Ischaemic optic neuropathy (ION) is usually quite easy to distinguish from optic neuritis clinically, if their presentations are typical. However, the distinction can be blurred in atypical cases. The diagnosis of optic neuritis can usually be confirmed on MRI (95% of acute optic neuritis cases show gadolinium enhancement on MRI). On the other hand, traditionally, there is no single diagnostic test for ION, particularly the non-arteritic type. ION is usually a clinical diagnosis, and sometimes a diagnosis of exclusion. In this article, the authors reviewed the basic principles behind MRI, particularly of diffusion MRI, and proposed that it can aide the diagnosis of ION. The signal intensity of diffusion MRI reflects the diffusion of water molecules within a tissue, and the instrument generates a map of "apparent diffusion coefficient" (ADC). Acute ischaemic brain injuries, including those of the optic nerve, are characterised by an increased (bright) diffusion-weighted image (DWI) signal and decreased (dark) ADC value, as early as 2 days after the insult. In contrast, optic neuritis causes an increase in ADC value. However, currently the clinical use of DWI in the diagnosis of ION is limited due to the relatively lower resolution of DWI, the small size of the optic nerve, possible eye movements during scanning, and high signal from the surrounding cerebrospinal fluid and orbital fat.
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