A compound muscle action potential (CMAP) is the summated action potential recorded from muscle during a motor nerve conduction study (NCS). Motor NCSs with recording of CMAPs may be used for several purposes in assessing neuromuscular diseases, including providing objective measurements of the extent and localization of the cause of weakness; determining the underlying pathological abnormality, such as conduction block or slowing of conduction at a localized area of neurapractic injury; identifying the changes associated with Wallerian degeneration and regeneration in the motor nerve; and assisting (along with needle EMG) in distinguishing peripheral nerve disease from lower motor neuron disease, neuromuscular junction disease, and myopathies. This chapter will review the concepts and techniques of motor NCS and CMAP recording, will describe the technique and measurements, and will discuss the findings in various neuromuscular diseases.
