Anterior ischemic optic neuropathy (AION) is a medical condition 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.

[edit] Introduction

The distinction between AAION and NAION 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 big-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.^[1]

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).^[2]

[edit] 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 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^[3]. 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.

[edit] Incidence

It is estimated that the incidence of AION is about 8,000/year in the U.S.^[4]

[edit] 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 called the cup to disc ratio. Hence there is extra space that acts as a margin of error. But some patients have no such margin, . Their optic disc appears "crowded" when seen by ophthalmoscopy. Nonetheless, most patients with this optic disc shape see well all of their lives.

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 (the "last straw") is marked fall of blood pressure during sleep (nocturnal arterial hypotenison)- 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, but in optic disc without a cup or a small cup, the crowded conditions don't allow space for this, so compression occurs and this leads to 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.

A number of studies have linked Viagra use with NAION.^{[5][6][7][8][9][10]}

[edit] 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.^{[11][12][13][14][15]} There are no current clinical trials for the treatment of NAION. So far we no evidence that the so-called neuroprotectors have any beneficial effect in NAION.

[edit] References

[edit] External links

• Online handbook of Ocular Disease Management at revoptom.com
• Department of Ophthalmology & Visual Sciences at University of Iowa
• Polymyalgia rheumatica article from National Institute of Arthritis and Musculoskeletal and Skin Diseases

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Anterior ischemic optic neuropathy".