Role of the blood-nerve barrier in experimental nerve edema

Nerve edema is a common response to the nerve injury seen in many peripheral neuropathies and is an important component of Wallerian degeneration. However, independent pathologic effects of nerve edema that aggravate or induce nerve injury extend the role of edema beyond that of an epiphenomenon of injury. New insights into the mechanism and impact of nerve edema come largely from animal models. In the following review, we discuss the cause and consequences of nerve edema with particular reference to endoneurial fluid pressure and its relevance to the nerve microenvironment. Experimental models of nerve edema include conditions with increased vascular permeability such as lead poisoning, experimental allergic neuritis, and murine globoid leukodystrophy. Increased perineurial permeability induced by local anesthetics and neurolytic drugs can also induce nerve edema sufficient to increase endoneurial fluid pressure. Both perineurial and vascular permeability are increased after damage induced by crush, freeze, or laser injury. One of the most important forms of nerve edema is induced by external compression; the significance of this change is that edema has local compressive effects that persist after the external pressure has been relaxed. Nerve edema and increased endoneurial fluid pressure also occur in conditions in which vascular permeability appears to be unchanged such as experimental diabetic neuropathy and in hexachlorophene intoxication. In both of these conditions, reduced nerve blood flow has been demonstrated in rats and is viewed as a consequence of increased endoneurial fluid pressure. Whatever its mechanism, endoneurial edema has important structural and functional consequences for nerve fibers. A clear understanding of the underlying pathology of the nerve microenvironment may provide useful insights into treatment of clinical neuropathies.