Figure 1: The supracerebellar transtentorial approach has numerous advantages over other alternative supratentorial operative corridors to reach the posterior basal temporal lobe: 1) a small paramedian craniotomy is minimally disruptive, and 2) only one cerebellar hemisphere is manipulated and the supratentorial cortices are left intact and not placed under retraction. The green arrow (upper illustration) points to the roadmap trajectory for this operative corridor, and the green and purple colored sections of the hippocampus illustrate the reach of this approach (upper image). The lower images show the operative view of the typical locations of the lesion in relation to the hippocampus (yellow shading) and temporal horn (blue shading).

Figure 2: Posterior parahippocampal lesions, such as this metastatic adenocarcinoma, may be exposed through the supracerebellar transtentorial approach. The location of this tumor is at the most anterior reach of this approach.

Figure 3: The regional anatomy for approaching the tentorium and its transection is shown. The window within the tentorium is tailored based on the location of the lesion and the extent of exposure necessary using navigation. A bilateral craniotomy is unnecessary. The mediobasal supratentorial regions become available after transection of the tentorium (A-upper image): Supracerebellar transtentorial approach on the left side after tentorial resection, demonstrating the operative corridor toward the basal surface of the temporal lobe (B-lower image). From de Oliveira JG, et al. Supracerebellar transtentorial approach-resection of the tentorium instead of an opening-to provide broad exposure of the mediobasal temporal lobe: Anatomical aspects and surgical applications. J Neurosurg 116: 764-772, 2012.

Figure 4: Sectioning a window of the left tentorium through a paramedian supracerebellar craniotomy exposes the posterior ambient cisterns, basal temporal lobe, and the relevant arterial anatomy. Note the generous exposure of the posterior parahippocampus and distal posterior cerebral artery branches through this route (images courtesy of AL Rhoton, Jr).

Figure 5: A left-sided suboccipital supracerebellar craniotomy is perfomed to expose the left tentorium. A skull clamp is used with the patient’s neck flexed and head turned slightly (15-20 degrees) toward the floor. The patient’s ipsilateral shoulder is allowed to fall forward and is taped away from the surgeon’s working zone. Intraoperative neuronavigation identifies the location of the midline, as well as the transverse and sigmoid sinuses. A paramedian vertical linear incision is made halfway between the inion and mastoid groove. This incision extends one-third above and two-thirds below the transverse sinus and is about 7–8 cm in length. Note that the Keen’s point is underneath the upper edge of the incision. The location of the transverse sinus is marked with the short horizontal line (left image).

Figure 6: A single burr hole is made at the inferior edge of the transverse sinus, approximately 2 cm lateral to the midline and torcula. A small bone flap is elevated while the entire width of the transverse sinus is exposed to allow room for later rostral mobilization of this sinus (left upper image). The dura is opened as a single curved flap based on the sinus. Two retraction sutures may be placed along the posterior aspect of the tentorium to mobilize and gently rotate the transverse sinus superiorly to expand the operative space through the supracerebellar corridor (right upper image).

Figure 7: One or two paramedian bridging veins may have to be sacrificed. Large midline bridging veins are left intact. Note the retraction sutures placed through the posterior tentorium. These sutures gently rotate and mobilize the transverse sinuses superiorly. Microdoppler ultrasonography can confirm the patency of the sinus and gauge the safe degree of retraction on the sinus.

Figure 8: Gradual release of cerebrospinal fluid through the lumbar drain or a ventriculostomy catheter allows gentle caudal mobilization of the lateral cerebellar hemisphere. The dura may be incised in a “T”-shaped pattern (hashed line) for smaller lesions or a “U”-shaped fashion (see Figure 10) for larger lesions. Intraoperative navigation guides the location of the tentorial incisions.

Figure 9: The steps in completing the tentorial incisions (“T”-shaped opening) for the tumor in Figure 2. The edge of the tentorium is elevated with a fine right-angled dissector, and microscissors are used to continue the transection process (left upper image). A small curved knife can facilitate cutting toward the operator (right upper image). The lower images demonstrate placement of one of the retraction sutures within the tentorium (left lower image) and the final extent of the operative corridor using retention sutures over cottonoid patties to protect the surface of the hemisphere (right lower image).

Figure 10: The dura may be incised in a “U”-shaped configuration for exposing wider regions of the mediobasal surface and resection of larger lesions. Retraction sutures also mobilize the incised section of the tentorium along with the cerebellum inferiorly. Note the location of the underlying temporal horn (blue) and hippocampus (yellow). Dissection of the arachnoid membranes over the medial dorsolateral mesencephalon will mobilize the cerebellum inferiorly and expand the operative corridor. The trochlear nerve is protected along the edges of tentorium.

Figure 11: Intraoperative navigation guides the borders of tentorial sectioning based on the exact location of the lesion. Because of unfamiliar operative angles, the surgeon can easily misinterpret or be disoriented regarding the location of the lesion in relation to the surface of the tentorium. After adequate surface of the posterior basal temporal lobe is exposed, navigation can guide the location of the corticotomy if the lesion is not apparent on the pial surface. Distal posterior cerebral artery en passage branches are numerous in this region and should be meticulously preserved. The thalamoperforating arteries can be injured during tumor manipulation. In addition, indiscriminate coagulation leads to undesirable thalamic and occipital lobe ischemia. Small cortical arteries overlying the lesion may have to be sacrificed.

Next, the surgeon can begin microsurgical removal of the tumor. This inferior-to-superior trajectory is beneficial for removing tumors that extend to the level of the temporal horn and Calcar avis.

Figure 12: The final appearance of the operative space after resection of the tumor.

Figure 13: Note the role of this route for resection of medial tentorial extra-axial tumors and, more specifically, meningiomas. The trochlear nerve must be protected along the lateral edge of the incisura during tentorial transection (upper inset image). Incision along the red hashed line will sacrifice the nerve—incision along the black hashed line is appropriate. Alternatively, a “T”-shaped incision may be made within the tentorium for intraparenchymal lesions within the posterior basal temporal lobe (see the above section). An intraoperative photograph during resection of a left-sided petrous apex meningioma demonstrates the location of the nerve as it enters the dura at the anterior edge of the coagulated tumor and tentorium (lower image).

Figure 14: Early exposure of the dorsolateral brainstem and surrounding neurovascular structures at the tentorial incisura allows for their protection by microdissection away from the tumor before significant tumor debulking is undertaken and the surgical field is obscured by bleeding. Extra-axial tumors can be devascularized early in surgery through cauterization of the undersurface of the tentorium.

Figure 15: A generous portion of the tentorium is then incised from the petrous ridge to the midline while the operator identifies and preserves the trochlear nerve along the entire anterior edge of the tentorium. Occasional bridging veins draining the occipital lobe and entering the superior aspect of the tentorium may be sacrificed. The medial tentorial cut should preserve the straight sinus and its tributaries. Venous lakes may be present, and venous bleeding through the leaflets of the tentorium should be controlled using thrombin-soaked gelfoam packing. Bipolar cauterization will exacerbate the bleeding by shrinking and tearing the tentorial edges.

Figure 16: This tentorial resection creates a wide corridor to the basal occipital and posteromedial temporal regions. An intra-axial tumor in this region can be similarly resected.