Anesthetic Management for Posterior Fossa Surgery

Sitting Position

To establish the sitting position, the patient’s skull is most often secured in a three-pin head holder. The arterial pressure transducer is zeroed at the skull base during positioning and throughout the procedure to make maintenance of adequate cerebral perfusion pressure (CPP) easier. Bony prominences should be well padded, the elbows supported by padding to avoid contact with the table or stretch on the brachial plexus, and the legs freed of pressure at the level of the common peroneal nerve just distal and lateral to the head of the fibula. Efforts to prevent cervical cord stretching and obstruction of venous drainage from the face and tongue include maintenance of at least a 2.5 cm space between chin and chest, avoidance of large oral airways or bite blocks in the pharynx, and avoidance of excessive neck rotation, especially in elderly patients. Abdominal compression, lower extremity ischemia, and sciatic nerve injury are prevented by avoidance of excessive flexion of the knees toward the chest.

A “lounge chair” modification of the sitting position, with the thoracic cage raised 30 to 45 degrees, may be used for lateral lesions. Access to more midline structures may be impeded by the degree of neck flexion required. Another modification, the lateral park bench, allows rapid head lowering to the left lateral decubitus position and continuation of the operation in the event of hypotension or persistent VAE.

For the anesthesiologist, the advantages of the sitting position include lower airway pressures, ease of diaphragmatic excursion and improved ability for hyperventilation; better access to the endotracheal tube and thorax for monitoring; improved access to the extremities for monitoring, fluid or blood administration and blood sampling; and better visualization of the face for observation of motor responses during cranial nerve stimulation.

Improved postoperative cranial nerve function has been reported in patients undergoing acoustic neuroma resection in the sitting position than in those operated on in horizontal positions. Relative contraindications to the sitting position are known intracardiac septal defects, known pulmonary arteriovenous malformations, severe hypovolemia, cachexia, or severe hydrocephalus.

Prone Position

The prone position is associated with a lower incidence of VAE. ^, However, the patient’s head is usually elevated above the heart to decrease venous bleeding, so the risk of VAE is not eliminated. Access to superior posterior fossa structures and ease of head manipulation is not as favorable as in the sitting position; the sitting position may also offer better operating conditions for high cervical decompression, in which neck flexion and weight-bearing on the head are detrimental.

When the patient is in the head-elevated position, placement of the shoulders at or above the edge of the operating table back prevents the face from becoming compressed against the cephalad edge of the table when it is inclined. Eye compression can produce blindness from retinal artery thrombosis; this risk is greater for prone and lateral patient positions, particularly when a padded facial headrest is used. Conjunctival edema is a benign consequence of the prone position that resolves quickly. Visual loss from a variety of mechanisms, usually perioperative ischemic optic neuropathy, is a rare but catastrophic outcome of operative intervention and may be of particular relevance in spinal fusion procedures. Venous pooling sufficient to impair venous return can occur in the lower extremities when they lie below the right atrium. Elderly, debilitated patients may not tolerate even a brief discontinuation of monitoring during the turn to the prone position without suffering severe hypotension. In these patients, monitoring cables and transducers should be oriented to allow uninterrupted electrocardiogram (ECG) and arterial blood pressure monitoring throughout the turn to the prone position and positioning adjustments.

Lateral, Three-Quarter Prone, and Park-Bench Positions

The lateral position is used for unilateral neurosurgical procedures in the upper posterior fossa. The three-quarter prone position, a modification of the prone and lateral positions, and the park-bench position are used for similar procedures to permit greater head rotation and access to more axial structures. The supine retrosigmoid position is often easier and quicker to perform. Although it provides inferior surgical exposure, it may be preferred because the time required for placing a patient in this position is less.

Risk–Benefit Analysis of Sitting Position Compared with Other Positions

The usefulness or appropriateness of the sitting surgical position for access to the posterior fossa is still a matter of debate among neurosurgeons and neuroanesthesiologists, because alternative positions can be used for posterior fossa access and the occurrence of VAE is more common and severe in posterior fossa procedures performed in the sitting position than in alternative positions. Investigators from different institutions have reported their experience with the sitting position, with particular emphasis placed on complications and outcome ( Table 12.1 ). ^, Some of the reported complications might have been prevented or reduced if the sitting position had not been used ( Table 12.2 ).

Induction of Anesthesia

Direct arterial blood pressure monitoring established before induction of anesthesia allows tighter control of blood pressure and CPP during induction and intubation, especially in patients at risk for increased ICP. The use of a low-dose, narcotic-based (4 to 6 μg/kg fentanyl), muscle relaxant technique with 0.5 to 1.0 MAC volatile inhalational anesthetic after intravenous induction with thiopental or propofol affords adequate analgesia and amnesia, preservation of autonomic nervous system activity, and rapid awakening after discontinuation of the inhalational anesthetics, allowing an early postoperative neurologic examination if desired. Some anesthesiologists continue to use nitrous oxide in oxygen (typically 50%) unless air embolism occurs, but with agents such as desflurane, propofol, dexmedetomidine, remifentanil, and sufentanil there appears to be little advantage to nitrous oxide. A propofol infusion (50–100 μg/kg/min) often provides better surgical access than inhalational anesthetic alone. Remifentanil and sufentanil infusions have a MAC sparing effect on both propofol and inhaled agents. Moreover, they are effective as akinetic agents for unparalyzed patients (eg, cranial nerve monitoring). β-Adrenergic blocking drugs and direct-acting vasodilators may be used alone or in combination to treat increases in blood pressure (instead of anesthetics). Use of long-acting antihypertensive drugs is avoided until the patient has been placed in the operating position. The need for vasopressor administration may arise after induction of anesthesia or positioning, especially in chronically hypertensive or debilitated patients. Short-acting drugs, such as small boluses of ephedrine or phenylephrine, are usually effective. Rarely, after all correctable derangements such as hypovolemia have been ruled out, inotrope infusions may be required throughout the surgical procedure, but a cause for an underlying mechanism should be sought.

Verification of appropriate placement of the endotracheal tube after final positioning, but before surgical incision, is of utmost importance, regardless of the position employed. Intraoperative access to the airway is limited by virtue of the proximity of the operative site, and neck flexion or extension can produce caudad or cephalad displacement of the endotracheal tube, respectively, by as much as 2 cm. Palpation of the endotracheal tube cuff above the sternal notch is a useful maneuver to ensure that the tip of the endotracheal tube rises above the carina, though this is not possible with many types of endotracheal tubes.

Maintenance of Anesthesia

Controlled positive-pressure ventilation with paralysis has the following advantages:

• Maintenance of lighter levels of anesthesia

• Hyperventilation, which diminishes PaCO ₂ , thereby decreasing both sympathetic stimulation and blood pressure at any given depth of anesthesia

• Cerebral vasoconstriction

• Less bleeding

• Lower ICP

• Less cardiovascular depression because of decreased anesthetic depth

• Less likelihood of patient movement.

The MAC for desflurane (and presumably other anesthetic drugs) is not altered by the sitting position. Excessive decreases in inhaled agent concentration as a strategy to combat hypotension may allow awareness. Intravenous anesthesia has been associated with smaller increases in CBF, and ICP, and less brain swelling, possibly improving surgical conditions. Intraoperative hypothermia should be avoided. Glucose-containing solutions are not used because of the possible detrimental effects of hyperglycemia on areas of the brain at risk for cerebral ischemia.

The administration of osmotic and loop diuretics for tumor resection and vascular procedures may predispose sitting patients to electrolyte disturbances or cardiovascular instability caused by hypovolemia. Also, the size of the pneumocephalus may be increased.