Reversible cerebral vasoconstriction syndrome (RCVS): RCVS encompasses a diverse group of conditions characterized by the acute onset of severe headaches associated with reversible segmental constriction of multiple cerebral arteries. Importantly, the underlying pathophysiology is reversible vasoconstriction rather than vasculitis. Of note, it is not possible to distinguish between RCVS and CNS vasculitis based on imaging. However, several clinical features are helpful in making the diagnosis. This is essential, as treatment and prognosis of RCVS and CNS vasculitis are quite different. The onset of RCVS is acute with thunderclap headache. However, the severe pain is short lived, lasting from a few minutes to a few hours. Attacks can be single, but patients have a mean of four attacks during a period of approximately 1–4 weeks. A self-limiting, benign outcome is the norm, and resolution is generally within 3 months. CSF analysis is usual normal or near normal. The neuroimaging hallmark of RCVS is vasoconstriction evident as segmental narrowing and dilatation (string of beads) of one or more arteries (Fig. 6.4). The appearance is similar to that of other forms of vasculitis, except the abnormalities resolve within a few months. Bilateral and diffuse anterior and posterior circulation changes are seen in the majority of patients. Although concomitant changes need not be present, subarachnoid hemorrhage, parenchymal edema, parenchymal hemorrhage, and infarction may occur. Of note, RCVS may be accompanied by posterior reversible encephalopathy syndrome (PRES), as these are not mutually exclusive. RCVS patients are treated with nimodipine or verapamil until resolution of angiographic findings.