Planning and positioning
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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 cardiac Doppler study is recommended to rule out any possible cardiac shunting or patent foramen ovale.
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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).
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A preoperative surgical navigation image is recommended as a surgical adjunct.
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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.
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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.
Figure 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 ).
