Ischaemic optic neuropathy

Ischaemic optic neuropathy (ION) is due to infarction of the optic nerve and presents with sudden painless persistent loss of visual acuity, a visual field defect that is classically altitudinal or arcuate but which may be central, and an afferent pupil defect.56 There is no significant sex predilection and the average age ranges from 57 to 69 years, with a peak range of 60-69 years.57

The critical regions of damage in both non-arteritic and arteritic anterior ischaemic optic neuropathy (AION) are the prelaminar, laminar, and immediate retrolaminar portions of the optic nerve which receive their blood supply from terminal branches of the ophthalmic artery, the lateral and medial posterior ciliary arteries, and from the pial circulation. In AION the ischaemic microvascular process may be solely or partially retrolaminar.58 Lessell59 suggests that the border zone that determines the site of infarction is between the lamina cribrosa and the centripetal branches of the pial vessels in the retrobulbar segment of the optic nerve. Besides vascular anatomical considerations, the best documented anatomical correlation is with cup-disc ratios. The discs at risk60 have small cup-disc ratios61 and cross-sectional areas,62 which imply that the optic nerve fibres are crowded within the scleral canal. This crowding is probably a contributing factor in the pathogenesis of non-arteritic AION.63 A small cup-disc ratio

may be a risk factor for development of AION in association with the use of sildenafil citrate.64

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