Midline Supracerebellar Craniotomy Free
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The supracerebellar craniotomy is one of the most underutilized surgical approaches in neurosurgery. Its flexibility as a far-reaching corridor to the posterior medial temporal lobe, mesencephalon, posterior third ventricle and thalamus has only been recently sufficiently explored.
The supracerebellar approach exploits the natural subdural space along the supracerebellar space to allow the surgeon to avoid brain transgression and reach the pineal region, posterolateral mesencephalon, and posterior third ventricle. The operative corridor is narrow, deep, and presents technical challenges.
A continuum of supracerebellar infratentorial approaches to the posterior and lateral tentorial incisural space have been explored as one moves further away from the midline. These approaches have coalesced into the classic midline approach, the lateral or paramedian approach, and the far lateral approach. Other approaches to the region include the transtentorial supracerebellar, occipital transtentorial, and transcallosal interhemispheric variations.
I do not use the occipital transtentorial and transcallosal interhemispheric variations as these approaches often place other normal supratentorial structures at risk. The infratentorial approach is very well tolerated by patients despite manipulation of the cerebellum and can be expanded to obviate the need for these other pathways for reaching almost all lesions of the area.
The midline bilateral suboccipital supracerebellar route is traditionally designed for exposing pineal region tumors. The limitations of this approach include limited lateral or inferior visualization caused by the angle of the tentorium and the obstructive apex of the culmen, respectively. Almost all midline bridging vermian veins are invariably sacrificed; this maneuver is not without risks.
I recently stopped using the midline bilateral suboccipital supracerebellar route for even large midline pineal region tumors in favor of the left-sided paramedian supracerebellar approach to offset some of these disadvantages. The trajectory over the lateral cerebellum or the quadrangular lobule is more direct and less steep than the one over the apex of the culmen. The paramedian corridor spares most midline vermian veins.
I believe the advantages of the paramedian versus a midline suboccipital craniotomy are similar to those of a frontolateral or pterional craniotomy versus a bifrontal craniotomy for resection of large olfactory groove meningiomas.
In this chapter, I describe the traditional midline supracerebellar approach. In the next chapter, I focus on the paramedian supracerebellar approach that provides all necessary operative access to the pineal, posterolateral mesencephalon, posterior third ventricle, and posterior basal temporal lobe.
Indications for the Approach
The supracerebellar approach is useful for exposing pineal region tumors such as germ cell tumors, pineoblastomas, astrocytomas, and other rare lesions such as meningiomas, epidermoid tumors, and pilocytic astrocytomas. Importantly, this is an alternative and my preferred approach for resection of posterior third ventricular tumors because gentle manipulation of the pulvinar is well tolerated by patients. Vascular lesions such as tentorial arteriovenous fistulae and malformations are amenable to this approach.
Other intraparenchymal posterolateral mesencephalic lesions, such as cavernous malformations and pilocytic astrocytomas, may be reached through this route. Distal superior cerebellar artery aneurysms may also be accessed.
The transtentorial extension of the paramedian supracerebellar operative corridor is innovative. Sectioning the tentorium through the supracerebellar space allows removal of the supratentorial extension of petrous apex meningiomas and avoids the need for a second-stage subtemporal surgery. Posterior hippocampal cavernous malformations, arteriovenous malformations, astrocytomas, and metastasis can be resected through this approach. This route is ideal for excising medial tentorial meningiomas while obviating the need for temporal lobe retraction to reach the medial tentorium through the subtemporal pathway.
Preoperative MR imaging discloses the extent of the tumor and the need to use a combined or transtentorial corridor. Obstructive hydrocephalus requires preparation of the Keen’s point or a preoperative frontal ventriculostomy. The paramedian incision can readily uncover the bony area corresponding to the burr hole for the Keen’s point.
If the surgeon is considering placing the patient in a sitting position during surgery to reach the pineal region, appropriate measures are required before surgery for detection and management of a possible venous air embolism. These measures may include a right heart catheter, transthoracic Doppler, and transesophageal echocardiography. A preoperative “bubble test” will reveal a patent foramen ovale. The sitting position may have its strongest indication for resection of pineal region tumors because gravity retraction mobilizes the cerebellum inferiorly and expands the operative corridor to the deep pineal region.
The transverse and sigmoid sinuses can have slightly variable courses and their preoperative study can enhance the safety of the craniotomy. Factors such as a steep tentorial angle and a very obese patient with a short neck, although not contraindications to the use of supracerebellar route, can make the operation more challenging. The patient’s neck flexion may ameliorate difficult working angles over the cerebellum, and it is recommended that the patient be placed in a sitting position in these situations.
MR images can provide critical information about the relationship of the deep venous structures (vein of Galen, basal vein of Rosenthal, internal cerebral veins, and straight sinus) to the operative trajectory and tumor. Occasionally, posterior thalamic and vermian tumors mimic pineal region masses and displace the diencephalic veins posteriorly; this configuration is a potential contraindication for use of the supracerebellar approach. The degree of tumor infiltration through the surrounding critical neural structures (e.g., midbrain, thalamus) must be studied before surgery.
MIDLINE SUPRACEREBELLAR CRANIOTOMY
As stated above, I prefer to use the paramedian supracerebellar approach to reach the pineal, posterolateral mesencephalon, and posterior third ventricular regions with the patient placed in a modified park-bench position. However, since most surgeons use the midline bilateral suboccipital supracerebellar craniotomy to reach these structures with the patient in the park-bench or sitting position, the midline approach is reviewed in the following sections.
The dura must be closed in a watertight fashion because occurrence of postoperative cerebrospinal fluid fistulae is a significant risk after tumor operations in the posterior fossa. I prefer to avoid using an allograft to reconstruct the dural defect and instead use a piece of pericranial autograft.
The bone may be replaced using cranial plates. I avoid placing the deep neck muscles under significant tension and minimize their strangulation by the deep sutures because this maneuver causes muscle necrosis and uncontrolled postoperative pain. The neck muscles are gently approximated. The fascia is closed in a watertight fashion.
The patient is observed in the intensive care unit for a day or two after surgery and then transferred to the ward. Steroids are administered prophylactically to minimize the risk of aseptic meningitis. If preoperative hydrocephalus was present and a ventricular catheter was implanted intraoperatively, this catheter should be left in place during surgery and weaned off after surgery.
Aggressive retraction on the cerebellum can lead to postoperative edema. This can be seen on imaging and can ultimately cause symptomatic posterior fossa tension and a need for decompression. Therefore, caution should be exercised during dural closure and bone flap replacement. If the brain appears swollen, the dural closure should not cause more tension, and the bone flap should not be replaced. This brain swelling can be potentially compounded by partial transverse sinus thrombosis and vermian veins sacrifice.
Pearls and Pitfalls
- Compared with the bilateral midline suboccipital supracerebellar approach, the paramedian supracerebellar approach is less invasive and provides adequate exposure for resection of large pineal region tumors while placing bilateral dural venous structures and cerebellar hemispheres at less risk.
- The use of tentorial retraction sutures to rotate and elevate the transverse sinuses expands the supracerebellar operative corridor.
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