Endoscopic Anatomy and Approach of the Cavernous Sinus

Background: Conventional approaches to the superior orbital fissure are usually represented by pterional modified, fronto-orbital and fronto-orbitozygomatic, and transfacial approaches.

Because, the expanded endoscopic endonasal approach is currently a valid minimally invasive alternative in the armamentarium of cranial base surgeon, and can be use for the treatment of midline and lateral expanded lesions, a detailed knowledge of the anatomy of this region is crucial.

Methods: Eight cadaveric specimens were prepared and anatomical dissections were performed using a rod lens endoscopes.

Results: To expose the lateral sphenoidal sinus with the expanded endonasal approach an extended midline approach must be undertaken. This includes the middle meatal route, in a combined endoscopic transsphenoidal-transethmoidal approach. The technique uses the maxillary sinus and the pterygopalatine fossa as part of the working corridor. Infraorbital/V2 and the vidian neurovascular bundles are used as landmarks and allow identification of the Mecke's cave The medial wall of the cavernous sinus has two parts: sellar and sphenoidal. The sellar part is a thin sheet that separates the pituitary fossa from the venous spaces in the cavernous sinus. The sphenoidal part is formed by the dura lining the carotid sulcus on the body of the sphenoid bone.

From our observations, the abducens nerve into the carvernous sinus can be subdivided into 2 subsegments: a proximal or paracarotid and a distal or infracarotid. In the smaller proximal cavernous or paracarotid subsegment, the nerve travels lateral and almost horizontal and perpendicular to the posteriorvertical segment of the intracavernous ICA along an anatomic triangular region bordered medially by the cavernous carotid, posteriorly by the posterior petroclinoidal fold, and lateroinferiorly by the petrolingual ligament. This particular portion of the nerve is not visible through an endonasal approach unless medial mobilization of the internal carotid artery is performed during a cavernous sinus or Meckel's cave approach.

The distal cavernous or infracarotid subsegment starts as soon as the nerve “emerges” from crossing the posterior vertical segment of the cavernous ICA. At this point the abducens nerve is firmly attached to the sympathetic fibers from the ICA and runs under the horizontal segment of the cavernous ICA.

The abducens nerve at the proximal cavernous or paracarotid segment is “protected” by the dural fold around the cavernous ICA itself. Therefore, damage to this particular segment is less likely because there is not direct exposure of this particular segment. Injury to the abducens nerve at the anterior or distal portion occurs not just from direct injury but also from a lesion at the carotid sympathetic nerve, just at the intersection with the abducens nerve at the apex of the anteromedial triangle of the cavernous sinus.

Conclusions: Expanded Endoscopic endonasal approach provides an important corridor for lesions located in the medial wall of the cavernous sinus. The abducens nerve is particularly at risk during endonasal endoscopic skull base approach. Anatomic landmarks to localize the abducens nerve intraoperatively such as V2 for Meckel's cave approach, are reliable and complementary to the use of intraoperative electrophysiological monitoring.