Ulnar motor conduction study is performed by placing active electrode at abductor digiti minimi (ADM) muscle, reference electrode about 3 cm distal to active electrode and ground electrode b/w stimulator and active electrode.
Ulnar nerve is stimulated at wrist, below elbow and above elbow. This is because elbow is a common site of nerve damage, thus identifying any evidence of focal demyelination in this segment of nerve is important. Usual evidence of focal demyelination is in the form of conduction block, which is defined as -fall in CMAP amplitude on proximal stimulation in excess of 50%, as compared to distal stimulation. Other evidence is in the form of presence of focal slowing. Identifying these features at elbow, and therefore stimulating the nerve both above and below elbow is important in this study.
Usually, a distal latency in excess of 3.0 ms is taken as abnormal. Similarly, CMAP amplitude of less than 5 mv or conduction velocity of less than 50 m/s is taken as abnormal. However, these values may vary b/w various populations, machines etc, thus it is advisable to generate a normative data for each centre.
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