More spontaneous activity and normal MUAPs

1. What are neuromyotonic discharges?
2. What are cramp discharges?
3. What are artifact potentials? What is the difference between noise and interference?
4. How is the MUAP amplitude measured and what might cause it to be abnormal?
5. What is the rise time and what does it represent?
6. What is the duration and what does it represent?
7. What is the difference between turns and phases?

Answers:
1. Neuromyotonic discharges arise from the motor axons and are clinically seen in Isaac's syndrome, but also in anterior horn cell disease, tetany, and anticholinesterase poisoning. There is continuous muscle fiber activity resulting in muscle rippling and stiffness with a progressive decrement in waveform amplitude due to single fiber fatigue. It sounds like a motorcycle and has a tornado-like appearance.
2. Cramp discharges are associate with involuntary repetitive firing of MUAPs in a large area of muscle, usually assoc with painfuly muscle contraction.
3. Artifact potentials are waveforms that obscure the real signal. Inference are signals from outside the system being studied. Noises is unwanted signals from within the system being studied, including the EMG instrument itself.
4. The amplitude is measured from peak to peak. It can be increased due to reinnervation or decreased due to loss of muscle fibers. Normal is 1mV with a range of 1uV to 2mV.
5. The rise time is the time it takes the MUAP to go from baseline to negative peak. It represents the proximity of the needle to the motor unit and normal is <500uV.
6. The duration represents the number of muscle fibers within the motor unit. It may be increased (>15ms) from collateral sprouting or decreased (<5ms) from loss of muscle fibers.
7. Turns (aka serrations) are changes in direction of waveform that do NOT cross the baseline, whereas phases are calculated as the number of baseline crossings +1. More than 4 phases is polyphasic and results from muscle fiber dropout, alterations in fiber CV, or collateral sprouting. With a monopolar needle, this can occur normally 30% of the time (15% in concentric needle), and more often in the elderly.