The oculomotor-tentorial triangle. Part 1: microsurgical anatomy and techniques to enhance exposure

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OBJECTIVE

Access to the ventrolateral pontomesencephalic area may be required for resecting cavernous malformations, performing revascularization of the upper posterior circulation, and treating vascular lesions such as aneurysms. However, such access is challenging because of nearby eloquent structures. Commonly used corridors to this surgical area include the optico-carotid, supracarotid, and carotid-oculomotor triangles. However, the window lateral to the oculomotor nerve can also be used and has not been studied. The authors describe the anatomical window formed between the oculomotor nerve and the medial tentorial edge (the oculomotor-tentorial triangle [OTT]) to the ventrolateral pontomesencephalic area, and assess techniques to expand it.

METHODS

Four cadaveric heads (8 sides) underwent orbitozygomatic craniotomy. The OTT was exposed via a pretemporal approach. The contents of the OTT were determined and their anatomical features were recorded. Also, dimensions of the brainstem surface exposed lateral and inferior to the oculomotor nerve were measured. Measurements were repeated after completing a transcavernous approach (TcA), and after resection of temporal lobe uncus (UnR).

RESULTS

The s1 segment and proximal s2 segment of the superior cerebellar artery (SCA) and P2A segment of the posterior cerebral artery (PCA) were the main contents of the OTT, with average exposed lengths of 6.4 ± 1.3 mm and 5.5 ± 1.6 mm for the SCA and PCA, respectively. The exposed length of the SCA increased to 9.6 ± 2.7 mm after TcA (p = 0.002), and reached 11.6 ± 2.4 mm following UnR (p = 0.004). The exposed PCA length increased to 6.2 ± 1.6 mm after TcA (p = 0.04), and reached 10.4 ± 1.8 mm following UnR (p < 0.001). The brainstem surface was exposed 7.1 ± 0.5 mm inferior and 5.6 ± 0.9 mm lateral to the oculomotor nerve initially. The exposure inferior to the oculomotor nerve increased to 9.3 ± 1.7 mm after TcA (p = 0.003), and to 9.9 ± 2.5 mm after UnR (p = 0.21). The exposure lateral to the oculomotor nerve increased to 8.0 ± 1.7 mm after TcA (p = 0.001), and to 10.4 ± 2.4 mm after UnR (p = 0.002).

CONCLUSIONS

The OTT is an anatomical window that provides generous access to the upper ventrolateral pontomesencephalic area, s1- and s2-SCA, and P2A-PCA. This window may be efficiently used to address various pathologies in the region and is considerably expandable by TcA and/or UnR.

ABBREVIATIONS AChA = anterior choroidal artery; AMZ = anterior mesencephalic zone; BA = basilar artery; OTT = oculomotor-tentorial triangle; PCA = posterior cerebral artery; PCoA = posterior communicating artery; PCP= posterior clinoid process; PMS = pontomesencephalic sulcus; SCA = superior cerebellar artery; STZ = supratrigeminal zone; TcA = transcavernous approach; UnR = resection of temporal lobe uncus.

Article Information

Correspondence Michael T. Lawton: Barrow Neurological Institute, Phoenix, AZ. michael.lawton@barrowbrainandspine.com.

INCLUDE WHEN CITING Published online June 29, 2018; DOI: 10.3171/2018.1.JNS173139.

Disclosures The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

© AANS, except where prohibited by US copyright law.

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Figures

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    Left: Cadaveric dissection showing the right tentorial incisura from a lateral perspective. The OTT (yellow shaded area) is formed between the cisternal segment of the oculomotor nerve medially, and the free edge of the tentorium laterally. The PCA and SCA are the main vascular contents of the triangle, although the BA may also be accessed through the OTT. Right: Exposure of the right OTT in another specimen after completing an orbitozygomatic craniotomy and developing a pretemporal corridor. The posterior aspect of the OTT (yellow shaded area) is delimited by the cerebral peduncle and PCA coursing around it medially, and uncus of the temporal lobe laterally. CN = cranial nerve; ICA = internal carotid artery; ON = optic nerve; PCP = posterior clinoid process. Copyright Michael T. Lawton. Published with permission. Figure is available in color online only.

