Variability of the C3-C4 Transitional Area of the Internal Carotid Artery: Transcranial and Endonasal Prosection with Neuroradiological and Histological Correlation

Background: Discussion continues about the segmentation and nomenclature of the intracranial internal carotid artery (ICA). Using the 1996 schema that defined the C1-C7 segments of the ICA, our study focuses on the transition of the lacerum (C3) and cavernous (C4) segments, including their meningeal relationships and Meckel's cave (MC). Using embryological, histological, microsurgical, and endoscopic analysis, we sought to better define the transition between the lacerum and cavernous segments, and address the controversy of another possible segment between C3 and C4.

Materials and Methods: In 5 adult cadaveric formalin-fixed heads injected with colored silicone, bilateral transcranial extradural and endonasal endoscopic CT-guided dissections were performed. Transcranially, the trigeminal root at the level of the porus trigeminus was sectioned and reflected anteriorly to expose cranial nerve (CN) VI, C3-C4 transition, sympathetic fibers, and petrolingual ligament (PLL). We defined a quadrilateral area medial to MC between CN VI, anterolateral and posterolateral borders of the ICA, and PLL. Endoscopically, the area was exposed through the “door” between MC and ICA using a 4-mm endoscope with 0°, 30°, and 45° angled-rod lenses. Measurements were made in situ through both approaches. Anatomical correlations were made with coronal histological sections of the sellar region (6-μm thick; hematoxylin-eosin and Masson's-trichrome stained) and neuroradiological images. In 64 dry skulls, we drew an arbitrary line in the PLL region, measuring from the lingula sphenoidalis to the anterior process of the petrosal bone (petrolingual line), and determined angulation relative to the Frankfurt horizontal line.

Results: In 70% of cases (7 sides), venous channels were absent on the dorsal aspect of C4 (subjacent to MC, inferior to CN VI) at the quadrilateral area. Meningeal attachments from MC were consistently observed and dissected from the PLL and ICA. Measurements differed slightly between endoscopic and transcranial perspectives. Angulation of the petrolingual line varied (i.e., opened dorsally 0° to 30° in 49% of specimens).

Conclusion: Clear understanding of the complex microsurgical anatomy of the C3-C4 transitional area and its consistently identifiable landmarks are important because of high risk of vascular and neural injury during transcranial and endoscopic surgeries. Absence of venous structures on the dorsal aspect of C4 suggested variability in the anatomical origin of the CS related to the inconsistency of MC and cranial venous system development. Moreover, variable angulation of the PLL inherently begets variability of the C3-C4 boundary. Our results suggest that the PLL does not constitute a fixed demarcation between the C3 and C4 segments of the ICA. Rather, variations in venous anatomy may determine this boundary and account for variability among studies.