Perioperative Ischemic Optic Neuropathy

A multidisciplinary task force consisting of anesthesiologists, orthopedic surgeons, neurosurgeons, and neuro-ophthalmologists from the American Society of Anesthesiologists (ASA) has been unable to define any specific ophthalmic or neuro-ophthalmic evaluations that might identify patients at risk for perioperative visual loss (PVL) related to ischemic optic neuropathy (ION), which has been reported. While there was consensus among the ASA Task Force on Perioperative Blindness that preoperative anemia, prolonged duration of surgical procedure (> 6.5 hours), substantial blood loss (defined as 44.7% of estimated blood volume), and secondary anemia increase the risk of PVL, a quantitative risk assessment and thresholds for prevention were not given in the group’s recently released practice advisory. One possible way to address this oversight is for operating surgeons to seek timely and appropriate informed consent from “high risk patients” (e.g., patients with anticipated prolonged procedure times, risk for substantial blood loss, pre-operative vasculopathic risk factors or anemia) undergoing non-ocular surgery. This is because perioperative ION has been reported in association with many different types of surgical procedures including lumbar spine surgery and cardiac bypass surgery (Anesthesiology. 2006;104:1319-1328). The purpose of this article is to examine the recommendations for care issued in the ASA task force’s practice advisory on ION after spine surgery and to define the possible role of the ophthalmologist in the care of these patients.

The High Risk Factors

The majority of the literature seems to implicate “substantial” blood loss with secondary anemia and systemic hypotension as the “predominant associated risk factors”^4-8 for perioperative ION. However, the ASA task force failed to reach consensus over the possible role of these factors versus the risk of “deliberate” intraoperative hypotension, even in high-risk patients with preexisting preoperative hypertension. Indeed, patients with preoperative hypertension may have a different threshold for hypotension, which might produce PVL, compared with normotensive individuals.

Although there is no clear consensus on the definition of hypotension, despite the existence of well defined hypertension parameters, the ASA task force suggests keeping the blood pressure of patients undergoing surgical procedures, especially with “deliberate” intraoperative hypotension, within 25% of the estimated baseline mean arterial pressure or within a minimum systolic blood pressure of 84 mm Hg. The ASA practice advisory also recommends that s ystemic blood pressure measurements be continually monitored in high-risk patients. Nevertheless, the use of “deliberate hypotensive techniques” during spine surgery (presumably to reduce blood loss and keep the surgical field clearer) was not proven to be associated with the development of PVL.

Intraoperative Ambiguity

The management of intraoperative blood loss and secondary anemia has been evolving over the years. Although there is consensus that hemoglobin or hematocrit levels should be periodically monitored in high-risk patients, the precise blood levels and thresholds for transfusion remain controversial. Indeed, the ASA task force concluded that “there is no documented lower limit of hemoglobin concentration that has been associated with the development of PVL,” and that “a transfusion threshold that would eliminate the risk of PVL related to anemia cannot be established at this time.” Additionally, while no studies have defined the precise benefit of monitoring intravascular volume in perioperative ION, the ASA task force has suggested that “intravascular volume should be continually monitored in high-risk patients.”

The use of vasopressors during surgical procedures has also been implicated as a risk factor for PVL. Unfortunately, no studies have established a cause and effect relationship. Therefore, the risk-benefit decision to use vasopressors (i.e., adrenergic agonists), even in “high-risk” patients, should be individualized.

Patient Position and Prolonged Surgery

The ASA task force has recommended that “high-risk” patients be “positioned so that the head is level with or higher than the heart,” and that the patient’s head be maintained in a neutral forward position (e.g., without significant neck flexion, extension, lateral flexion, or rotation), if possible. This is because patient positioning, especially the face down position, has been postulated as a possible risk factor for PVL. While the likelihood that direct globe compression results in perioperative ION results is unlikely, the task force’s recommendations may help avoid central retinal artery occlusion.

Prolonged duration of surgery has also been implicated as another potential risk factor for PVL (Anesthesiology. 1996;85:1020–1027). This may be an epiphenomenon rather than a true cause-effect relationship, since longer procedures are associated with longer durations of hypotension, greater intravenous fluid replacement, more blood loss, worse secondary anemia, prolonged face down positioning, and/or more complex or difficult surgery. Consequently, the ASA task force and specialty society members have considered establishing the following intraoperative parameters:

		Procedures are considered to be prolonged “when they exceed an average 6.5 hours (range, 2–12 hours) in duration”
		Blood loss is considered substantial above an “average of 44.7% (range, 10–200%) of estimated blood volume”

Physicians who wish to stage prolonged, complex procedures or surgery may do so, if there is a high likelihood for significant blood loss, hypotension, or other high risk conditions. The possible benefits of this course of action must be weighed against the increased risks associated with staging (e.g., anesthesia risks, infection, postoperative recovery time) as well as the additional financial costs.

Summary

Although anterior (i.e., optic disc edema present acutely) and posterior (i.e., retrobulbar) ION have both been reported after many different, non-ocular surgical procedures, patients who undergo identical procedures and identical operative times and who have identical blood loss, blood pressure, and risk factors have not experienced perioperative ION. In those patients afflicted with perioperative ION, the condition can occur unilaterally or bilaterally, and PVL often occurs in the first few days postoperatively.

Unfortunately, there is no evidence that any treatment is effective for perioperative or postoperative ION. Corticosteroid therapy, anti-platelet agents, and intraocular pressure (IOP) lowering agents may demonstrate various theoretic benefits, but there is no proven role or clear evidence to support their use. In addition, treating blood loss by transfusion introduces certain risks (e.g., infection, transfusion reaction, costs) that must be weighed against the only theoretic benefit in PVL. For example, higher intraoperative blood pressures (including the avoidance of deliberate hypotension) might lead to more intraoperative blood loss and greater levels of anemia or a bloodier surgical field and prolonged duration of surgery. That is to say, treatment of one purported risk factor (e.g., hypotension) might exacerbate another potential risk factor (e.g., surgical time, anemia).

Ophthalmologists who are called to see a patient with PVL have several responsibilities (Table 1). They should be aware of the differential diagnoses, perform a timely and complete exam, and assist the primary medical or surgical services with the management. Since there is no proven effective therapy for PVL, future research should concentrate on establishing ways to reduce the risk for predisposed patients with specific risk factors.

Table 1. Ophthalmologist’s role in perioperative visual loss.

References

Author Disclosure

This work was supported in part by ophthalmology departmental grants from Research to Prevent Blindness, Inc. in New York, New York.