Abnormal MUAPs

1. What causes long duration, large amplitude polyphasic potentials?
2. What causes short duration, small amplitude polyphasic potentials?
3. How do neuropathic potentials differ from myopathic potentials?
4. What causes unstable potentials?
5. What are satellite potentials?
6. What are multiplet potentials and what causes them?

Answers:
1. Collateral sprouting that occurs with reinnervation, resulting in increased number of muscle fibers per motor unit. This is most commonly seen in neuropathic disease.
2. Dropout or dysfunction of muscle fibers. This is most commonly seen in myopathic diseases or NMJ disorders, but may also be seen in severe neuropathic injury leading to nascent motor units.
3. Neuropathic potentials are long duration, large amplitude MUAPs. Myopathic potentials are short duration, small amplitude MUAPs.
4. Unstable potentials have variations in MUAP amplitude and duration, and is most commonly seen in NMJ disorders, neuropathic disorders, or muscle trauma.
5. Satellite potentials are small potentials originating from a few muscle fibers that occur approximately 10-15ms after a MUAP. They are caused by incomplete myelin formation and immature terminal sprouts from chronic reinnervation or myopathy.
6. Multiplet potentials are 2 or more MUAPs firing together, seen in ischemia, hyperventilation, tetany, MND, or metabolic disease.