Subspecialty Management |
|Chapter 52:||Neurosurgical Anesthesia|
The transsphenoidal approach to the pituitary is used for the excision of tumors that lie within the sella or that have extension to the immediate suprasellar area. The most common lesions are prolactin-secreting microadenomas (Table 52–13). These patients are usually women who present with secondary amenorrhea. There are three other less common pituitary tumors: growth hormonesecreting lesions result in acromegaly; adrenocorticotropic hormone (ACTH)secreting tumors cause Cushing disease; and a very rare thyroid-stimulating hormone (TSH)secreting lesion that results in hyperthyroidism.
TABLE 52–13. Tumors of the Pituitary Region
The important preoperative considerations relate to the patients endocrine status. In general, as a pituitary lesion expands and compresses the pituitary tissue, the sequence in which hormonal function is lost is as follows: (1) gonadotropins; (2) growth hormone; (3) ACTH; and (4) TSH. The precise definition of the adrenal status of these patients is often not important because, in general, they all receive adrenal hormone supplementation at least temporarily. However, profound hypocortisolism, with associated hyponatremia, should be corrected preoperatively. It is, in fact, uncommon for thyroid deficiency to occur. However, hypothyroidism should be sought and corrected preoperatively because hypothyroid patients have a diminished tolerance for the cardiovascular-depressant effects of anesthetic agents. Patients with advanced acromegaly can develop an enlarged tongue, and the airway should be evaluated.
Many practitioners place an arterial catheter, but it is not absolutely necessary. Blood sampling access is a valuable adjunct to postoperative care if diabetes insipidus develops. Blood loss is usually modest. However, the cavernous sinus is an immediate lateral relation of the pituitary and may be entered during the resection of large tumors. In addition, in some patients, there is a venous sinusoid that lies in front of the pituitary gland and connects the two cavernous sinuses. This can be the origin of substantial blood loss. It has, on occasion, actually precluded this approach to the pituitary gland.
The latitudes are broad with respect to choice of agent, although tumors with suprasellar extension can cause hydrocephalus and thereby can add increased ICP constraints to the anesthetic technique. The procedure is performed with the patient in a supine position, usually with some degree of head-up posture to avoid venous engorgement. A pharyngeal pack prevents an accumulation of blood in the stomach (which causes vomiting) or in the glottis (which contributes to coughing at extubation). An RAE-type tube secured to the lower jaw at the corner of the mouth opposite the surgeons dominant hand, such as the left corner of the mouth for a right-handed surgeon, is suitable. A small esophageal stethoscope and a temperature probe can lie with the endotracheal tube. Covering the entire bundle with a towel drape (a plastic sheet with an adhesive edge) placed just below the patients lower lip so that it hangs from the lower jaw like a veil protects it from the preparation solutions.
The procedure requires a C-arm image intensifier (lateral views), and the patients head and arms are relatively inaccessible once the patient is draped. It is appropriate to establish the nerve stimulator at a lower extremity site. The surgical approach is via the nasal cavity through an incision made under the upper lip. During the approach, the mucosal surfaces within the nose are infiltrated with a local anesthetic and epinephrine solution, and the patient should be observed for the occurrence of dysrhythmias.
Surgical preferences for CO2 management vary. In some instances, hypocapnia is requested to reduce brain volume and thereby to minimize the degree to which the arachnoid bulges into the sella. One of the important surgical considerations is the avoidance, when possible, of opening the arachnoid. Postoperative CSF leaks can be persistent and are associated with a considerable risk of meningitis. By contrast, when there is suprasellar extension of a tumor, a normal or increased CO2 helps to deliver the lesion into the sella for excision. As an alternative way to accomplish this, some surgeons have resorted to pumping of saline or air into the lumbar CSF space. 229, 230
Diabetes insipidus is a potential complication of this procedure. The antidiuretic hormone (ADH) is synthesized in the supraoptic nuclei of the hypothalamus and is transported down the supraoptic-hypophyseal tract to the posterior lobe of the pituitary. This portion of the pituitary gland is frequently spared. Even when it is excised, water homeostasis commonly normalizes, presumably because the ADH is released from the cut end of the tract. However, even when the posterior lobe of the pituitary is left intact, transient diabetes insipidus may occur. This disorder usually occurs 4 to 12 hours postoperatively and very rarely arises intraoperatively. The clinical picture is one of polyuria in association with a rising serum osmolality. The diagnosis is made by comparison of the osmolalities of urine and serum. Hypo-osmolar urine in the face of an elevated and rising serum osmolality strongly supports the diagnosis. Urine specific gravity is a useful bedside test. In the presence of bona fide diabetes insipidus, specific gravity is low, that is, less than or equal to 1.002.
When the diagnosis of diabetes insipidus is established, an appropriate fluid management regimen is hourly maintenance fluids plus two-thirds of the previous hours urine output. (An acceptable alternative is the previous hours urine output minus 50 mL plus maintenance.) The choice of fluid is dictated by the patients electrolyte picture. In general, the patient is losing fluid that is hypo-osmolar and relatively low in sodium. Half-normal saline and 5 percent dextrose in water (D5 W) are commonly used as replacement fluids. Beware of hyperglycemia when large volumes of D5 W are employed. An unacceptable fluid regimen that has been employed calls for maintenance fluids plus the previous hours urine output. This regimen has the potential to create a vicious circle. Should the patient become iatrogenically fluid overloaded, this regimen precludes a return to isovolemia, and, in fact, when the maintenance fluid allowance is generous, it guarantees that the patient will become increasingly hypervolemic. If the hourly requirement exceeds 350 to 400 mL, ADH is usually administered.
A smooth emergence from anesthesia (see earlier) is desirable especially if the CSF space has been opened (and resealed with fibrin glue or by packing the sphenoid sinus with fat or muscle). Repeated, intense Valsalva maneuvers, such as with coughing or vomiting, may contribute to the reopening of a CSF leak and may worsen the risk of subsequent meningitis. The airway should be cleared of debris including formed clot. Some clinicians routinely inspect the pharynx using a laryngoscope. This also permits one to assess whether or not there is still active bleeding. This allows one more confidently to extubate the patient promptly at the first signs of reactivity to the endotracheal tube. In situations in which there is concern that a persistent CSF leak may occur, some surgeons place a lumbar CSF drain to maintain CSF decompression in the early postoperative period.
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