SPECIFIC PROCEDURES

Many of the considerations relevant to individual neurosurgical procedures are generic ones that have already been presented in the section on recurrent issues. The descriptions that follow will highlight only procedure-specific issues

Supratentorial Tumors

Craniotomies for excision or biopsy of supratentorial tumors are among the most common neurosurgical procedures performed. Gliomas and meningiomas are among the most frequent tumors. Relevant preoperative considerations include the patient's ICP status (see the earlier section "Control of Intracranial Pressure/Brain Relaxation") and the location and size of the tumor. Location and size will give the anesthesiologist an indication of the surgical position and the potential for blood loss and, occasionally, will reveal a risk for air embolism. The risk of VAE developing is quite low for most other supratentorial tumors. However, lesions (usually convexity meningiomas) that

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Patients with craniopharyngiomas and pituitary tumors with suprasellar extension may undergo procedures that involve dissection in and around the hypothalamus. Irritation of the hypothalamus can elicit sympathetic responses, including hypertension. Damage to the hypothalamus can result in a spectrum of disturbances in consciousness varying from lethargy to obtundation. Disturbances in water balance may also occur. Diabetes insipidus is the most likely, although the cerebral salt-wasting syndrome can potentially occur. The latter has been very infrequent. The various disturbances in water balance typically have a delayed onset and begin 12 to 24 hours postoperatively rather than in the operating room. Postoperative temperature homeostasis may also be disturbed.

Patients who undergo a craniotomy involving a subfrontal approach may, on occasion, manifest a disturbance of consciousness in the immediate postoperative period. Retraction/irritation of the inferior surfaces of the frontal lobes can result in a patient who is lethargic and will not awaken "cleanly." Patients exhibiting this phenomenon are sometimes referred to "frontal lobey." The phenomenon is more likely to be evident in the event of bilateral subfrontal retraction than when it occurs only unilaterally. The anesthetic implication is that the clinician should be more inclined to confirm the return of consciousness before extubating the patient than to extubate expectantly. A further implication taken by us (though not confirmed by any systematic study) is that less liberal use of fixed agents (narcotics, benzodiazepines) may be appropriate when a bilateral, subfrontal retraction is to be performed. This recommendation is based on the rationale that the low residual concentrations of these drugs that would be compatible with reasonable recovery of consciousness in most patients may be less well tolerated in this population. Subfrontal approaches will be most commonly used in patients with olfactory groove meningiomas and those with suprasellar tumors (craniopharyngiomas and pituitary tumors with suprasellar extension).

Preoperative Preparation

Patients with a significant tumor-related mass effect, especially if tumor-related edema is present, should receive preoperative steroids. If the patient is not receiving steroids, it is the anesthesiologist's responsibility to ask why. A 48-hour course is ideal (see the earlier section "Steroids"), although 24 hours is sufficient for a clinical effect to be evident. Dexamethasone is the most commonly used steroid. A regimen such as 10 mg intravenously or orally followed by 10 mg every 6 hours is typical. Because of concern about producing carbon dioxide retention in a patient whose intracranial compliance is already abnormal, patients with any substantial mass effect are not usually premedicated with anything beyond full-dose reassurance.

Monitoring

Frequently, the nature of the procedure does not require anything more than routine monitoring. However, some situations do argue for invasive monitors (see the section "Monitoring"). Preinduction placement of an arterial line may be appropriate in patients with a severe mass effect and little residual compensatory latitude. It is the period of induction during which hypertension, with its attendant risks in a patient with impaired compliance and autoregulation, is most likely to occur. Procedures with a potential for substantial blood loss (tumors encroaching on the sagittal sinus, large vascular tumors) may also justify arterial or CVP catheters, or both. Is ICP monitoring ever warranted for intraoperative management? In our opinion the answer is no. We have sufficient understanding of the potential impact of anesthetic drugs and techniques that we should be able to manage induction of anesthesia "blind." Then, once the cranium is open, observation of conditions in the surgical field provides equivalent information.

Management of Anesthesia

The principles governing the choice of anesthetics are presented under the subheading "Selection of Anesthetics" in "Control of Intracranial Pressure/Brain Relaxation."