The vast majority of reported series of IAD patients come from Asian countries and IAD affects the posterior circulation more frequently than the anterior circulation in these series. This contrasts with CeAD and saccular intracranial aneurysms, which most commonly affect the anterior circulation. Due to treatment and publication bias little is known about the natural history of IAD. Overall, IAD is assumed to have a more severe course than CeAD, with a more ominous outcome in patients with SAH IAD than in patients with non-SAH IAD. Furthermore, little information is available on the risk of recurrent IAD as well as risk for recurrent ischaemic and haemorrhagic events. Radiological diagnosis of IAD can be challenging given the small size of intracranial arteries, and the subtle and non-specific radiological signs which tend to evolve with time.