The location of the sigmoid sinus within the mastoid cavity can be variable. An anteriorly displaced vertical segment of the sigmoid sinus constitutes an uncommon but potentially dangerous anatomical variation that surgeons can encounter during surgery. In this, the sigmoid sinus lays underneath a very thin bony flap, which can be easily damaged. Thus, an abrupt bleeding might occur. We present a case where the vertical segment of the left sigmoid sinus was encountered just underneath the posterior wall of the external auditory canal during evaluation of a case of COM. Anatomical variations of the sigmoid sinus are not uncommon, and the otolaryngologist should be aware of such variations to prevent intra- operative surprises.

Keywords: Sigmoid sinus; Abnormally displaced; Anterior course; Anatomical variation.