Endonasal endoscopic approach to the petrous apex: an image-guided quantitative anatomical study

The petrous apex is a relatively inaccessible region, deeply situated within the skull base. Removal of lesions from this area, traditionally accomplished via lateral approaches, can cause significant morbidity. We undertook an anatomical study to investigate the surgical anatomy of the petrous apex through an endonasal endoscopic approach, which has been sporadically described in the literature, to investigate its feasibility and to characterise clear and consistent surgical landmarks for access.

Cadaveric dissections were performed on five heads. Pre-dissection computed tomography scans were used, with the BrainLab navigation system, to verify entry into the petrous apex. Surgical landmarks were characterised in relation to fixed sphenoid sinus structures, and surgical access before and after drilling the sphenoid sinus rostrum was quantitatively compared.

The landmark for entry into the petrous apex was the intersection of a vertical line halfway between the medial surface of the internal carotid artery and the midline, with a horizontal line one-third of the way up from the postero-inferior floor of the sphenoid sinus. The dimensions of the postero-superior sphenoid sinus were characterised by the inter-carotid distance, pituitary-to-sphenoid-floor distance and the width of the sphenoid sinus floor, which were 15 +/- 3 mm, 16 +/- 3 mm and 26 +/- 1.6 mm respectively. The surface area of surgical access was 193 +/- 28 mm(2), increasing to 316 +/- 39 mm(2) after drilling of the sphenoid rostrum (P < 0.001; paired t-test).

Endoscopic approach to the petrous apex is anatomically feasible, and, aided by image navigation, could extend the scope of endonasal surgery to access highly-selected lesions in the middle cranial fossa.