Review
Non-arteritic anterior ischaemic optic neuropathy: A review and update

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Abstract

Non-arteritic anterior ischaemic optic neuropathy (NAION) is the most common acute optic neuropathy in people aged 50 years and older. The condition is caused by infarction of the laminar or retrolaminar portion of the optic nerve head supplied by the short posterior ciliary arteries (SPCAs). The underlying aetiology and pathophysiology is poorly elucidated. Factors that have been implicated include nocturnal hypotension, impaired autoregulation of the microvascular supply, vasculopathic occlusion, and venous insufficiency. These factors are thought to result in axonal oedema causing a compartment syndrome in a structurally crowded optic disc leading to axonal degeneration and loss of retinal ganglion cells via apoptosis. Clinically NAION is characterised by sudden, usually painless, loss of vision in one or both eyes. Examination findings include decreased visual acuity, a visual field defect, decreased colour vision, a relative afferent pupillary defect, and optic disc swelling. Despite significant research, treatment options for NAION remain limited.

Introduction

Non-arteritic anterior ischaemic optic neuropathy (NAION) is an important cause of acute visual loss in the middle-aged and elderly populations. However, many aspects of this disease remain poorly understood. In particular the exact pathophysiology leading to infarction of the optic nerve head remains to be elucidated. Furthermore the efficacy of medical and surgical treatments for NAION are yet to be proven. The purpose of this review is to synthesise current knowledge on the clinical presentation, risk factors, pathophysiology, and management of NAION.

Section snippets

Clinical features

The clinical picture may vary significantly from the classic presentation of sudden, painless, unilateral visual loss, frequently noticed on awakening from sleep (Table 1). It is now recognised that approximately 12% of patients experience ocular pain or headache which complicates the differentiation of NAION from optic neuritis.1 In over two-thirds of patients visual loss is noticed on awakening from sleep, or at the first opportunity to use vision critically after sleeping, suggesting that

Epidemiology

NAION is the most common acute optic neuropathy in people aged 50 years and over. The condition is estimated to affect between 2.3 and 10.3 people per 100 000 individuals per year.7, 8 Affected individuals are typically between 60 and 70 years of age although the condition is not uncommon in younger patients.9 In a retrospective study 23% of patients with NAION were under 50 years of age.10 NAION is predominately a disease of Caucasian people who account for nearly 95% of cases.

Small cup-to-disc ratio

Approximately 97% of people who develop NAION have a small optic disc (less than 1.2 mm) with a small or absent physiological cup (cup-to-disc disc ratio  0.2), a so-called “disc-at-risk”.11, 12, 13, 14, 15, 16 It is believed that crowding at the level of the lamina cribrosa predisposes to a compartment syndrome phenomenon whereby axoplasmic stasis and oedema following a microvascular ischaemic event leads to further ischaemia through compression of capillaries among nerve fibre bundles due to

Pathophysiology

The exact pathophysiology of NAION is unclear and highly controversial. It is generally accepted that the site of the infarction is located in the retrolaminar portion of the optic nerve head which is supplied by the short posterior ciliary arteries (SPCAs) as this has been demonstrated histologically.48 Doppler studies have also corroborated these findings by showing reduced blood flow in the SPCAs.49 The location of impaired blood flow has been further refined by fluorescein and indocyanine

Arteritic anterior ischaemic optic neuropathy

It is important that NAION is differentiated from AAION (Table 3) due to the propensity of the latter to cause profound bilateral visual loss without expeditious systemic corticosteroid treatment. Unlike NAION, which is not uncommon in patients under the age of 60 years, AAION almost exclusively affects patients 60 years of age or older. AAION typically presents with sudden, profound, visual loss and may become bilateral over hours to days; usually in the setting of delayed diagnosis. In AAION

Investigations

The diagnosis of NAION is predominately clinical and no specific investigations are required to confirm the diagnosis. However, in younger patients it can be difficult to differentiate the 12% of patients with NAION who experience pain from patients with optic neuritis. In this situation MRI can be helpful as the optic nerve is frequently normal with NAION in contrast to optic neuritis. In one study optic nerve abnormalities were present on MRI in 31 of 32 patients with optic neuritis compared

Management

Currently there is no effective treatment for NAION. Optic nerve decompression surgery has been investigated however the results have been disappointing. The IONDT study was a randomized, single-masked, multicenter trial that investigated the safety and efficacy of optic nerve decompression surgery compared with careful follow-up alone in patients with NAION. While the study was underpowered to determine whether progressive NAION benefited from such intervention, it did establish that the

Prognosis

Over 6 to 12 weeks the optic disc swelling subsides leaving a pale and atrophic optic disc. Unlike AAION there is no significant enlargement of the cup-to-disc ratio.86 In a longitudinal study of the natural history of NAION there was improvement in 41% of patients who had a visual acuity of 6/21 or better at presentation.4 The prognosis for visual recovery is better for patients less than 50 years of age.10 Although it should be remembered that apparent improvements in visual acuity may simply

Conclusion

NAION is an important cause of acute visual loss in the middle-aged and elderly populations. Multiple medical and surgical treatments have been investigated; however to date the results have been disappointing primarily because of the lack of understanding of the underlying pathophysiology. Application of improved neuroimaging modalities may allow a greater understanding into the underlying pathophysiology of NAION. Further areas of research should focus on the mechanisms of neurodegeneration

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