New neurological entities, either syndromes or diseases, continue to be described and defined in the 21st century. Neuromyelitis optica, postulated by some to be a variant of multiple sclerosis even in the late 20th century, has more recently been defined as a separate disease with its own pathogenesis, clinical description, diagnostic testing, and treatment.1
Reversible cerebral vasoconstriction syndrome (RCVS) has now been defined as a syndrome, but not a clear-cut disease entity.2 The syndrome typically presents with repeated thunderclap headaches over days to weeks. Cerebrovascular imaging reveals diffuse segmental constriction and dilatation of large and medium-sized intradural arteries. These abnormalities are transient and resolve within 3 months of clinical presentation. Convexity subarachnoid hemorrhage, intracerebral hemorrhage, seizures, and posterior reversible encephalopathy syndrome (PRES) occur in a minority of patients early, usually in the first week. Transient ischemic attacks and cerebral infarcts occur later.3 However, these complications are usually transient and resolve.2 Few patients are left with permanent sequelae. Results of cerebrospinal fluid testing are normal or show only a slight pleocytosis and elevated protein levels.