Aneurysmal rupture and secondary vasospasm: A difficult diagnostic dilemma may arise when an intracranial aneurysm is identified on angiography, despite the changes being attributable to RCVS. In this situation, the distribution and extent of subarachnoid hemorrhage and the location of cerebral vasospasm are invaluable in ascertaining the diagnosis. In general, subarachnoid hemorrhage is associated with RCVS and basal subarachnoid hemorrhage with aneurysmal rupture. Subarachnoid hemorrhage could be associated with ruptured distal mycotic aneurysm; however, there is usually a history of intravenous drug use or infective endocarditis that predisposes to septic emboli.

CNS vasculitis: This may be primary in etiology, in which the arteritis is confined to the CNS vessels, or secondary, where there is a history of systemic vasculitis, most often systemic lupus erythematosus (SLE). In the latter situation, CNS involvement is unlikely to be the first manifestation of the disease, as there is almost always a history of systemic vasculitis. While the radiological features of primary CNS vasculitis can overlap with RCVS, the clinical onset of CNS vasculitis is more insidious, the headache is chronic and progressive, and there are usually accompanying cerebrospinal fluid abnormalities. Multiple ischemic lesions can be encountered on MRI (Figs. 10.2 and 10.3).