What structure is pointed to by the white arrow?
Abducens nerve (cranial nerve VI).
What canal does it enter superiorly, before passing into the cavernous sinus?
What are the margins of this canal?
Superiorly it is bounded by the petrosphenoidal ligament (Gruber's ligament). Inferiorly, it is bound by a bony notch
What is Gradenigo's syndrome?
Gradenigo syndrome consists of the triad of : 1) petrous apicitis, with 2) abducens nerve palsy, secondary to involvement of the nerve as it passes through Dorello's canal, and 3) retro-orbital pain, or pain in the cutaneous distribution of the trigeminal nerve, due to extension of inflammation into Meckel's cave.
Annotated high resolution FIESTA sequence through the medulla and pons demonstrates the normal course of the abducens nerve (CN VI) (white arrow) as it ascends from the ponto-medullary junction to Dorello's canal.