A 28-year-old woman presents with acute vision loss in the left eye over 48 hours, associated with periocular pain on eye movements. On examination, visual acuity is 6/36 in the affected eye, pupil is relatively afferent, and fundoscopy reveals hyperaemia and blurring of the optic disc margins with peripapillary flame haemorrhages. Intraocular pressure is normal. What is the most appropriate next step in management?