The Glossopharyngo-Cochlear Triangle-Part I: Quantitative Anatomic Analysis of High-Riding Posterior Inferior Cerebellar Artery Aneurysms Exposed Through the Extended Retrosigmoid Approach

A system of anatomical triangles defining dissection routes to brainstem cavernous malformations: definitions and application to a cohort of 183 patients

OBJECTIVE

Anatomical triangles defined by intersecting neurovascular structures delineate surgical routes to pathological targets and guide neurosurgeons during dissection steps. Collections or systems of anatomical triangles have been integrated into skull base surgery to help surgeons navigate complex regions such as the cavernous sinus. The authors present a system of triangles specifically intended for resection of brainstem cavernous malformations (BSCMs). This system of triangles is complementary to the authors' BSCM taxonomy that defines dissection routes to these lesions.

METHODS

The anatomical triangle through which a BSCM was resected microsurgically was determined for the patients treated during a 23-year period who had both brain MRI and intraoperative photographs or videos available for review.

RESULTS

Of 183 patients who met the inclusion criteria, 50 had midbrain lesions (27%), 102 had pontine lesions (56%), and 31 had medullary lesions (17%). The craniotomies used to resect these BSCMs included the extended retrosigmoid (66 [36.1%]), midline suboccipital (46 [25.1%]), far lateral (30 [16.4%]), pterional/orbitozygomatic (17 [9.3%]), torcular (8 [4.4%]), and lateral suboccipital (8 [4.4%]) approaches. The anatomical triangles through which the BSCMs were most frequently resected were the interlobular (37 [20.2%]), vallecular (32 [17.5%]), vagoaccessory (30 [16.4%]), supracerebellar-infratrochlear (16 [8.7%]), subtonsillar (14 [7.7%]), oculomotor-tentorial (11 [6.0%]), infragalenic (8 [4.4%]), and supracerebellar-supratrochlear (8 [4.4%]) triangles. New but infrequently used triangles included the vertebrobasilar junctional (1 [0.5%]), supratrigeminal (3 [1.6%]), and infratrigeminal (5 [2.7%]) triangles. Overall, 15 BSCM subtypes were exposed through 6 craniotomies, and the approach was redirected to the BSCM by one of the 14 triangles paired with the BSCM subtype.

CONCLUSIONS

A system of BSCM triangles, including 9 newly defined triangles, was introduced to guide dissection to these lesions. The use of an anatomical triangle better defines the pathway taken through the craniotomy to the lesion and refines the conceptualization of surgical approaches. The triangle concept and the BSCM triangle system increase the precision of dissection through subarachnoid corridors, enhance microsurgical execution, and potentially improve patient outcomes.

A taxonomy for brainstem cavernous malformations: subtypes of pontine lesions. Part 1: basilar, peritrigeminal, and middle peduncular

OBJECTIVE

Brainstem cavernous malformations (BSCMs) are complex, difficult to access, and highly variable in size, shape, and position. The authors have proposed a novel taxonomy for pontine cavernous malformations (CMs) based upon clinical presentation (syndromes) and anatomical location (findings on MRI).

METHODS

The proposed taxonomy was applied to a 30-year (1990-2019), 2-surgeon experience. Of 601 patients who underwent microsurgical resection of BSCMs, 551 with appropriate data were classified on the basis of BSCM location: midbrain (151 [27%]), pons (323 [59%]), and medulla (77 [14%]). Pontine lesions were then subtyped on the basis of their predominant surface presentation identified on preoperative MRI. Neurological outcomes were assessed according to the modified Rankin Scale, with a score ≤ 2 defined as favorable.

