A 28-year-old man is referred to the neuro-ophthalmology clinic with a 2-week history of progressive bilateral blurred vision and headaches. On examination, both optic discs are swollen with obscured margins, and the swelling is more pronounced at the superior and inferior poles. Visual acuity is preserved at 6/6 bilaterally. Fundoscopy shows no haemorrhages or exudates. Which investigation is most appropriate to differentiate papilloedema from bilateral papillitis and identify the underlying cause?