Differential diagnosis of optic neuropathies can be challenging but is crucial for the correct management of the underlying disease and prevention of further damage. Bruch’s membrane opening (BMO) was recently discovered as the anatomical border of the optic disc. BMO minimum rim width (BM-MRW) seems to be a reliable representation of the neuro-retinal rim. The authors present a retrospective observational study whose purpose was to evaluate the discriminative capability of BMO-MRW in different optic neuropathies and to access the diagnostic ability of a new index, based on the relationship between MRW and peripapillary retinal nerve fibre layer, for these different optic neuropathies. Results demonstrate the ability of BMO-MRW to differentiate between glaucomatous and non-glaucomatous optic neuropathies. In addition, the authors propose an MRW ratio, which may allow discrimination of open angle glaucoma from either non-arteritic anterior ischaemic optic neuropathy or compressive optic neuropathy.