RESULTS

The 323 pontine BSCMs were classified into 6 distinct subtypes: basilar (6 [1.9%]), peritrigeminal (53 [16.4%]), middle peduncular (MP) (100 [31.0%]), inferior peduncular (47 [14.6%]), rhomboid (80 [24.8%]), and supraolivary (37 [11.5%]). Part 1 of this 2-part series describes the taxonomic basis for the first 3 of these 6 subtypes of pontine CM. Basilar lesions are located in the anteromedial pons and associated with contralateral hemiparesis. Peritrigeminal lesions are located in the anterolateral pons and are associated with hemiparesis and sensory changes. Patients with MP lesions presented with mild anterior inferior cerebellar artery syndrome with contralateral hemisensory loss, ipsilateral ataxia, and ipsilateral facial numbness without cranial neuropathies. A single surgical approach and strategy were preferred for each subtype: for basilar lesions, the pterional craniotomy and anterior transpetrous approach was preferred; for peritrigeminal lesions, extended retrosigmoid craniotomy and transcerebellopontine angle approach; and for MP lesions, extended retrosigmoid craniotomy and trans-middle cerebellar peduncle approach. Favorable outcomes were observed in 123 of 143 (86%) patients with follow-up data. There were no significant differences in outcomes between the 3 subtypes or any other subtypes.

CONCLUSIONS

The neurological symptoms and key localizing signs associated with a hemorrhagic pontine subtype can help to define that subtype clinically. The proposed taxonomy for pontine CMs meaningfully guides surgical strategy and may improve patient outcomes.

Modified Retromastoid Approach and Clipping of "High-Riding" VA-PICA Junction Aneurysm: An Operative Video

Background and Introduction

Clipping an aneurysm on an elongated and tented V4 segment near the origin of the posterior inferior cerebellar artery (high-riding VA-PICA junction aneurysm) can be challenging.

Objective

We demonstrate the microsurgical clipping technique of such an aneurysm using a modified retromastoid approach (MRMA) and glossopharyngeal-cochlear triangle (GCT).

Surgical Technique

A 50-year-old female with a ruptured high-riding left VA-PICA junction aneurysm underwent an MRMA. Using segmental vessel isolation with proximal and distal temporary clips, this aneurysm was occluded through the GCT by applying a tandem clipping technique while preserving the PICA.

Results

The procedure was uneventful. Apart from transient ataxia, she recovered completely and maintains a good status at follow-up.

Conclusion

In high-riding VA-PICA junction aneurysms, a conventional far lateral approach may create awkward viewing and working angles. An MRMA with a horizontal trajectory through the GCT may be a more appropriate strategy.

Extended Retrosigmoid Craniotomy and Approach Through the Glossopharyngeal Cochlear Triangle for Clipping of a High-Riding Vertebral-Posterior Inferior Cerebellar Artery Aneurysm: 2-Dimensional Operative Video

The far lateral transcondylar (FL) craniotomy is the standard approach for posterior inferior cerebellar artery (PICA) aneurysm exposure through microsurgical dissection in the vagoaccessory triangle (VAT).1,2 However, the extended retrosigmoid (eRS) craniotomy and dissection through the glossopharyngeal-cochlear triangle (GCT) may be more appropriate when the patient has an aneurysm arising from a high-riding vertebral artery (VA)-PICA origin.3-5 We present a case of a 41-yr-old woman with hypertension presenting with left occipital pain and left-side hearing loss and past facial spasm and pain. Computed tomography angiography and digital subtraction angiography demonstrated an unruptured 8.4 × 9.0 × 10.2 mm saccular aneurysm at the left VA-PICA junction. Surgical clipping was chosen over endovascular therapy given the relationship of the PICA origin to the aneurysm neck as well as the history of cranial neuropathy. It was noted that the VA-PICA junction and aneurysm was high-riding at the level of the internal auditory canal. An eRS craniotomy was performed with dissection through the GCT, and the aneurysm was clipped as shown in the accompanying 2-dimensional operative video. Postoperative angiography demonstrated complete occlusion of the aneurysm and patency of the left VA and PICA without stenosis, and the patient had a favorable postoperative course although her left-sided hearing remained diminished. The eRS craniotomy allowed direct exposure via the GCT for clipping of the high-riding VA-PICA junction aneurysm and decompression of the cranial nerves. The traditional FL craniotomy and exposure through the VAT would likely have resulted in a less desirable inferior trajectory. The patient gave informed consent for the operation depicted in the video. Animation at 2:43 in video is used with permission from Barrow Neurological Institute, Phoenix, Arizona.