Hyperintense vessel sign on magnetic resonance imaging at the time of presentation in reversible cerebral vasoconstriction syndrome

•Reversible cerebral vasoconstriction syndrome (RCVS) can cause thunderclap headache.•RCVS often lacks abnormality on initial imaging studies.•RCVS can present only with hyperintense vessel sign on initial imaging.•Hyperintense vessel sign is useful for early diagnosis of RCVS.•Ischemic complications should be noted in RCVS with hyperintense vessel sign.