The Blood-Nerve Barrier and the Pathologic Significance of Nerve Edema

Edema occurs in peripheral nerves when excessive fluid accumulates either in the interstitium or within the myelin sheath, creating a disturbance in the endoneurial microenvironment. Nerve edema is concomitant with many peripheral neuropathies and can alter the endoneurial microenvironment by increasing pressure, reducing blood flow, or altering electrolyte concentrations in the endoneurial fluid (EF). Not only is it a complication of several diseases of peripheral nerve, but it can independently injure the nerve fiber. This is due to the vulnerability of the vasa nervorum to internal and external pressure. These vessels are characterized by numerous anastomoses that form the epineural and perineurial vascular plexuses (Figs. 1 and 2). Since they pierce the perineurium, pressure at this point causes ischemia, damaging nerve fibers. This chapter reviews new findings concerning the physiology of the endoneurial environment, mechanisms of increased endoneural fluid pressure (EFP), and their pathologic complications.