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    Depiction of the measurement of the dimensions of the ventrolateral pontomesencephalic area through the OTT. Point A is at the root exit zone of the oculomotor nerve just medial to the cerebral peduncle (CP); point B is the inferior-most exposed point on the brainstem surface along a vertical line passing through point A; point C is the lateral-most exposed point on the cerebral peduncle at the axial level of point A; point D is the inferior-most exposed point on brainstem surface along a vertical line passing through point C. The lengths of lines A-B, A-C, and C-D were calculated. Copyright Michael T. Lawton. Published with permission. Figure is available in color online only.

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    Exposure and stepwise enhancement of the left OTT. A: The OTT is exposed after completing a pretemporal approach. The s1- and proximal s2-SCA and P2A-PCA are exposed via the OTT. The dashed line designates the medial tentorial edge. B: The PMS is also exposed through the OTT after slight manipulation of the SCA and PCA. Note that the tentorial edge and temporal uncus are the major obstacles to exposing further posterior segments of the SCA and PCA toward the ambient cistern. The tentorium also blocks exposure of the brainstem surface below the level of PMS. C: After completion of a TcA, the cavernous segment of the oculomotor nerve is exposed (double-arrow line), which enables further mobilization. This maneuver enables better exposure of the STZ that is located inferior to the PMS at the sagittal level of the root exit zone of the oculomotor nerve. Further posterior segments of the SCA and PCA are exposed as well. However, the temporal uncus is still a major obstacle in exposing the SCA and PCA toward the ambient cistern. D: Resection of the uncus enables more distal segments of the s2-SCA and P2A-PCA. MCA = middle cerebral artery. Copyright Michael T. Lawton. Published with permission. Figure is available in color online only.

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    Depictions of exposed surface of the brainstem on a cadaveric specimen through the OTT with a transsylvian pretemporal approach (yellow shaded area), and its enhancement following a TcA (red shaded area) and resection of the temporal uncus (green shaded area). Anterior (left) and anterolateral (right) perspectives are shown. Areas 1 and 2 designate the anterior mesencephalic and supratrigeminal safe entry zones of the brainstem. It is evident that the AMZ is sufficiently exposed using the transsylvian pretemporal approach, whereas optimal exposure of the STZ may require expanding the OTT with a TcA. The resection of the temporal uncus after the TcA enables further posterior exposure of the ventrolateral cerebral peduncle, but does not extend to the lateral mesencephalic sulcus (LMS) that is typically exposed during a subtemporal approach. The dashed line shows the PMS. The black arrow represents the course of the pontocerebellar (transverse pontine) fibers that pass through the middle cerebellar peduncle (MCP) to reach the cerebellum. Copyright Michael T. Lawton. Published with permission. Figure is available in color online only.

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    Graph representations of the area of the OTT (A), and exposed lengths of the BA (B), PCA (C), and SCA (D) through the OTT during a transsylvian pretemporal (TS) approach, and following completion of the TcA and UnR. Brackets show statistically significant increases (paired t-test).

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    Graph representations of the exposed dimensions of the brainstem through the OTT during a TS approach, and following completion of the TcA and UnR. A: Height of the brainstem exposed at the sagittal level of the root exit zone of the oculomotor nerve (line A-B, Fig. 2). B: Width of the cerebral peduncle at the axial level of the root exit zone of the oculomotor nerve (line A-C, Fig. 2). C: Height of the brainstem exposed at the sagittal level of the lateral-most exposed point of the width of the cerebral peduncle (line C-D, Fig. 2). Brackets show statistically significant increases (paired t-test).

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