Perineurial window: demyelination in nonherniated endoneurium with reduced nerve blood flow.

The perineurial window, created by surgical incision of the perineurial sheath allowing its contents to herniate into the epineurial space, provides an experimental model of primary demyelination, the cause of which is unclear. Because the injury is localized and involves distortion of tissue at the lesion site, ischemia is suspected as a cause of demyelination. To study the mechanism of demyelination in the perineurial window model, we measured nerve blood flow (NBF) with a laser Doppler flowmeter before and after perineurial rupture in rat sciatic nerve and assessed the spatial distribution of demyelinated fibers, particularly in the nonherniated portion of the endoneurium. Nerve blood flow at the site of the perineurial window was reduced significantly with an average level of NBF approximately 50% of presurgical values 10 minutes, 60 minutes and 6 hours after surgery. By light microscopic examination, most nerve fibers that herniated through the perineurial window underwent demyelination by 7 days. In addition, focal lesions of subperineurial demyelination were found in the nonherniated endoneurium in the adjacent subperineurial region and proximally and distally to the perineurial window. Endoneurial vessels adjacent to the perineurial incision appeared to be compressed. We suggest that ischemia contributes to the process of demyelination in the perineurial window model.