This chapter is concentrated on such secondary events, particularly on changes taking place in the fluid environment. Oedema is by definition an accumulation of fluid in a tissue. It may occur in all the compartments of a nerve. We can thus speak of epineurial, perineurial and endoneurial oedema depending on the location of the fluid. The term vasogenic peripheral nerve oedema is used when there are signs of increased vascular permeability to protein molecules.
Nonvasogenic oedema is used for a swelling with normal vascular permeability to proteins.
This can be of extracellular type , as exemplified by the oedema occurring in galactose neuropathy or of intramyelinic type , as in hexachlorophene intoxication. There are several ways of studying oedema in peripheral nerves. In all these techniques it is hard to selectively sample endoneurial and epineurial compartments. Variations in the amount of adipose tissue in the epineurium may also disturb determinations with the microgravimetric test. Nevertheless, the methods may give valuable information in experiments on peripheral nervous system oedema.