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Illustration showing the normal nerve and its components and the changes that occur with chronic nerve compression: blood nerve barrier changes lead to connective tissue changes, which then lead to localized nerve fiber changes and eventually severe diffuse fiber changes with Wallerian degeneration (degeneration of the axon farthest from the site where the nerve was cut). Illustration from Surgery of the Peripheral Nerve by Mackinnon and Dellon, reprinted with permission of Thieme Medical Publishers, Inc.

Over time, with compression of any nerve, there is a breakdown of the blood-nerve barrier. This protective barrier within the nerve keeps the inside of the nerve isolated from anything outside of it. When the barrier is broken down — which can happen with nerve compression, repetitive motion or even trauma — it causes leakage into the nerve and fluid will accumulate. This accumulation, in turn, causes swelling and pressure within the nerve. The swelling can lead to inflammation and scarring, and the scarring can interfere with the function of the nerve itself.

Often, if the pressure on the nerve is released, this cascade effect can be broken. The nerve will begin to heal, and the barrier will be restored. Injury to the axons or the myelin also will heal. Most often, symptoms that result from compression of a nerve can be reversed, even in long-standing cases. But when noticeable motor loss or severe sensory loss occurs, full recovery may not be realized.

Carpal tunnel syndrome, tarsal tunnel syndrome, cubital tunnel syndrome, compression of the peroneal nerve in the knee and a herniated disk in the spine all are examples of nerve compression. Occasionally, this type of nerve compression can occur with thoracic outlet syndrome.