The reported incidence of blindness following posterior spine surgery ranges from 0.013 to 0.36 % [4, 5], which suggests that the event is rare. However, this incidence is misleading for a variety of reasons. First, because until recently there was no required national reporting mechanism, not all cases were included in retrospective analyses of blindness after spine surgery. Second, there are many spine operations that are minor in terms of duration, extent of the surgical intervention, and blood loss (i.e.: diskectomy), which skew the denominator in the calculation of incidence, and greatly lower the true incidence for patients undergoing major posterior spine surgery such as spinal fusion with instrumentation. A better analysis of incidence would be to compare the number of cases of blindness seen with the number of cases of major posterior spinal surgery exceeding a specific time period, or with time as a continuous variable. Third, although we do not know the total number of major posterior spine surgeries performed between 1987 and 2004, the fact that the Registry was able to identify 83 cases of ION in that interval suggests that it is an ongoing problem of some magnitude. Finally, events labeled as “rare”, regardless of severity, tend to receive less attention in busy medical practices because of the expectation that it will not happen. A recent study by Rubin et al. suggests that the incidence of ION decreased 2.7 fold from 1998 to 2012 [6]. The authors did not determine the cause(s) of this decrease. They did confirm earlier findings that aging, male sex, transfusion and obesity all increase the risk of ION. However, regardless of incidence, it is essential to do all that we can as anesthesia providers to minimize or prevent this event. To achieve this goal, certain issues need clarification and a plan of action developed that has the consensus of the anesthesia community.

Blindness following posterior spine surgery may result from either central retinal artery occlusion (CRAO) or anterior or posterior ischemic optic neuropathy (AION/PION) [7] (Fig. 13.1). Since the clinical manifestations of AION and PION are similar, they are typically considered together as ischemic optic neuropathy (ION). ION is more common than CRAO. In the Registry study, 83 of 93 patients had ION; the remaining ten had CRAO. CRAO is usually unilateral whereas ION is almost always bilateral. It is generally believed that CRAO is caused by prolonged external pressure on the globe, while there is no established etiology for ION. Although blindness can occur at any age, it is most common in patients over age 40. Recovery of meaningful vision virtually never occurs with either injury.

Since CRAO is believed to be due to prolonged external pressure on the eyes, prevention is relatively straightforward. Placing the head in Mayfield 3-point fixation will eliminate any such pressure, and is a good choice if a prolonged operation is anticipated. Properly applied, the risks from using this head holder are inconsequential relative to the benefit. Alternatively, there are several available foam or gel-filled head holders with cutouts for the eyes, nose and chin. There is no evidence that one type is better than another, so long as the head is held in a neutral position, and all of the downward pressure is applied on the forehead and cheeks, and not on the eyes, nose or chin. The Task Force could not agree on whether eye checks should be performed by the anesthesia provider and how often. Certainly if technically feasible , it is probably prudent to do so periodically, recognizing that the examination itself may cause undesired head movement resulting in ocular pressure. Head movement for visualization of the eyes can be minimized by using a mirrored head holder (e.g. ProneView^TM) (Fig. 13.2).

Since the etiology of ION is unknown, it is impossible to provide any scientifically based recommendations for what to do to prevent it. The Task Force was not able to identify any predisposing medical conditions or any preoperative tests that might indicate susceptibility to ION. However, it did offer eight advisories for the management of “high risk” patients undergoing posterior spine surgery.