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NERVES
STUDIED IN THE UPPER EXTREMITIES 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.
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| 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.
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| 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.
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| 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.
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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.
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