Adventitial Adversity: Histopathology of the Distal Dural Ring

Introduction: The distal dural ring (DDR) is a well-studied anatomic structure of significant neurosurgical relevance, defined as the point of entry of the internal carotid artery (ICA) into the subarachnoid space. The relationships between the DDR and the surrounding structures have been the subject of some controversy, including debate over whether the ICA can safely be separated from the DDR. These relationships have been subject to multiple gross anatomic investigations, but the only preceding histologic study did not specifically evaluate the relationship between the ICA and the DDR.

Methods: Ten consecutive specimens donated for anatomic study with no known intracranial or cerebrovascular pathology were included in the study. Specimen harvest was completed via bilateral frontotemporal craniotomy, extradural anterior clinoidectomy, and ligation of the ICA at its intracavernous and intradural segments. A wide dural margin was cut at the level of the DDR, and specimens were fixed in formalin and stained with hematoxylin and eosin (H&E), as well as the Masson trichrome stain. Histopathologic relationships between the arterial and dural layers were examined and qualitatively assessed.

Results: In all 10 specimens, extensive microscopic investment by the DDR of the underlying ICA adventitia was observed, and a clear plane could not be defined between them. Low power (20×) H&E and Masson trichrome histopathology of the ICA in longitudinal section at the level of the DDR shows the gross insertion point of dural collagen (arrowheads), tangent to the arterial wall (Fig. 1A). Higher magnification (40×) demonstrates the convergence of collagen fiber from both the dural and adventitial layers (Fig. 1B). Under highest magnification (100×), the inner arterial layers of intima and muscularis are visualized with a clear boundary between the muscularis and adventitia (arrows); by contrast, the adventitial later is fully invested by dural collagen, and there is no evidence of a clear plane separating adventitia from dura (Fig. 1C).

Conclusion: Convergence of collagen from the dural and adventitial layers was widely observed, suggestive of an intimate interrelatedness between the ICA and DDR. Correspondingly, a true plane may be difficult to establish between these structures, and direct dissection of the DDR from the ICA may carry a higher risk of carotid rupture. In cases that demand mobilization of the ICA, careful circumferential trimming of the DDR is preferable to attempting to establish a true dissection plane between the DDR and ICA.

Fig. 1