# Superior and Inferior Petrosal Sinuses

> Superior and inferior petrosal sinus anatomy for MBBS Anatomy: drainage of cavernous sinus, Dorello's canal, basilar plexus and Gradenigo syndrome explained.

- Canonical URL: https://prepelephant.com/topics/mbbs/anatomy/petrosal-sinuses
- Exam / course: MBBS · Subject: Anatomy
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Superior and Inferior Petrosal Sinuses", PrepElephant, https://prepelephant.com/topics/mbbs/anatomy/petrosal-sinuses

## Direct answer

Two paired channels drain the cavernous sinus backwards towards the jugular foramen: the superior petrosal sinus runs along the upper border of the petrous temporal bone in the attached edge of the tentorium to join the transverse–sigmoid junction, while the inferior petrosal sinus runs in the groove between the petrous temporal bone and the clivus to open into the superior bulb of the internal jugular vein. The abducent nerve reaches the cavernous sinus through the inferior petrosal sinus in Dorello's canal, and the trigeminal root lies beneath the tentorial edge near the superior petrosal sinus. Petrous apicitis irritates both relations, producing the Gradenigo triad loved by exam setters.

## What you must remember

- The superior petrosal sinus begins at the posterior end of the cavernous sinus, runs along the superior border of the petrous temporal bone in the tentorial attachment, and drains into the junction of transverse and sigmoid sinuses.
- Its terminal part crosses the root of the trigeminal nerve, which indents the tentorial dura just below it — the landmark for Meckel's cave.
- The inferior petrosal sinus runs in the petro-occipital groove (between petrous temporal bone and clivus), and leaves the skull through the anteromedial compartment of the jugular foramen with the ninth, tenth and eleventh nerves.
- The abducent nerve pierces the dura and passes beneath the petroclinoid ligament (Gruber's ligament) through Dorello's canal, in company with the inferior petrosal sinus, before entering the cavernous sinus.
- The basilar plexus lies on the clivus, interconnects the two inferior petrosal sinuses and communicates with the vertebral venous plexus — a route for spread of pelvic malignancy to the skull base.
- The inferior petrosal sinus drains the internal acoustic meatus veins, cerebellar veins and the cavernous sinus.
- Gradenigo syndrome: diplopia from sixth nerve palsy, retro-orbital or frontal pain from the trigeminal ganglion, and ear discharge from petrous apicitis.

## A typical case that stitches these relations together

A twelve-year-old with a discharging ear and mastoiditis develops severe pain behind the right eye and, three days later, double vision on looking to the right. Each sign is a structure irritated at the petrous apex. Pneumatisation of the petrous apex becomes infected (petrous apicitis); the inflammatory exudate reaches the dura at the petroclinoid ligament, where the abducent nerve crosses with the inferior petrosal sinus in Dorello's canal, and the nerve fails — the affected eye will not abduct, and an esotropia appears. Immediately posterior and superior lies the trigeminal ganglion in Meckel's cave under the tentorial edge beside the superior petrosal sinus, so the forehead and cheek burn with pain referred along V1 and V2.

Work the pathway further as an examiner would. If infection tracks along the inferior petrosal sinus itself, thrombosis of the cavernous sinus follows — chemosis, proptosis and ophthalmoplegia — because the petrosal sinuses are the cavernous sinus's principal outlets. Across the clivus, the basilar plexus links both inferior petrosal sinuses and continues through the foramen magnum into the vertebral venous plexus, which has no valves; this is the anatomical licence for retrograde spread from pelvic veins to the cranial cavity. Management of the case is antibiotic therapy directed at the otitis with mastoid drainage per current otolaryngology practice, the anatomy dictating which nerve will recover last.

## Where students slip

Candidates routinely attach Dorello's canal to the wrong nerve, crediting it to the trochlear or the trigeminal; it belongs to the abducent nerve with the inferior petrosal sinus, and this pairing is a standard one-mark question. The second error is making the superior petrosal sinus "drain into the cavernous sinus" — flow runs the other way, from cavernous sinus backwards to the sigmoid system, which is why cavernous sinus thrombosis can propagate into it. Third, students forget that the inferior petrosal sinus exits through the jugular foramen even though it is not, strictly, "in" the pars vascularis with the sigmoid bulb; it passes anteromedial to the ninth, tenth and eleventh nerves, and this compartment detail separates a good answer from a passing one.

## Frequently asked questions

### Which sinus accompanies the abducent nerve in Dorello's canal?
The inferior petrosal sinus; the abducent nerve passes under Gruber's petroclinoid ligament with it before entering the cavernous sinus.

### Where does the superior petrosal sinus drain?
Into the junction of the transverse and sigmoid sinuses, after crossing the root of the trigeminal nerve near Meckel's cave.

### What are the components of Gradenigo syndrome?
Diplopia due to abducent palsy, retro-orbital trigeminal pain and otorrhoea — the triad of petrous apicitis.

### How can pelvic infection or malignancy reach the skull base?
Through the valveless basilar plexus and vertebral venous plexus, which connect the pelvic veins to the inferior petrosal sinuses on the clivus.

### Through which part of the jugular foramen does the inferior petrosal sinus leave the skull?
The anteromedial compartment, in front of the ninth, tenth and eleventh cranial nerves.