The Effect of Table Position on Intraocular Pressure (IOP) and Ocular Perfusion Pressure (OPP) During Prone Spine Surgery

Postoperative permanent visual loss is a rare but devastating complication of surgery estimated to occur after approximately 1/60,000 anesthetics. After procedures involving cardiopulmonary bypass and prone spinal surgery, the estimates are higher, 1/1600 to 1/1100, respectively and have led to the formation in July of 1999 of the Postoperative Visual Loss (POVL) Registry under the auspices of the American Society of Anesthesia (ASA) Committee on Professional Liability. The majority of reported cases as of early 2003 were associated with spine surgery (67%).

Of the spine cases, the majority were due to ischemic optic neuropathy (ION) (81%) followed by central retinal artery occlusion (13%) and unknown diagnosis (6%). Central retinal artery occlusion is characterized by periorbital edema, a cherry red spot at the fovea and monocular blindness. It is thought to be due to direct prolonged extraocular pressure on the globe and thus is preventable. Direct pressure on the eye is the etiology most often mentioned by spine surgeons in an attempt to explain all forms of postoperative visual loss.

Post anesthetic ION, affecting both the anterior and posterior portions of the optic nerve, however, is the more common diagnosis. The etiology is unclear but hypo perfusion of the optic nerve has been associated with multiple risk factors. The four patient factors are obesity, hypertension, diabetes and low preoperative hematocrit. There are five surgical factors, which include an operation of long duration, large blood loss, prone position, deliberate hypotension, and blood replacement strategies which increase the tissue fluid compartment while decreasing the hematocrit. ION occurs in patients who had their heads suspended in Mayfield tongs (18% of ION cases), virtually eliminating any source of external pressure. In addition, 58% of these patients had bilateral disease, making direct pressure less likely.

Thus, most POVL cases appear to be directly related to a change in retinal and/or optic nerve perfusion. The visual loss associated with anterior ION is caused by infarction in the watershed zones between the areas supplied by the posterior ciliary arteries, which are end arteries without anastomosis. Posterior optic neuropathy is thought to be caused by decreased oxygen delivery to the posterior portion of the optic nerve between the orbital apex and the entrance of the central retinal artery.

Critical to any discussion of perfusion to the eye is the concept of ocular perfusion pressure (OPP), defined as the difference between the mean arterial pressure (MAP) and the intraocular pressure (IOP). Unopposed decreases in MAP, increases in IOP or a combination of the two may result in hypo perfusion of the eye and can cause an ocular infarction at the level of the retina or optic nerve, leading to varying degrees of visual loss which is frequently bilateral and irreversible.