Mammalian peripheral nerve sheath has unique responses to chronic elevations of endoneurial fluid pressure

In acute experiments with fibroelastic tubes such as blood vessels there is a linear ΔP:ΔV relationship followed by a steeper ΔP:ΔV relationship for further increments of volume. Endoneurial fluid pressure (EEP) of peripheral nerve also increases with increases in endoneurial fluid volume. We monitored the effects of volume changes on ΔEFP during a protracted period of time (6 weeks to 1 year on 16 control and 16 experimental rats) to study if a similar relationship occurred in nerve. Nerves were rendered edematous using parenteral and oral galactose administration, EFP was monitored using an active servonull system, and endoneurial volume and subperineurial area (SPA) was measured on fixed tissue. Marked endoneurial edema was produced but EFP did not exceed 6 mm Hg at any time. There was a linear ΔP:ΔV relationship for a limited range of volumes followed by a reduced ΔP:ΔV slope. Because these changes evolved over a long time course we examined the response of ΔEFP/ΔSPA as a function of time. There was an exponential reduction with time, thus underlying the importance of time-dependent processes in the production of a reduced ΔP:ΔV slope. We conclude that neuropathic changes are unlikely to be due to ischemia by compression of capillaries. Instead, in edematous states there is a very low shear modulus (i.e., a small ΔEFP produces a major volume change with time) and certain types of deformations appear very likely to cause demyelination.