We gratefully acknowledge the interest that Drs. Kabbara, Larson, and Weiskopf et al.  have shown in our article on spine surgery and postoperative visual loss (POVL).1It is only through the continued interest and investment of time and resources by anesthesiologists, ophthalmologists, and surgeons that we will develop preventative strategies and/or treatment for this devastating perioperative complication. These letters provide an opportunity to discuss and expand on topics that space limitations would not allow in the original article.

Dr. Kabbara makes an insightful deduction in noting that our current lack of proven risk factors for ischemic optic neuropathy (ION), and its possible multifactorial etiology, would make an intraoperative monitor of optic nerve function a logical means to prevent ION. Unfortunately, previous studies have demonstrated that anesthetics diminish or ablate visual evoked potentials, making their intraoperative reliability poor.2,3Additional technical problems include poor sensitivity of the light-flash as opposed to pattern-evoked potentials. Further research and technical advances will be required before the sensitivity and specificity of this monitor for detecting optic nerve dysfunction are acceptable for routine clinical use. Moreover, some patients do not develop clinical visual deficits until several days after surgery, and it is unclear how useful an “intraoperative” optic nerve monitor would be in these situations.

Dr. Larson summarized his personal experience over many years and his personal beliefs about cause-and-effect relations regarding ION. Unfortunately, there is no way to validate the summary statements and beliefs derived from his anecdotal experience. Moreover, our clinical experience makes us concerned that limiting fluids to a specific amount, without regard to urine output or blood loss, may lead to underresuscitation and increase the risk of organ failure.

Although the American Society of Anesthesiologists POVL Registry has provided detailed descriptive characteristics of patients who develop ION after major spine surgery, it cannot be used to determine risk factors because there are no denominator data and no unaffected patients for comparison. Because of the nature of complex spine surgery, it is possible that patients who do not develop ION after major spine surgery have received similar amounts of crystalloid. The American Society of Anesthesiologists recently reviewed the scientific evidence and expert opinion regarding the anesthetic management to reduce the risk of perioperative visual loss in prone spine surgery. Because of the lack of scientific literature, an advisory, not a guideline, resulted. Although the advisory recommended the use of both colloid and crystalloid, specific amounts of these solutions could not be recommended because of the absence of any evidence-based literature.4 

Dr. Weiskopf et al. ’s point about frequent eye checks to prevent central retinal artery occlusion from globe compression is appreciated. Because the focus of our article was ION, we did not explicitly state, but do completely agree, that frequent eye checks during major prone spine surgery are of unquestionable value to prevent globe compression. Data on inspired oxygen concentration and arterial oxygen were not collected and therefore could not be examined with respect to anemia. The clinical use of high inspired oxygen concentration in the potential presence of ischemia remains controversial because of theoretical risks of reactive oxygen species tissue damage.5Other details regarding clinical care of spine patients at the authors’ institution were noted, including limitation of crystalloid infusion, but again, the benefit of this practice with respect to prevention of ION cannot be validated based on the literature.

We agree with Dr. Weiskopf that consenting patients undergoing major spine surgery for the risk of POVL is challenging, but our experience reading closed claims files for POVL has repeatedly revealed that patients believe that they should have been consented for the risk of blindness associated with major spine surgery. The fact that the authors have made four significant intraoperative interventions aimed at preventing POVL demonstrates that it is of great concern to anesthesiologists and surgeons. Rest assured that it is of even more concern for patients. There is no widely accepted threshold of incidence of complications to preclude discussion of risks. Most states use the “reasonable patient” standard for consent as described by O’Leary, in which a physician is required “to disclose information that a reasonable patient under similar circumstances would want to know to make an informed decision.”6These risks would include common side effects and complications of low severity, and those that are less common, but with significant impact, such as blindness.

The data are clear regarding the types of spine cases in which ION occurs: prolonged operations in the prone position with large blood loss.1We, like others, speculate that the physiologic basis for these findings may have more to do with the prone position in which venous pressures are elevated and the time that it takes for optic nerve axons to become dysfunctional. Large blood loss increases the potential for hypovolemia and the occurrence of anemia, and increases fluid administration and transfusion of blood products, all of which may affect oxygen delivery to the tissues. However, any theory of causation for ION remains to be proven. We agree with the authors that maintenance of normovolemia is important and would be useful data to analyze, but this assessment is subject to varied interpretation, particularly in the prone position. This information would have to be collected in a prospective fashion with rigid criteria and uniform monitoring. We would like to clarify that we did not advocate a change in surgical practice, except for consent, without a randomized controlled trial comparing the effects of staged surgery for major spine procedures with single-stage surgery, because this alternative also has the potential for significant morbidity. We agree that surgeons and anesthesiologists must work together to minimize potential contributing factors to the development of POVL for our patients. The data demonstrate that two of these factors are prolonged spine surgery in the prone position and large blood loss.1 

Finally, we would like to reiterate that the clinical phenomenon of perioperative ION occurs at such a low frequency (highest incidence reported to date 0.1%)7that prospective clinical studies randomizing patients to treatment arms would require a multicenter, long-term, costly study. Currently, there is no evidence-based medicine to support any causative (or preventative) statements regarding the development of ION. Because of the low incidence of ION, and the predominance of these cases in spine operations of 6 h or longer and blood loss of 1,000 ml or greater,1most anesthesiologists are fortunate enough to have never encountered this complication, regardless of their anesthetic management. However, good fortune should not be equated with best practice when the etiology and prevention of ION remain unproven.

*University of Washington, Seattle, Washington. lorlee@u.washington.edu

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