To the Editor: 

After publication of the first report by the American Society of Anesthesiologists Visual Loss Registry Study Group,1I submitted a Letter to the Editor in which I stated that administration of excessive volumes of crystalloid fluid may be the cause of ischemic optic neuropathy (ION) and recommended that crystalloid fluid therapy not exceed 40 ml/kg, regardless of the duration of the posterior spinal surgery.2In reply, Dr. Warner stated that my recommendation was dogmatic and unsubstantiated, which was true, but he also did not provide any documentation that it was invalid. Now we have additional evidence that my recommendation regarding crystalloid fluid therapy was on target.

In their recent report, the Visual Loss Study Group did a retrospective comparison of a number of variables in their database of 80 patients with ION with those from 315 carefully selected, matched control subjects who underwent posterior spinal surgery but did not experience ION.3The Study Group identified a number of highly significant differences (P = 0.001) between the ION and control subjects. Three significant differences that are interrelated stand out. The total volume of fluid replacement and the total nonblood fluid replacement were greater in the ION patients, and the administration of colloid as a percentage of the total nonblood replacement was less in the ION patients. The only remaining fluid in this analysis would be crystalloid. These findings directly support the concept that the crystalloid fluid volume was significantly greater in the ION patients, although a direct comparison of the volume of crystalloid administered in the two groups did not reach significance.

Other significant differences between the two groups included gender, obesity, use of the Wilson frame, duration of anesthesia, and estimated blood loss. Both the Study Group and Dr. Warner in his editorial4suggested that ION may be less common in women than men because of the protective effect of estrogen. A simpler and more reasonable explanation for the difference is that most anesthesia providers are more likely to give larger volumes of crystalloid fluid to men weighing 80–120 kg than they are to women weighing 60–80 kg. With respect to obesity, the Study Group suggested that positioning the obese patient prone may increase intraabdominal, intrathoracic, intraocular, and venous pressures and produce ischemia of the optic nerve by a variety of mechanisms. Another more plausible explanation would be that if prone positioning did increase venous pressure in the obese patients, it would be manifest most profoundly as blood loss at the operative site, which in turn, would necessitate greater fluid administration, including crystalloid fluid. Finally, the Study Group suggested that the reason that ION was more common with use of the Wilson frame was because the head is more dependent with its use. However, this explanation is only conjecture because the exact positioning of the head was not documented in all of the patients who experienced ION while on the Wilson frame. When using the Wilson frame, the head need not be dependent because it can be supported in the neutral position with pillows and head supports, and this may have been done in some of the ION patients on the Wilson frame. I do not believe that exactness in head position is necessary provided crystalloid fluid volume administration is limited. We do a large number of robotic-guided, laparoscopic, retropubic radical prostatectomies with the patients in a very steep Trendelenburg position for 4–6 h. The crystalloid fluid volume is limited to less than 1 l until the patient is returned to the level position to avoid fluid collection in the bladder, which will obscure the operative field when the bladder is opened. We have not had a case of ION in this population. Two things stand out in the reported cases of ION occurring after prostate surgery: the patients were in a Trendelenburg position for 4–6 h, and they received approximately 5–10 l crystalloid fluid.

The recent report of the American Society of Anesthesiologists Task Force on Perioperative Visual Loss5advocates the use of both colloid and crystalloid fluids but does not recommend any limit on the latter. Based on the evidence to date, which admittedly is mostly circumstantial, I would urge anesthesia providers to strongly consider limiting crystalloid fluid therapy to less than 40 ml/kg regardless of operative length. With this change alone, I believe that we will experience a measurable decrease in the incidence of ION.

Lee LA, Roth S, Posner KL, Cheney FW, Caplan RA, Newman NJ, Domino KB: The American Society of Anesthesiologists Postoperative Visual Loss Registry: Analysis of 93 spine surgery cases with postoperative visual loss. ANESTHESIOLOGY 2006; 105:652–9
Larson CP Jr: Excessive crystalloid infusion may contribute to ischemic optic neuropathy [letter]. ANESTHESIOLOGY 2007; 106:1249
Postoperative Visual Loss Study Group: Risk factors associated with ischemic optic neuropathy after spinal fusion surgery. ANESTHESIOLOGY 2012; 116:15–24
Warner MA: Cracking open the door on perioperative visual loss. ANESTHESIOLOGY 2012; 116:1–2
American Society of Anesthesiologists Task Force on Perioperative Visual Loss: Practice advisory for perioperative visual loss associated with spine surgery: An updated report by the American Society of Anesthesiologists Task Force on Perioperative Visual Loss. ANESTHESIOLOGY 2012; 116:274–85