Resection of a Pituitary Tumor
A 41-year-old woman presents to the operating room for resection of a 10-mm pituitary tumor. She had complained of amenorrhea and had started noticing some decrease in visual acuity.
What hormones does the pituitary gland normally secrete?
Functionally and anatomically, the pituitary is divided into two parts: anterior and posterior. The latter is part of the neurohypophysis, which also includes the pituitary stalk and the median eminence.
The anterior pituitary is composed of several cell types, each secreting a specific hormone. Anterior pituitary hormones include adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), growth hormone (GH), the gonadotropins (follicle-stimulating hormone [FSH] and luteinizing hormone [LH]), and prolactin (PRL). Secretion of each of these hormones is regulated by hypothalamic peptides (releasing hormones) that are transported to the adenohypophysis by a capillary portal system. The secretion of FSH, LH, ACTH, TSH, and their respective releasing hormones is also under negative feedback control by the products of their target organs. For example, an increase in circulating thyroid hormone inhibits the secretion of TSH-releasing factor and TSH.
The posterior pituitary secretes antidiuretic hormone (ADH, also called vasopressin) and oxytocin. These hormones are actually formed in supraoptic and paraventricular neurons, respectively, and are transported down axons that terminate in the posterior pituitary. Hypothalamic osmoreceptors, and, to a lesser extent, peripheral vascular stretch receptors, regulate secretion of ADH.
What is the function of these hormones?
ACTH stimulates the adrenal cortex to secrete glucocorticoids. Unlike production of mineralocorticoids, production of glucocorticoids is dependent on ACTH secretion. TSH accelerates the synthesis and release of thyroid hormone (thyroxine). Normal thyroid function is dependent on production of TSH. The gonadotropins FSH and LH are necessary for normal production of testosterone and spermatogenesis in males and cyclic ovarian function in females. GH promotes tissue growth and increases protein synthesis as well as fatty acid mobilization. Its effects on carbohydrate metabolism are to decrease cellular glucose uptake and utilization and increase insulin secretion. PRL functions to support breast development during pregnancy. Dopamine receptor antagonists are known to increase secretion of PRL.
Through its effect on water permeability in renal collecting ducts, ADH regulates extracellular osmolarity and blood volume Oxytocin acts on areolar myoepithelial cells as part of the milk letdown reflex during suckling and enhances uterine activity during labor.
What factors determine the surgical approach in this patient?
The pituitary gland is attached to the brain by a stalk and extends downward to lie in the sella turcica of the sphenoid bone. Anteriorly, posteriorly, and inferiorly, it is bordered by bone. Laterally, it is bordered by the cavernous sinus, which contains cranial nerves III, IV, V1, and VI, as well as the cavernous portion of the carotid artery. Superiorly, the diaphragma sella, a thick dural reflection, usually tightly encircles the stalk and forms the roof of the sella turcica. In close proximity to the stalk lie the optic nerves and chiasm. The hypothalamus lies contiguous and superior to the stalk.
Tumors less than 10 mm in diameter are usually approached via the transsphenoidal route, whereas tumors greater than 20 mm in diameter and with significant suprasellar extension are approached via a bifrontal craniotomy. With the use of prophylactic antibiotics, morbidity and mortality rates are significantly less with the transsphenoidal approach; the operation is carried out with the aid of a microscope through an incision in the gingival mucosa beneath the upper lip. The surgeon enters the nasal cavity, dissects through the nasal septum, and finally penetrates the roof of the sphenoid sinus to enter the floor of the sella turcica.
What are the major problems associated with the transsphenoidal approach?
Problems include (1) the need for mucosal injections of epinephrine-containing solution to reduce bleeding, (2) the accumulation of blood and tissue debris in the pharynx and stomach, (3) the risk of hemorrhage from inadvertent entry into the cavernous sinus or the internal carotid artery, (4) cranial nerve damage, and (5) pituitary hypofunction. Prophylactic administration of glucocorticoids is routinely used in most centers. Diabetes insipidus develops postoperatively in up to 40% of patients but is usually transient. Less commonly, the diabetes insipidus presents intraoperatively. The supine and slightly head-up position used for this procedure may also predispose to venous air embolism.
What type of tumor does this patient have?
Tumors in or around the sella turcica account for 10% to 15% of intracranial neoplasms. Pituitary adenomas are most common, followed by craniopharyngiomas and then parasellar meningiomas. Primary malignant pituitary and metastatic tumors are rare. Pituitary tumors that secrete hormones (functional tumors) usually present early, when they are still relatively small (<10 mm). Other tumors present late, with signs of increased ICP (headache, nausea, and vomiting) or compression of contiguous structures (visual disturbances or pituitary hypofunction). Compression of the optic chiasm classically results in bitemporal hemianopia. Compression of normal pituitary tissue produces progressive endocrine dysfunction. Failure of hormonal secretion usually progresses in the order of gonadotropins, GH, ACTH, and TSH. Diabetes insipidus can also be seen preoperatively. Rarely, hemorrhage into the pituitary results in acute panhypopituitarism (pituitary apoplexy) with signs of a rapidly expanding mass, hemodynamic instability, and hypoglycemia.
This patient has the most common type of secretory adenoma—that producing hyperprolactinemia. Women with this tumor typically have amenorrhea, galactorrhea, or both. Men with prolactin-secreting adenomas may have galactorrhea or infertility, but more commonly present with symptoms of an expanding mass.
What other types of secretory hormones are seen?
Adenomas secreting ACTH (Cushing’s disease) produce classic manifestations of Cushing’s syndrome: truncal obesity, moon facies, abdominal striae, proximal muscle weakness, hypertension, and osteoporosis. Glucose tolerance is typically impaired, but frank diabetes is less common (<20%). Hirsutism, acne, and amenorrhea are also commonly seen in women.
Adenomas that secrete GH are often large and result in either gigantism (prepubertal patients) or acromegaly (adults). Excessive growth prior to epiphyseal fusion results in massive growth of the entire skeleton. After epiphyseal closure, the abnormal growth is limited to soft tissues and acral parts: hands, feet, nose, and mandible. Patients develop osteoarthritis, which often affects the temporomandibular joint and spine. Diabetes, myopathies, and neuropathies are common. Cardiovascular complications include hypertension, premature coronary disease, and cardiomyopathy in some patients. The most serious anesthetic problem encountered in these patients is difficulty in intubating the trachea.
Are any special monitors required for transsphenoidal surgery?
Monitoring should be carried out in somewhat the same way as for craniotomies. Visual evoked potentials may be employed with large tumors that involve the optic nerves. Precordial Doppler sonography may be used for detecting venous air embolism. Venous access with large bore catheters is desirable in the event of massive hemorrhage
What modifications, if any, are necessary in the anesthetic technique?
The same principles discussed for craniotomies apply, particularly if the patient has evidence of increased ICP. Intravenous antibiotic prophylaxis and glucocorticoid coverage (hydrocortisone, 100 mg) are usually given prior to induction. Many clinicians avoid nitrous oxide to prevent problems with a postoperative pneumocephalus (see above). Intense neuromuscular blockade is important to prevent movement while the surgeon is using the microscope. In some circumstances, the surgeon may request placement of a lumbar intrathecal catheter to drain CSF, thereby facilitating surgical exposure.