Osmotic diuresis paradoxically worsens brain shift after subdural grid placement

Abstract

The purpose of this study was to assess for peri-operative factors associated with brain shift following craniotomy for subdural grid electrode placement. A retrospective analysis of cases operated at a single institution was undertaken, examining 63 consecutive patients undergoing craniotomy for subdural grid placement for seizure monitoring between 2001 and 2007. Peri-operative records were reviewed in order to assess for intraoperative employment of osmotic duiresis. Postoperative MRI scans were analyzed for shift of the midline and brain displacement. One patient was excluded due to gross hemispheric atrophy confounding the midline, and four patients were excluded due to lack of available imaging. Hence 58 patients were radiographically reviewed. The employment of osmotic diuresis during grid placement appeared to be the most significant peri-operative factor influencing brain shift. Osmotic diuresis was administered in only 14 patients. Midline shift of the third ventricle was greater in the osmotic diuresis group (2.3 ± 0.3mm vs. 1.5 ± 0.2mm, p = 0.037). Moreover, the volume of shifted brain was significantly higher in the osmotic diuresis group (7.9 ± 0.5cm(3) vs. 4.7 ± 0.5cm(3), p = 0.003). There was no significant difference in the rates of neurological complications between patients who received osmotic diuresis and those who did not. Employment of osmotic diuresis during grid placement appears to be associated with a paradoxical increase in the volume of shifted brain. This may be due to a combination of the resultant "sagging" of the brain and the pressure exerted by the grid, suggesting that osmotic diuresis might not improve mass effect as intended when employed within this context.