Figure 1: The paramedian supracerebellar approach has numerous advantages over the midline approach: 1) the craniotomy is less invasive and does not place the frequently more dominant right transverse sinus and torcula at risk, 2) only one cerebellar hemisphere is manipulated, 3) the vermian bridging veins are often protected, and 4) the lower slope of the lateral cerebellum provides a more inferior trajectory to the inferior pole of the tumor. Repeat surgery may be performed through the intact contralateral supracerebellar route.

Figure 2: Large lesions in the pineal region (left image) and posterolateral mesencephalon (right image) can be readily exposed through the paramedian supracerebellar route.

Figure 3: Transtentorial petrous apex meningiomas (upper images) and medial tentorial meningiomas (lower images) may be resected with a single-stage surgery using the paramedian supracerebellar transtentorial route.

Figure 4: The regional anatomy for approaching the pineal region is shown. Note the relationship of the posterior basal temporal lobe and occipital lobe to the pineal area (A). These supratentorial regions become available after transection of the tentorium through the supracerebellar route. The surface anatomy of the cerebellum (B) and posterior diencephalon (C and D) are shown. The vascular arterial (E) and venous (F) anatomy are evident (E)(Images courtesy of AL Rhoton, Jr).

Figure 5: Higher magnified images of the cerebrovascular anatomy within the pineal region through the paramedian supracerebellar approach are shown. The splenium is anterior to the vein of Galen. As the operator follows the contours of the tentorium toward the pineal region, he or she may be led to inadvertently dissect around the vein of Galen. This error must be avoided and the operative trajectory readjusted inferiorly toward the pineal region. Note the generous view of the pineal region through the lateral trajectory (E)(Images courtesy of Rhoton, AL).

Figure 6: Compared with the midline approach (left image), the paramedian supracerebellar approach (right image) avoids most of the midline vermian bridging veins.

Figure 7: The midline route (left image) depends upon retraction of the culmen, whereas the paramedian route (right image) reaches over the more inferiorly situated lateral cerebellum. Cranial nerve IV is situated at the lower edge of the dissection field.

Figure 8: Sectioning a window of the left tentorium through a paramedian supracerebellar craniotomy exposes the posterior ambien cisterns, basal temporal lobe, and the relevant arterial anatomy (B, C, and D). Note the generous exposure of the posterior parahippocampus and distal posterior cerebral artery branches through this route (Images courtesy of AL Rhoton, Jr).

Figure 9: The left-sided suboccipital craniotomy protects the torcula and the often more dominant right-sided veins and dural sinuses, including the transverse sinus. A skull clamp is used with the patient’s neck flexed and head turned slightly (15-20 degrees) toward the floor. The location of the tumor is marked with X on the upper image.

The patient’s ipsilateral shoulder is allowed to fall forward and is taped away from the surgeon’s working zone. Intraoperative neuronavigation may be used to identify 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 the mastoid groove. This incision extends one-third above and two-thirds below the transverse sinus and is about 7–8 cm in length. Note the Keen’s point (arrow, lower left image) is within the upper edge of the incision.

Figure 10: 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. The dura is opened as a single curved flap based on the sinus. Two retraction sutures are 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.

Gradual release of cerebrospinal fluid through the lumbar drain or ventriculostomy catheter allows gentle caudal mobilization of the lateral cerebellar hemisphere.

Figure 11: Traditional bone removal and dural opening restrict the working angles of the surgeon and limit the corridor within the supracerebellar space (top image). Placement of a fixed retractor on the tentorium, despite skeletonization of the sinuses, does not significantly improve the operative corridor. In fact, it may interfere with the working angles of the instruments (middle image). The retraction sutures anchored on the tentorium elevate the tentorium, mobilize/rotate the transverse sinus, and expand the operative corridor (bottom image).

Figure 12: 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 patency of the sinus and gauge the safe degree of retraction on the sinus.

Figure 13: Dissection of the arachnoid membranes over the medial dorsolateral mesencephalon will mobilize the cerebellum downward and open the corridor toward the midline. Cranial nerve IV is evident in the inferolateral corner of the exposure.

Figure 14: Upon opening the arachnoid membranes over the tumor, the surgeon can begin microsurgical removal of the tumor. This “cross court” trajectory is beneficial since the tentorial surface of the cerebellum is highest along its anteromedial apex and slopes down laterally. Therefore, the lateral paramedian operative corridor provides a more inferior trajectory to the posterior midline and posterolateral mesencephalon as compared with the midline trajectory to access the inferior extension of large pineal region tumors.

Dynamic retraction of the cerebellum using the suction apparatus allows exposure and resection of the inferior extent of the tumor without the use of fixed retractors. The suction apparatus allows a more controlled expanded view of the working zone at the exact location of the dissection, whereas the wide blade of the retractor may at times compromise the deep exposure because of its less flexible vector of retraction. Generous exposure of the contralateral tectum is readily available through this unilateral working channel and near the falx cerebelli.

Figure 15: Note the role of this route for resection of medial tentorial tumors. Cranial nerve IV must be protected along the lateral edge of the incisura during tentorial transection (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.

Figure 16: Early exposure of the posterior brainstem and surrounding neurovascular structures at the tentorial incisura allows for their protection by microdissection away from the tumor before significant tumor debulking is performed 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 17: A generous portion of the tentorium is then incised from the petrous ridge to the midline while identifying and preserving 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.

Sectioning of the tentorium as described above will further devascularize the tumor and allow a relatively bloodless field to debulk the tumor and microsurgically mobilize it from the surrounding cortex.

Figure 18: This tentorial resection creates a wide corridor to the basal occipital and posterior/medial temporal regions. An intra-axial tumor in this region can be similarly resected.