Ischemic Optic Neuropathy (ION)

by James W. Ogilvie, MD

Thursday, November 26th, 2009

 Ischemic optic neuropathy (ION) is a disorder than can occur following surgical procedures. There is partial or complete loss of vision as the result of a vascular insult. It has several possible etiologies including thrombosis of the central retinal artery most commonly associated with giant cell arteritis. Direct trauma to the orbit and cortical blindness must also be considered. ION has also been reported with acute non-surgical blood loss and the use of Viagara™. Hippocrates gives an account of someone with acute hematemesis who subsequently lost their sight, perhaps the first report of ION.

The least common and most enigmatic cause of post-operative vision loss is an ischemic episode to the optic nerve heads which are supplied by the short posterior cilliary arteries. The diagnosis of ION is made by fundoscopic examination of the eye in someone who reports a visual field defect following surgery. Emboli in the retinal vessels (posterior ION) can be visualized while in anterior ION (That which occurs anterior to the cribriform plate.) there are no initial diagnostic findings. After several months there is visible atrophy of the optic nerve heads resulting in a pale retina.

Because there may be effective therapies for other causes, it is important to differentiate ION from other etiologies of visual loss. An ophthalmologic consultant can accurately make the diagnosis. To date there is no effective treatment for anterior ION. Many therapeutic trials have been performed including the use of steroids, osmotic agents, hyperbaric oxygen, vasodilators and surgical decompression, all without benefit. There may be some spontaneous improvement in visual fields, but recovery from no light perception is very rare.

The causes of ION are not well understood, but acute blood loss is the most constant finding. ION has been reported with surgery in the supine, sitting and prone position. Prolonged spinal surgery in the prone position is the other commonly reported factor. Long surgical procedures resulting in facial edema when accompanied by hypotension or low hematocrit is often encountered in cases of ION. While atherosclerosis or diabetes may be predisposing factors, the relationship has not been studied in a scholarly fashion and ION has been reported in adolescents undergoing scoliosis surgery.

There is speculation that with acute blood loss there is an idiosyncratic response from released endogenous vasoconstrictors which may cause vasospasm of the short posterior cilliary vessels. It is not a sympathetic nervous system response due to the fact that sympathetic nerves do not supply the short posterior cilliary arteries. There may also be a congenital predisposition to ION due to a reduced ratio of capillary vessels to optic nerve heads. Unfortunately, there are no pre-operative tests to identify those with an increased susceptibility to ION.

Prevention of ION is clearly preferable. Reducing facial edema with the use of the reverse Trendelenburg position, limiting the use of crystalloids for fluid resuscitation and avoiding hypotension or anemia may lessen the incidence of ION.

ION frequently results in a medical liability action. If there are irregularities in the anesthetic record such as prolonged anemia or hypotension, use of large amounts of crystalloid for fluid resuscitation resulting in facial edema or improper patient positioning, the surgical team is often held liable. The issue of informed consent is often raised. What responsibility for discussing visual loss lies with the surgeon and anesthesiologist? There are no absolute answers to this issue, however this question should be settled long before discovery depositions are taken, preferably prior to the surgery itself.

     The guest author of this article for is Dr. James Ogilvie, a board certified orthopaedic surgeon.  He is Professor, Department of Orthopaedic Surgery, at the University of Utah in Salt Lake City and Professor Emeritus, Department of Orthopaedic Surgery, at the University of Minnesota in Minneapolis, MN. He is Staff Surgeon / Attending Staff at Shriners Hospital Intermountain Unit in Salt Lake City.

     A more detailed article on ION by Dr. Ogilvie can be found by clicking on the following link to it in the October 2009 issue of the American Academy of Orthopaedic Surgeons newsletter “AAOS Now”.

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