The patient is positioned according to the surgeon’s preference and the tumor location.
- Prone position: This position gives access to the cerebellar hemisphere, floor of the fourth ventricle, and the cervicomedullary junction, while the inferior aspect of the tentorium and the upper cerebellar vermis are less easily approached.
- Seated position: This position offers the same exposure of the posterior fossa as the prone position but with more direct access to rostral regions.
- Concorde position: This is an alternative option to the prone or seated position for reaching the upper vermis.
- Park bench position: This may be used for hemangioblastomas confined to the cerebellopontine angle.
Midline suboccipital approach
The midline suboccipital approach is the most frequently used since hemangioblastomas are located in the posteromedial aspect of the cerebellar hemisphere (70–75% of the cases) or in the dorsal aspect of the brainstem (95–100%) (53, 54).
- Diuretics: Intravenous mannitol and/or furosemide administration at the time of incision can be useful to decrease ICP in anticipation of the dural opening and the initial tumor excision.
- Midline skin incision: The midline incision is made from inion to C3 (or further inferiorly if necessary). Fascia and muscles are divided, placing the incision along the avascular midline of the ligamentum nuchae.
- Expose C1: The arch of C1 is exposed in the subperiosteal plane to enlarge the surgical field. C1 periosteum stripping is required to avoid injury of the vertebral arteries once the muscles are retracted and occipital squama is exposed.
- Suboccipital craniotomy vs. craniectomy: The choice is made according to the surgeon’s preference. In children, craniotomy is preferred as it allows a nearly complete and physiological healing of the occipital squama and reduces the risk of postoperative suboccipital pseudomeningocele. It is performed with two bur holes, placed just below the transverse sinus of each side, and an osteotomy is then made by high-speed craniotome to join the two bur holes and each bur hole with the foramen magnum. The craniectomy is made with a bone ronguer.
- Laminectomy: The posterior arch of C1 is then removed to expose the lower cerebellum and the cervicomedullary junction.
- Imaging of the posterior fossa: Intraoperative ultrasound is useful at this point to verify the adequacy of the bone removal and to localize the tumor through the dura mater. This will also guide the surgeon in removal of fluid from a tumor cyst, when present, before opening the dura, thereby avoiding herniation and strangulation of the cerebellar hemispheres in the case of elevated ICP.
- Dura mater opened: The dura is usually opened with a Y-shaped incision, the vertical leg extending to the upper cervical region.
- Removal of CSF: Brain relaxation can be obtained by withdrawal of CSF from a previously placed EVD and/or by opening the cisterna magna.
When the hemangioblastoma is located elsewhere in the posterior fossa, other approaches may be preferred. These alternatives include the retrosigmoid approach for a CP angle hemangioblastoma, posterior subtemporal or suboccipital transtentorial approach, or infratentorial supracerebellar approach for an upper vermian hemangioblastoma.
- Goal of complete resection: Complete eradication of the solid tumor is the goal to prevent recurrences. Tumor resection is performed similarly to that for arteriovenous malformations of the brain. Lesions are removed circumferentially if possible with sequential coagulation of feeding arteries. Entry into the tumor should be avoided as it may result in hazardous bleeding. Venous drainage is interrupted after interruption of the arterial supply. It is not necessary to excise the cyst (52, 59), which disappears after removal of the solid tumor and does not typically recur.
- Usually easily found: Cerebellar hemangioblastomas are large, hemispheric tumors easily detected by macroscopic inspection. When the tumor is superficial (60–70% of the cases), it is visualized through the pial surface. When it is deeply located, it is revealed by the distortion of the hemisphere, the distension of the cerebellar folia, and the asymmetric descent of the cerebellar tonsils. Intraoperative ultrasound or neuronavigation will facilitate the search for deep midline or small hemangioblastomas when visual inspection is not adequate.
- Cortical incision: Superficial hemangioblastomas of the cerebellar hemisphere are approached by a circular incision of the pia mater surrounding the tumor surface, while deep tumors are reached through a cerebellar cortisectomy. Cortisectomy is realized parallel to the folia above the more superficial aspect of the tumor.
- Avoid cyst collapse early: The tumor’s cyst should not be entered initially or emptied completely since it facilitates the dissection maneuvers.
- Solid tumor dissected free in its surrounding gliotic plane: The interface between the hemangioblastoma and surrounding cerebellar tissue is a soft gliotic tissue. It is generally found so that the tumor is dissected using such an interface. The red color and noninfiltrative nature of the tumor favor the visualization of this cleavage plane. The dissection is carried out under magnification, proceeding circumferentially into its deeper regions, and inserting cottonoids to progressively demarcate the interface between the hemangioblastoma and neural tissue. The solid portion is excised en bloc, and the cyst is entered to search for possible tumor remnants.
- Cauterize feeding vessels when encountered: All feeding vessels are carefully coagulated and interrupted, following the same rules as for AVMs: feeding arteries are interrupted first (coagulation close to the tumor not to interfere with brain blood supply), then the tumor is dissected, and finally the draining veins are occluded. Premature closure of the veins may result in severe intraoperative brain swelling and tumor bleeding.
- Shrink tumor: Shrinkage of the tumor by cauterization of its walls is used to favor mobilization and traction of very large hemangioblastomas, although it can make the tumor less distinguishable from the surrounding tissue. Alternatively, the tumor core can be coagulated and removed, leaving intact the tumor-cerebellum interface (15). This maneuver should be carefully weighed as it could result in profuse bleeding.
- Check for residual tumor: Intraoperative indocyanin green videoangiography has been recently proposed to identify feeding vessels and to rule out tumor remnants (36).
Intramedullary brainstem tumors are most commonly solid and not cystic. They are usually large (mean diameter 3–4 cm) and have a midline location that often displaces the cerebellar vermis and cerebellar hemispheres. Care should be taken not to damage the tumor vessels during the dural opening.
- Exposure of the ventricle floor: A hemangioblastoma in the pons requires division of the vermis or a telovelar approach to be reached through the floor of the fourth ventricle.
- Tumor located: Brainstem hemangioblastomas are prevalently located at the dorsal portion of the lower brainstem. The tumor often can be visualized on the dorsal surface of the medulla oblongata or on the floor of the fourth ventricle. A pure intramedullary location is less common (54, 59). When the tumor is completely covered by normal tissue, an asymmetric bulging of the dorsal surface will be apparent. An ultrasound also may be used to locate the tumor.
- Tumor dissection and resection: Dissection is similar to that for cerebellar hemangioblastomas but with some important differences. Brainstem hemangioblastomas are often exophytic and well distinguishable from the surrounding white and lucent tissue. However, a cleavage plane is often missing unless a hemorrhage has previously occurred. Therefore, dissection should start from the visible part of the tumor and proceed along its peripheral portion, paying attention to spare the neural tissue and to limit cauterization as much as possible to prevent heat-induced injury of the brainstem.
- En bloc tumor resection optimal: En bloc resection is the optimal way to remove brainstem hemangioblastomas. It also allows for the reduction of cauterization. Piecemeal tumor removal can be attempted to limit brainstem manipulation in case of large hemangioblastomas. A piece-by-piece excision may need meticulous cauterization and may result in uncontrollable bleeding; therefore, it should be used only in favorable cases.
- Standard dural closure: Watertight closure is accomplished with silk or prolene sutures.
- Standard bone flap replacement: Osteosynthesis is carried out with titanium or resorbable plates or with silk sutures.
- Muscle and skin: Closure of the muscles and the superficial layers is carried out in a standard fashion (layer by layer, resorbable sutures) after careful hemostasis.
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