The sigmoid sinus is the S-shaped continuation of the transverse sinus as it descends behind the mastoid process and the posterior petrous temporal bone, passing anterior to the mastoid air cells before exiting the skull through the jugular foramen to become the internal jugular vein. It lies immediately deep to the mastoid cortex, in close relationship to the facial nerve canal and the posterior semicircular canal.
The sigmoid sinus is the critical vascular landmark in mastoid and posterior petrous surgery. During cortical mastoidectomy, the posterior limit of dissection is the sigmoid sinus, which is identified by its characteristic blue colour through thin bone or by unroofing the overlying cortex carefully. Skeletonisation of the sinus provides access to the posterior fossa for the translabyrinthine approach to the internal auditory canal. Sigmoid sinus thrombosis from mastoiditis was historically called otitic hydrocephalus. Inadvertent sinus entry during mastoidectomy requires immediate packing and haemostatic control.
Inadvertent entry into the sigmoid sinus during mastoidectomy or cochlear implant surgery causes profuse venous haemorrhage managed by immediate bone wax packing and surgicel compression; uncontrolled injury may require intracranial venous repair.
Congenital absence of bone overlying the sigmoid sinus, a normal anatomical variant in up to 10% of individuals, produces a pulsatile tinnitus that is a useful diagnostic clue when planning mastoid surgery; CT scanning before operation identifies the variant and prevents inadvertent sinus injury.
