The techniques described previously can be applied to almost any peripheral nerve. Schematic section of peripheral nerve showing osmotic pressure increase with axonal swelling at center. In the patient, the nerve is composed of thin fibers only and black-stained myelin is hardly visible. From Essential Neurology, Pryse-Phillips and Murray, Medical Examination Publishing Co, Inc, 1978. Judging whether a given set of nerve conduction data are normal requires comparing them to values obtained from that nerve in normal subjects.
Normal values are well established for motor and sensory conduction in most major peripheral nerves.
Of course, the neurologist must select the correct nerve or nerves to test, depending on each patient's clinical problem. If the nerve conduction studies are abnormal, one can also get some idea of how severely a nerve has been damaged or injured. Nerve conduction studies may also allow one to judge whether the main site of damage is the axon or the myelin sheath. Axonal neuropathies produce a decrease in CMAP and SNAP amplitudes but have relatively little effect on conduction velocities. By contrast, in a demyelinating neuropathy , conduction velocities are significantly slowed , while the CMAP and SNAP amplitudes are little changed.