In Reply: The Glossopharyngo-Cochlear Triangle-Part II: Case Series Highlighting the Clinical Application to High-Riding Posterior Inferior Cerebellar Artery Aneurysms Exposed Through the Extended Retrosigmoid Approach.

Abstract

To the Editor: We thank the authors1 of this letter for their comments and insights. We hope this 2-part study successfully described the elucidation and execution of a patient-specific, microsurgical approach using the glossopharyngo-cochlear triangle (GCT) to treat a select subset of posterior inferior cerebellar artery (PICA) aneurysms.2,3 We wholeheartedly agree that 2-part studies—in which part I3 provides an in-depth discussion of the microsurgical anatomy and results of cadaveric dissections and part II2 highlights the clinical utility and the nuances of “real-world” surgical application of the anatomic principles—are “indispensable prerequisite(s).”1 We thank the editorial board of Operative Neurosurgery for continuing to publish and support these important endeavors. We believe that combining these 2 distinct studies into a single manuscript would have truncated and obscured the salient points of both studies, potentially diluting the significance and limiting the educational value and clinical applicability of the findings. Innovation in cerebrovascular and skull base microneurosurgery is driven by applying honed microsurgical techniques and skills to novel clinical scenarios, resulting in improvements in surgical outcomes and patient care. This innovation can be accomplished only by the continued enhancement of our knowledge of microsurgical anatomy; commitment to laboratory dissection, where we can continue to expand our armamentarium and safely push the limits of what is possible; and critical evaluation of our surgical and clinical outcomes. It is through spending time in the anatomy laboratory that we developed the microsurgical skills necessary to safely and efficaciously treat challenging lesions, such as distal PICA aneurysms, despite their proximity to delicate neurovascular structures and their location in deep surgical corridors. We welcome the idea that novel microsurgical approaches may provide an even more efficacious route or technique for treating these challenging lesions; if these new approaches could somehow obviate the need to manipulate the neighboring cranial nerves, all the better. We agree with the authors that such approaches can be devised, and their application critically evaluated, only by 2-part studies such as these. This point further highlights the importance of anatomic laboratory dissections, which are critical to neurosurgical education during residency and fellowship training and beyond. With regard to the definition of a “high-riding PICA aneurysm,” we found a measurement greater than 23 mm above the foramen magnum to be the most straightforward clinically applicable measurement.2,3 However, this is just one of many anatomic clues that can guide approach selection and patient-specific surgical intervention. In our clinical manuscript, in addition to reporting increased distance of the aneurysm from the foramen magnum, we reported how the distances from the internal auditory canal (IAC) and jugular foramen (JF) to the aneurysm differed significantly between typical and high-riding PICA aneurysms.2 High-riding PICA aneurysms had a mean distance below the IAC of 3.7 mm (range 0-6.8 mm) and a mean distance above the JF of 8.2 mm (range 6.9-9.4 mm). In contrast, the control group of typical PICA aneurysms had a mean distance below the IAC of 11.6 mm (range 7.0-18.0 mm) and a mean distance above the JF of 1.9 mm (range −7.3 to 7.3 mm). Identifying an aneurysm's location relative to these anatomic landmarks and using these results as a reference can guide surgical approach planning. These additional measurements can also assist in scenarios where the angle of gantry of the computed tomography angiography prevents direct measurement of distance in a single image slice. An additional solution to this issue can be to measure the distance on consecutive image slices (as was done in the illustrative case described in part I of our study).3 Although measuring on multiple slices may have a minor effect on the calculated distances, it is unlikely that this possible submillimetric difference would drastically affect approach selection. Importantly, much like our philosophy on anatomic triangles used to guide subarachnoid dissection, these measurement guidelines are meant to help predict approach corridors and are not intended to be rigid barriers. We agree that an operative video highlighting this technique would be of clinical interest and would clarify the nuances of the surgical approach and patient selection. Indeed, at the time of this response, we have a narrated video submitted for review, which we hope the authors of this letter and others will find interesting and clinically beneficial.4 Again, we would like to thank the authors1 of this letter for their comments and their thoughtful response. Funding This study did not receive any funding or financial support. Disclosures The authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article.