Anterior ischemic optic neuropathy overview

# 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.

# Causes
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.

# Epidemiology and Demographics
It is estimated that the incidence of AION is about 8,000/year or 2.55 out of every 100,000 people in the U.S.

# Risk Factors
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

## Secondary Prevention
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.

## Future or Investigational Therapies
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.