Evaluation of Optic Nerve Morphology in Non-glaucomatous Optic Neuropathies with Quantitative Optic Nerve Imaging Modalities

Abstract

Purpose Non-glaucomatous optic neuropathies produce patterns of injury to the optic nerve head that have not been studied objectively because of the lack of quantitative imaging modalities. This related series of studies aimed to explore the role of new quantitative optic nerve imaging technologies (optical coherence tomography, OCT; Heidelberg Retinal Tomography, HRT; and scanning laser polarimetry, GDx) in non-glaucomatous optic neuropathies focusing on ischaemic and compressive optic neuropathies and to evaluate whether the technologies may identify characteristic structural changes to the optic nerve head and whether these changes correlate with the degree of visual field damage. Methods Subjects with anterior ischaemic optic neuropathy (AION) (both non-arteritic anterior ischaemic optic neuropathy (NAION) and arteritic anterior ischaemic optic neuropathy (AAION)), glaucoma, and compressive optic neuropathy were recruited. Subjects were assessed with complete neuro-ophthalmic assessment, visual field sensitivity (Humphrey visual field testing), OCT, HRT, and GDx. Structure-function correlations were calculated using retinal nerve fibre layer thickness (RNFL), macular thickness, optic nerve head morphology measures and visual field sensitivity. Results The ocular imaging modalities of OCT, HRT, and GDx are able to globally and sectorally identify a significant difference in AION eyes compared with controls. RNFL thickness correlates strongly with the extent and pattern of visual filed loss. The optic nerve head morphology in glaucoma is quantitatively distinct from both AAION and NAION with glaucoma producing larger, deeper cups, smaller rims, more cup volume, and less rim volume. OCT macular parameters also show robust correlation with visual field sensitivity. For patients who have had a prior parachiasmal lesion treated with surgery, the OCT RNFL correlates with the residual visual field defect. Correlations improved significantly with a longer post-operative course. Thinner preoperative RNFL thickness is associated with worse visual acuity and visual field outcome. Conclusions Quantitative imaging modalities are useful for identifying the extent and pattern of injury in non-glaucomatous optic neuropathies and may make a significant contribution to the preoperative assessment of patients with compressive lesions.