Papilloedema refers to bilateral swelling of the optic nerve head (where the optic nerve enters the eye to transmit visual information to the brain) caused by raised intracranial pressure and is an important clinical sign that may mean that a patient has a life-threatening condition such as a brain tumour, or that they are at risk of losing vision. An unknown proportion of the population have optic nerve heads that look swollen but are not – termed pseudo-papilloedema. It is not known how many people have pseudo-papilloedema and it is therefore not possible to say how useful papilloedema is as a clinical sign in all cases.