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• This approach provides excellent exposure for lesions of the pineal region, posterior third ventricle, and posterior mesencephalon.
• The angle of the tentorium is an important consideration; this approach is not suitable for patients with a steeply angled tentorium. For such situations, alternative approaches such as the occipital transtentorial approach should be considered.
Planning and positioning
• Preoperative planning includes assessment of the patient’s cardiopulmonary status, evaluation of comorbidities, and basic laboratory tests, including a basic metabolic panel, complete blood count, coagulation profile, and type and screen. Baseline chest x-ray and electrocardiogram are also useful. A preoperative bubble cardi1ac Doppler study is recommended to rule out any possible cardiac shunting or patent foramen ovale.
• Preoperative magnetic resonance imaging (MRI) including magnetic resonance venography is obtained; particular attention is paid to the relationship of the deep venous structures (vein of Galen, basal vein of Rosenthal, internal cerebral veins, and straight sinus) in relation to the trajectory and tumor. Imaging is also assessed for degree of tumor infiltration into surrounding critical neural structures (e.g., midbrain, thalamus).
• A preoperative surgical navigation image is recommended as a surgical adjunct.
• For patients with preoperative hydrocephalus, an intraventricular catheter is placed before soft tissue dissection; this can be placed at the mid-pupillary line on the lambdoid suture.
• We prefer to use the sitting position for this approach. The upright positioning permits the cerebellum to fall with gravity away from the tentorium, in addition to preventing pooling of venous blood in the operative field. The prone position is the only recommended position if the patient has a patent foramen ovale, given the risk of pulmonary air embolism with the sitting position. An intraoperative discussion should be held with the anesthesia team to perform cardiac Doppler during the procedure to prevent a venous air embolism. Precordial Doppler ultrasonography is the most sensitive of the generally available monitors capable of detecting intracardiac air. Placement of a central venous catheter with multiple orifices is strongly recommended as a means of aspirating air from the circulation should a venous air embolism occur.
Infratentorial Approach 10-1: The patient is first placed supine on the operative table (with reverse orientation) (A). After application of Mayfield holder, the bed is maneuvered to raise the patient’s back and flex the legs. The head is flexed to place the tentorium parallel to the floor (B).
Infratentorial Approach 10-2: The skin incision is marked from above the inion down to approximately C2. Registration with surgical navigation can be performed at this point.Procedure
Infratentorial Approach 10-3: Suboccipital exposure is performed with dissection of the suboccipital musculature; the musculature is not detached and is preserved from the spinous processes of C1-2. A craniotomy is performed. Burr holes are placed on each side of the superior sagittal sinus (right above the t1orcular Herophili), and superior and inferior to each transverse sinus a few centimeters distal to the torcular Herophili. A craniotome is used to connect the burr holes to create a bone flap. If there is evidence of preoperative tonsillar descent, the foramen magnum can be removed in addition to a C1 laminectomy. A semilunar or cruciate dural incision is made based on the transverse sinuses and torcular Herophili and reflected superiorly with tenting sutures. The surgeon should be cognizant of the retraction placed on the venous sinus when reflecting the dural flap.
Infratentorial Approach 10-4: Arachnoid adhesions and bridging veins between the cerebellum and tentorium are divided to open the supracerebellar infratentorial corridor. These bridging veins should be divided close to the cerebellum to prevent retraction of inaccessible bleeding sources back into the tentorium. As this process of dissection proceeds, the cerebellum falls with gravity, and a retractor can be placed on the tentorium if necessary.
Infratentorial Approach 10-5: Thickened arachnoid overlying the pineal gland and quadrigeminal cistern is exposed and sharply dissected open. In this process, the precentral cerebellar vein is visualized draining into the vein of Galen—this is the only deep venous structure that should be cauterized and divided.
Infratentorial Approach 10-6: Normal anatomy when exposure is achieved and cerebellar retraction occurs. Depending on the pathology for which this approach is chosen, the vascular structures and neural structures are shifted to nonanatomic positions.
Tips from the masters
• The angle of the tentorium and relationship of venous structures to the tumor are crucial to the success of this approach versus other alternatives.
• The placement of an intraventricular catheter is not only useful for treatment of preoperative hydrocephalus, but also facilitates brain relaxation and decompression of posterior fossa.
• Special preoperative cardiac work-up and planning should be considered if the sitting position is to be used. Communication with the anesthesia team should occur regarding the need of a cardiac Doppler examination intraoperatively with a multiple-channel central line for dealing with a potential air embolism.
• In the event of an intraoperative air embolism, the transverse torcular Herophil1i sinus area should be covered with laparotomy pads, and the field should be flooded with irrigation.
The primary limitation of the sitting position is the risk for air embolism. All patients should have intraoperative monitoring via end-tidal CO2 monitoring and Doppler ultrasound. Formation of emboli is halted by flooding the field with irrigation and lowering the patient’s head. A central venous catheter can be used to retrieve any large emboli.
• If the angle of the tentorium is too steep, the craniotomy can be extended for an occipital transtentorial approach, or the tentorium can be cut and retracted via the supracerebellar approach.