Anterior ischemic optic neuropathy

Overview
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.

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 medium-sized blood vessels 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 "AION" henceforth will be used to refer to non-arteritic anterior ischemic optic neuropathy.

Differential Diagnosis

 * Central Retinal Vein Occlusion
 * Ocular Hypotony
 * Papilledema
 * Pseudopapilledema

Symptoms and diagnosis
AION 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. There is no pain. There may be a slight improvement of visual acuity over the course of the next month, but generally, there is little change. There is a serious risk of a similar event occurring in the fellow eye over the next few years. Fortunately, it may not be terribly devastating as the visual acuity may remain only moderately impaired. Furthermore, most cases of AION involve the loss of a hemifield (either the upper or lower half of the visual field, but not both). A few cases of AION 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 AION must be evaluated to exclude AAION (symptoms: painful jaw muscle spasms, scalp tenderness, unintentional weight loss, fatigue, myalgias and loss of appetite). Furthermore, AION 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 AION used to be quite controversial. However, the experts in the field (neuro-ophthalmologists) have come to a consensus that most cases involve the convergence of two problems. The first is a predisposition in the form of a type of optic disc shape. The optic disc is the most anterior (forward) portion of the optic nerve, the bundle of nerves that carries 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. 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 "hit" involves cardiovascular risk factors. The most common are diabetes, hypertension and high cholesterol levels. In patients with "a disc at risk", these vascular risk factors lead to ischemia (poor blood supply) to a portion of the optic disc. The disc then swells, but 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 ophthalmologist will look at the good eye to assess the anatomical predisposition.

There is evidence that genetic factors may play a role in NAION. (see the OMIM link)

A number of studies have linked Viagra use with NAION.

Treatment
Once AION happens, there is no accepted treatment to reverse the damage, but prevention of further damage may be possible. 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 AION is suspected, then ideally a neuro-ophthalmology consultation should be obtained. Some rare causes of AION are treatable.

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 AION.