Occipital Transtentorial Approach
• An occipital transtentorial craniotomy can provide excellent exposure for falcitentorial meningiomas and any lesion arising from the precentral cerebellar fissure, posterior incisural space, and adjoining structures.Contraindications
• Standard medical contraindications for prone positioning• Patent foramen ovale with positive bubble study for sitting position, owing to risks arising from venous air embolismPlanning and positioning
• Standard preoperative magnetic resonance imaging (MRI) is needed as well as magnetic resonance venography or angiography to confirm patency of the straight sinus. Preoperative visual field testing is required as a baseline for all patients with larger tumors and greater risk of transient cortical blindness.
Occipital Transtentorial Approach 11-1: For small tumors (<3 cm), a unilateral approach with the ipsilateral lobe down is sufficient. For most patients, a lateral or semilateral position is adequate. An approach from the right is preferred because a right hemianopsia, resulting from a left-sided approach, produces greater difficulty with reading. For the lateral position, us1ing arm extension allows the shoulder to drop down avoiding collision of the chin with the clavicle. For larger patients, a modified park bench position is necessary.
Occipital Transtentorial Approach 11-2: For larger tumors (>3 cm), a bilateral occipital transtentorial approach is needed. Patients can be placed prone or in the sitting position. Patients with a large body habitus benefit from the sitting position because high intrathoracic pressures in the prone position can complicate exposure.
• Preoperative embolization is safe if the blood supply arises from external branches and the meningohypophysial branches of the internal carotid arteries.
• The operating room setup may include bipolar cautery and bovie cautery, operating microscope (foot pedal for focus and zoom, mouthpiece for fine adjustments), chair with arm rests and floor wheels, and neurophysiologic monitoring with somatosensory evoked potentials.• Anesthesia includes 1 g of ceftriaxone, 10 mg of dexamethasone (Decadron), and 1 g/kg of mannitol on skin incision. Cerebral perfusion pressure should be maintained at greater than 70 mm Hg to prevent ischemia from brain retraction. Severe hypertension should be treated aggressively (e.g., propofol, thiopental, vasoactive drugs).
Positioning for Occipital Transtentorial Craniotomy
• Patient prone or sitting with head fixed in Mayfield head holder
• Prone position: neck extended on the chest, head flexed on the neck
• Armored endotracheal tube to prevent kinking• Bilateral kidney rests to allow operating table to be laterally rotated
Occipital Transtentorial Craniotomy
• An external ventricular drain via a parietooccipital trajectory or a lumbar drain should be placed.
Occipital Transtentorial Approach 11-3: After adequate positioning, a U-shaped, inferiorly based incision is made extending supratentorially and infratentorially. This flap provides excellent exposure to both occipital lobes and both cerebellar hemispheres.
Tips from the masters
• For larger tumors, a two-surgeon team is recommended for efficient tumor removal and minimal surgeon fatigue.
• The foramen magnum is opened at the surgeon’s discretion with significant infratentorial components.
• The infratentorial portion is more often the exophytic component of the tumor.
• Patience is necessary when dissecting the vein of Galen.
• The pial surface is covered with rubber dams during the procedure to minimize retraction injury.
• Patients and families should be counseled preoperatively regarding the risks of transient cortical blindness. Several days of transient cortical blindness should be expected after larger tumor resections.
Retraction edema can be minimized by careful placement of the initial retractor, brain relaxation with release of cerebrospinal fluid, and intermittent placement and replacement of the retractors.
Avoidance of venous infarction requires extensive preoperative vascular imaging, including magnetic resonance venography or formal angiography or both to ascertain patent and occluded structures.
The most difficult portion of the case is finding and preserving the straight sinus when it is patent.
Dural leaflets may be expanded by tumor and create false walls within the tumor.
• Uncontrolled external cerebral herniation after decompression can risk primary closure of the scalp. If this possibility is suspected in advance, it is wise to obtain hemostasis and be prepared to close before the dura is opened. • In especially urgent cases, such as cases with recent development of anisocoria and an underlying subdural hematoma, making a cruciate opening in the dura through the first burr hole may provide some relief of intracranial hypertension during the craniotomy.