Upper Extremity Nerves

Nerve Conduction Studies General Rules

As a rule, latencies and conduction velocities are affected most. With few exceptions, the sensory fibers are affected first. The sensory nerve action Nerve Conduction Studies General Rules

A few rules make nerve conduction studies easy to perform and greatly reduce the amount of examiner errors.

– All recording and stimulation points must be carefully marked with clearly visible ink. Such markings will allow easy rechecking of the stimulating and recording points, but most importantly, will allow the remeasuring of the distance when motor or sensory conduction velocities appear artifactually slowed.

– Distances should always be measured with the tape closely apposed to the skin and the anatomical course of the nerve carefully adhered to. In sensory studies, set distances are used between the stimulator’s cathode and the active recording electrode because only one-point stimulations are routinely performed. These set distances enable you to compare the results with the lab’s normal values obtained at these same distances.

– In motor conduction studies, the proximal stimulation yields a response which, though identical to the distal in most respects, has a slightly longer duration (due to the temporal dispersion of conduction along the nerve fibers) and therefore slightly lower amplitude. The drop in amplitude from distal to proximal stimulation however is less than 2 mv in normal nerves with the exception of the posterior tibial. There the drop can reach 4 mv because of the deep lying position of the nerve in the popiteal fossa.

– The sensory fibers can either be studied orthodromically (in the direction of physilogical nerve conduction) or antidromically (in the opposite direction of physilogical nerve conduction). While there are good arguments for both, we use the antidromic technique for the simplicity of performance and easy reproducibility.

Temperature Effects

Minimal changes in temperature can greatly affect nerve conduction studies, and extra care should be taken to monitor skin temperature during nerve conduction studies. At lower skin temperatures, sensory and motor amplitudes become higher and distal latencies are prolonged. Both motor and sensory conduction velocities are slowed. In neuromuscular transmission defects, decrements may altogether disappear at lower temperatures. Optimal skin temperature is 35 C and the extremities should be warmed if it falls below that. In our lab we use disposable adhesive temperature strips applied over the dorsum of the hand and the dorsolateral aspect of the foot during the study.