To the Editor:
The recent article and editorial regarding intraoperative vision loss in the prone position continue to promote real advances in understanding and reducing the occurrence of this devastating complication.1,2The importance of prone positioning, obesity, gender, and use of the Wilson frame clearly invite the conclusion of perioptic venous back pressure and edema formation as causative mechanisms. However, the commonality for all four factors is perhaps, at least partially, one of simple geometry. The gender factor may not be hormonal differences. Women typically are shorter than men. Fixation on body mass index as the index of obesity obscures the gender difference in absolute height, also imparting a different absolute “thickness.” The combination of increased thickness and length together may contribute to significant differences in periocular congestion and edema. The Wilson frame's absolute height may be fixed, but measurements of “thicker” and longer males will result in a greater total prone body height as measured from base (eyes) to apex (skin incision site) geometrically (Pythagorus). This is minimized by Jackson style frames, where the shoulders and hips are preferentially supported in a level position. The shortest female's face may never reach to the base of the Wilson's arch.
Geometry has been implicated as a significant factor in vision loss in prolonged supine surgical positioning: robotic prostatectomy.3,4Certainly the prone and head-down positions impart increased ventilation pressures to increase central venous pressure and venous pressure in the optic area, with prolonged surgery promoting intensification of edema accumulation. If we accept the geometry theory of this process, the rational conclusion to eliminate ischemic optic neuropathy is clear: Perform prolonged spinal surgery only in the left lateral position! The head is now uniformly placed above the heart, facilitating minimal venous back pressure from gravity and ventilation, while maximizing the filling pressure of the now “dependent” heart. Can geometric considerations drive a change in “routine standard neurosurgical practice?” Is the prone position primarily used for obsolete “historical reasons?” Geometry considerations have reduced sitting craniotomy numbers to an unparalleled historical minimum only by exposing the dangers of air embolism, which was also a “rare event.” Is ischemic optic neuropathy any less devastating? Can the authors examine the geometry factors in their available data because the published material is inadequate in this regard? Can surgeons be led to use the lateral position, especially for prolonged surgical procedures? What problems would be introduced or need solutions? Is it time to reexamine the premise and study this theory prospectively as optimal preventive strategy?
The suggestion that staging procedures may represent a preventative strategy deserves consideration here. Staging recently has been demonstrated to impart increased morbidity and possibly mortality in major spinal surgery.5The multicenter retrospective data indicate, but do not prove, that increased morbidity and mortality, prolonged hospital stay, increased costs, and infections are to be expected. It is also possible that nonarteritic ischemic optic neuropathy occurring during prone surgeries simply reflects coincidental occurrences found in the general nonsurgical population, given the relatively similar frequency of occurrence.6Vasopressors commonly used during these surgeries or delayed detection in the intensive care unit with causative association to surgery may also play a role.7Clearly, comparing prone to lateral surgery in a prospective fashion may be the single most effective means to improve patient outcome and clarify cause versus effect in this devastating surgical complex.
