Instrumentation, Quality, and Professional Practice in EDX Medicine
Instrumentation and quality control
Reliable EDX data depend on technique as much as interpretation. Surface active and reference electrodes are used for NCS; concentric or monopolar needles are used for EMG. Keep electrode impedance low, generally below 5 kOhm, by cleaning skin and using conductive paste. The amplifier's high common-mode rejection ratio rejects 60-Hz noise; check the ground electrode, avoid ground loops, and turn off nearby electrical devices. Set filters appropriately: motor studies 2 Hz-10 kHz; sensory studies 20 Hz-2 kHz; EMG 20 Hz-10 kHz. Use sweep speeds of 2-5 ms/div for motor studies and 1-2 ms/div for sensory studies, adjusting sensitivity to the waveform.
Quality requires supramaximal stimulation, consistent distance measurement, limb temperature above 32 C, limb immobilization, and clear documentation of side and muscles. Electrical safety requires an isolated stimulator, no current path through the heart, and avoidance of stimulation over the carotid sinus. Infection control requires gloves, single-use needles, and sharps disposal. Professional practice includes informed consent, accurate documentation, billing compliance, and recognition of scope.
Traps: using a notch filter to hide 60-Hz noise instead of fixing the source; high electrode impedance causing amplifier saturation; incorrect filter settings clipping sensory responses; failing to check the ground; measuring distance loosely along the skin; and not controlling limb temperature.
Worked example: a motor study shows a broad, noisy baseline at 60 Hz. Check the ground and electrode impedance first, then remove nearby electrical devices; do not simply turn on the notch filter, which can distort the CMAP.
Rule of thumb: 'Fix the source of noise before changing filters; low impedance and a good ground solve most artifacts.'