Studies on the permeability of potassium ions across the dura and arachnoid mater of the rat spinal cord

Cortical Spreading Depolarization (CSD) Recorded from Intact Skin, from Surface of Dura Mater or Cortex: Comparison with Intracortical Recordings in the Neocortex of Adult Rats

In cerebral cortex of anesthetized rats single waves of spreading depolarization (CSD) were elicited by needle prick. CSD-related changes of DC (direct current) potentials were either recorded from the intact skin or together with concomitant changes of potassium concentration with K⁺-selective microelectrodes simultaneously at the surface of the dura mater or of the cortex ([K⁺]_s) and in the extracellular space at a cortical depth of 1200 µm. At the intact skin CSD-related DC-shifts had amplitudes of less than 1 mV and had only in a minority of cases the typical CSD-like shape. In the majority these DC-shifts rose and recovered very slowly and were difficult to identify without further indicators. At dura surface CSD-related DC shifts were significantly smaller and rose and recovered slower than intracortically recorded CSD. Concomitant increases in [K⁺]_s were delayed and reached maximal values of about 5 mM from a baseline of 3 mM. They rose and recovered slower than simultaneously recorded intracortical changes in extracellular potassium concentration ([K⁺]_e) that were up to 65 mM. The results suggest that extracellular potassium during CSD is diffusing through the subarachnoid space and across the dura mater. In a few cases CSD was either absent at the dura or at a depth of 1200 µm. Even full blown CSDs in this cortical depth could remain without concomitant deflections at the dura. Our data confirmed in principle the possibility of non-invasive recordings of CSD-related DC-shifts. For a use in clinical routine sensitivity and specificity will have to be improved.

Postmortem image analysis of sheep cortical leptomeningeal space and vasculature: theoretical implications on brain surface dialysis

The vascular surface distribution of the subarachnoid space has not been studied extensively. The aim of our study was to develop a method of computer-assisted estimation of the distribution of the vascular network in the cortical leptomeninges and subarachnoid space and model it to aid the study of the physiology of brain surface dialysis. Nine sheets of leptomeningeal tissue were obtained from adult sheep. Fourteen image sample areas of 4 cm² each were acquired and processed with ImageJ. The vascular and nonvascular areas of the cortical subarachnoid space were identified using a "projected surface" approach. The modeling equations were used to predict the behavior of brain surface dialysis processes. The mean surface area of identified subarachnoid vessels was 0.354 ± 0.02 cm² per 1 cm² of tissue. The mean meningeal area with unidentified vessels was 0.646 ± 0.02 cm²/1 cm², and the difference between these surfaces was significant (p < 0.0001). The modeling equations used predict that modifying the vessel diameter of the subarachnoid space could alter the efficiency of brain surface dialysis. The computer-assisted modeling of the vascular surface of the cortical subarachnoid space may be a useful tool in depicting its morphology and assessing the physiology during brain surface dialysis.

Surface dialysis after experimental brain injury: modification of edema fluid flow in the rat model

Object

This study was undertaken to determine if dialysis of damaged brain surface can reduce cerebrospinal fluid (CSF) pressure and progressive brain edema. The authors secondarily determined if local brain cooling was simultaneously possible.

Methods

Telemetric pressure transmitters were implanted into the lumbar subarachnoid space of 58 young adult male rats. Cryogenic brain injury was created and 2 hours later decompressive craniectomy was performed. An osmotic cell with a semipermeable dialysis membrane placed on the damaged brain surface was perfused with dextran 15% solution for 2 or 4 hours. Water content was determined in the cerebral hemispheres using the wet-dry weight method. Evans blue–albumin spread was measured morphometrically. Brain temperature was measured bilaterally.

Results

The CSF pressure increased after cryogenic injury and decreased after craniotomy. Two hours of brain dialysis significantly reduced CSF pressure in comparison with craniotomy alone and sham dialysis. Injured brain had higher water content, but this was not affected by dialysis. Spread of Evans blue–albumin, however, was significantly reduced by the treatment. Cooling of the dialysis solution caused significant local brain cooling.

Conclusions

Surface dialysis of cryogenically injured rat brain controls CSF pressure and reduces intraparenchymal spread of edema fluid in the acute period after injury. The authors postulate that edema fluid moves into the osmotic cell rather than spreading through the uninjured brain. Long-term experiments will be needed to prove that this combination therapy is effective.