thoroughly familiar with all of the steps in this process, including potential shortcomings.