Summary
Nonarteritic anterior ischaemic optic neuropathy (NAION) is a condition of the optic nerve that tends to affect the middle-aged and elderly, although people of any age may be affected. Patients present with sudden onset of either blurred central vision in one eye and/or blurring of peripheral vision. Part or all of the optic disc is observed to be swollen. Although only one eye is affected initially, the other may be involved at later date in about 40% of patients. Some improvement in vision may occur in about 40% of patients, usually within 6 months of presentation. No specific treatment is available.

Introduction
This is a condition resulting from an interruption to the blood supply to the optic nerve with a mean of age of onset of 61 years standard deviation 12 years ie 97% of patients present between the ages of 37 and 85. The incidence of AION is about 1600/year in the UK which means that we see about 20-30 patients per year with this condition in West Kent.

Patients present with sudden onset of either blurred vision in one eye or blurring in part of the visual field of one eye. About 50% of patients present with visual acuity of better or equal to 6/12, whereas about 25% present with vision of counting fingers. Both eyes are affected in some 40% of patients, usually sequentially and within a period of about 5 years.

Examination of the eye shows that part or all of the optic disc is swollen. In some cases, the papillomacular fibres supplying the fovea are not involved at all, which explains the normal visual acuity of many eyes. To fully evaluate the effect on vision, therefore, both visual acuity and visual field need to be examined. The most common visual field anomaly is combination of relative inferior altitudinal defect with an absolute inferior nasal defect.

NAION needs to be distinguished from arteritic ION, which is due to giant cell arteritis, an inflammatory disease of medium-sized blood vessels.  The latter occurs in a slightly older age group and generally causes complete loss of vision, whereas the visual loss can be relatively mild in patients with NAION.

Aetiology
The primary event in NAION is ischaemia or hypoxia of the optic nerve. In patients with a small scleral opening for the optic nerve, a vicious cycle of swelling of the nerve cells (axoplasmic flow stasis), capillary compression, further axoplasmic stasis leads to visual loss. Most cases of NAION occur in patients with a small opening in the sclera at the back of the eye for the optic nerve. This hole is usually about 30% larger than the optic nerve, but in patients with NAION, is often less than 10% larger. This means that the blood vessels supplying the optic nerve (short posterior ciliary arteries) get squeezed, reducing the supply of blood to the nerve. We should emphasise that most patients with small optic discs do not develop NAION. Most patients do, however, have several cardiovascular risk factors, such as high blood pressure or diabetes, which in patients with small optic discs, increase the risk of NIAON. Some studies have linked the use of Viagra with NAION. It was previously thought that patients with hypermetropia (long eyesight) were more likely to have small optic canals and therefore small cup diameter:disc diameter (CD) ratios. However, it has now been shown that higher degrees of both myopia and hypermetropia have larger CD ratios – thus hypermetropia is not a risk factor for NAION.

In summary, the sequence of events in NAION is probably:-
i) optic nerve head ischaemia or hypoxia causes axoplasmic flow stasis
ii) the swollen axons, if in a restricted space, compress the capillaries causing secondary vascular changes
iii) nocturnal arterial hypotension further reduces blood flow, which is why visual loss is most often first noted on awakening.

Associated conditions
Systemic hypertension (45%)
Ischaemic heart disease (20%)
Diabetes (33%)
Raised cholesterol (70%)
Current or previous smoker (50%)
from Hayreh and Zimmerman Ophthalmology 2008;115:298-305

Presenting features
Patients usually presents with painless blurring of vision on awakening.  They might also note a shadow in the top or bottom half of their vision due a visual field defect. Most cases of NAION involve the loss of either the upper or lower half of the visual field.

Investigations
We need to exclude arteritic ischaemic optic neuropathy (AION) by means of clinical history (absence of jaw claudication – pain in the jaw on eating, headaches, tenderness over the temple).  Specific tests include:-

i) ESR and CRP – these are markers of inflammation and are raised in temporal arteritis and AION
ii) Very occasionally a temporal artery biopsy to exclude temporal arteritis
iii) A visual field test to assess your peripheral vision
iv) Blood pressure
v) 24 hour blood pressure to exclude nocturnal low blood pressure
v) Fasting blood sugar to exclude diabetes
vi) Very rarely, sleep studies to exclude sleep apnoea syndrome.

Treatment
Unfortunately, there is no treatment for this condition. We usually treat underlying conditions which might predispose to the condition such as high blood pressure, low nocturnal blood pressure, diabetes and sleep apnoea syndrome.

Prognosis
i) Presenting with vision better or equal to 6/21
Improvement 41%
Stable 40%
Deterioration 19%

iii) Presenting with moderate or severe visual loss (eg vision worse than 6/21)
Improvement 26%
Stable 59%
Deterioration in 15%

Improvement tends to occur early, with most visual field change in the first 2 weeks and lesser change up to 6 months. After this time, there is relatively little change. Frequently, patients don’t acutally get to see an ophthalmologist until after 2 weeks, when the changes in vision become less marked. Some of the improvement in visual acuity noted is due to the patient using eccentric fixation rather than their being a genuine improvement in visual field. Real improvement in optic disc function is always reflected in improvement in central visual field as well as visual acuity.
When the second eye develops NAION, the first affected eye may paradoxically improve, if the vision in the second eye is worse. This may be because the first eye has developed a degree of amblyopia because of misuse. When the second eye is involved, the first eye has to start being used again from Hayreh and Zimmerman Ophthalmology 2008;115:298-305

The shape of the optic disc and thickness of the retinal nerve layer after NIAON was studied using HRT and GDx respectively (Saito et al. Ophthalmology 2008;115:1585-1590). These authors demonstrated significant cupping of the optic disc post NAION in affected eyes compared to the patients’ unaffected eyes. As expected, the disc size of affected eyes were significantly smaller than age- and refraction-matched normal controls.