**5. Electrophysiology**

Electrophysiologic testing is part of the evaluation, but it never substitutes a complete history and a thorough physical examination. These tests can detect physiologic abnormalities in the course of motor and sensory axons. There are two main electrophysiologic tests: needle electromyography and nerve conduction, which permit differentiating between a focal mononeuropathy, a radiculopathy, and a plexopathy, or the discovery of a more diffuse process, such as a systemic peripheral neuropathy or motor neuron disorder.

The electromyography detects the voluntary or spontaneous generated electrical activity. The registry of this activity is made through the needle insertion, at rest and during muscular activity to assess duration, amplitude, configuration and recruitment after injury. Recruitment will be affected if demyelination occurs, but will not result in abnormal spontaneous activity. Meanwhile, axonal injury will result in both recruitment and abnormal spontaneous activity, which will not be seen on needle electromyography until 2 weeks after the initial insult [4].

Nerve conduction assesses for both sensory and motor nerves. This study consists in applying a voltage simulator to the skin over different points of the nerve in order to record the muscular action potential, analyzing the amplitude, duration, area, latency and conduction velocity. The amplitude indicates the number of available nerve fibers. Some authors consider diminished amplitude below 50% to be suggestive of compression. In such cases, we will find a normal response to distal stimulation but no response proximal to the site of entrapment. If the compression progresses, our results will be compatible with axonal degeneration with diminished amplitude of the response with relative preservation of the conduction velocity and distal latency until the remaining axons are completely damaged [5].
