Anterior ischemic optic neuropathy

Anterior ischemic optic neuropathy (AION) is a form of neuropathy involving loss of vision due to damage to the optic nerve from insufficient blood supply. AION is generally divided into two types: arteritic AION (or AAION) and non-arteritic AION (NAION or simply AION). This article will focus primarily on non-arteritic AION.

Introduction
The distinction between AAION and non-arteritic AION was made to highlight the different etiologies of anterior ischemic optic neuropathy. AAION is due to temporal arteritis (also called giant cell arteritis), an inflammatory disease of medium-sized blood vessels (Chapel-Hill-Conference) that occurs especially with advancing age. In contrast, NAION results from the coincidence of cardiovascular risk factors in a patient with "crowded" optic discs. Non-arteritic AION is more common than AAION and usually occurs in a slightly younger group than AAION. While only a few cases of NAION result in near total loss of vision, most cases of AAION involve nearly complete vision loss.

Beyond this introduction, this article will focus on non-arteritic AION. For a discussion on arteritic AION see the separate article arteritic anterior ischemic optic neuropathy. Though the term "AION" can be used to describe either anterior ischemic optic neuropathy in general or non-arteritic AION specifically, in this article "NAION" henceforth will be used to refer to non-arteritic anterior ischemic optic neuropathy. Nonarteritic anterior ischemic optic neuropathy (NAION) is an isolated white-matter stroke of the optic nerve (ON). NAION is the most common cause of sudden optic nerve-related vision loss, affecting more than 10,000 Americans every year, often bilaterally. No clinically effective treatments exist, largely because little is known about its pathophysiology, and there are few histopathological studies of the acute condition.

An exhaustive review article published in March 2009 described the latest information on arteritic and non-arteritic ischemic optic neuropathy, both anterior (A-AION and NA-AION) and posterior (A-PION, NA-PION, and surgical).

Symptoms and diagnosis
NAION typically presents suddenly and upon awakening. The patient notes seeing poorly in one eye. Vision in that eye is obscured by a dark shadow, often involving just the upper or lower half of vision, usually the area towards the nose. There is no pain. In approximately 6 months following the infarct visual acuity improves by 3 or more lines of vision on the Snellen Chart (the chart with smaller letters on each lower line) in 42.7% of patients. In addition, vision had worsened by 3 lines or more in 12.4% of patients. Second eye involvement occurs in approximately 15% to 20% of patients with NAION within 5 years. Fortunately, it may not be terribly devastating as the visual acuity may remain only moderately impaired. Furthermore, most cases of NAION involve the loss of a hemifield (either the upper or lower half of the visual field, but not both). A few cases of NAION involve almost total loss of vision.

Since arteritic AION is similar in presentation to non-arteritic AION, patients over the age of 50 diagnosed with NAION must be evaluated to exclude AAION (symptoms: painful jaw muscle spasms, scalp tenderness, unintentional weight loss, fatigue, myalgias and loss of appetite). Furthermore, NAION patients over the age of 75 should often be blood tested regardless.

Incidence
It is estimated that the incidence of AION is about 8,000/year in the U.S.

Causes and risk factors
The mechanism of injury for NAION used to be quite controversial. However, the experts in the field (neuro-ophthalmologists) have come to a consensus that most cases involve two main risk factors. The first is a predisposition in the form of a type of optic disc shape. The optic disc is where the axons from the retinal ganglion cells collect into the optic nerve. The optic nerve is the bundle of axons that carry the visual signals from the eye to the brain. This optic nerve must penetrate through the wall of the eye, and the hole to accommodate this is usually 20-30% larger than the nerve diameter. In some patients the optic nerve is nearly as large as the opening in the back of the eye, and the optic disc appears "crowded" when seen by ophthalmoscopy. A crowded disc is also referred to as a "disc at risk". While a risk factor, the vast majority of individuals with crowded discs do not experience NAION.

The second major risk factor involves more general cardiovascular risk factors. The most common are diabetes, hypertension and high cholesterol levels. While these factors predispose a patient to develop NAION, the most common precipitating factor is marked fall of blood pressure during sleep (nocturnal arterial hypotension)- that is why at least 75% of the patients first discover visual loss first on waking from sleep. These vascular risk factors lead to ischemia (poor blood supply) to a portion of the optic disc. The disc then swells, and in a crowded optic disc, this leads to compression and more ischemia.

Since both eyes tend to have a similar shape, the optometrist or ophthalmologist will look at the good eye to assess the anatomical predisposition. The unaffected eye has a 14.7% risk of NAION within five years.

A number of studies have linked Viagra use with NAION.

Treatment
Once NAION happens, it was thought that there was no accepted treatment to reverse the damage. However, a recent large study has shown that if patients are treated with large doses of corticosteroid therapy during the early stages of NAION, in eyes with initial visual acuity of 20/70 or worse, seen within 2 weeks of onset, there was visual acuity improvement in 70% in the treated group compared to 41% in the untreated group (odds ratio of improvement: 3.39; 95% CI:1.62, 7.11; p ¼ 0.001) (Graefes Arch. Clin. Exp. Ophthalmol 2008;246,:1029–1046.). That study and a natural history study on NAION (Ophthalmology 2008;115: 298–305.) showed that visual acuity can improve up to 6 months and not after that. To minimize the risk of further visual loss in the fellow eye or the same eye, it is essential to reduce the risk factors. Common sense dictates trying to control the cardiovascular risk factors for many reasons, including protection from this happening to the second eye. Sudden vision loss should lead to an ophthalmological consultation. If NAION is suspected, then ideally a neuro-ophthalmologist's consultation should be obtained.

There is much research currently underway looking at ways to protect the nerve (neuroprotection) or even regenerate new fibers within the optic nerve. There are no current clinical trials for the treatment of NAION. So far there is no evidence that the so-called neuroprotectors have any beneficial effect in NAION.

In addition to such research, patents have been applied for by Pfizer, The University of Southern California, Otsuka Pharmaceutical and other individual inventors for innovations related to the treatment of anterior ischemic optic neuropathy.