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CLINICALLY NONFUNCTIONING PITUITARY TUMOR

CLINICALLY NONFUNCTIONING PITUITARY TUMOR

Clinically nonfunctioning pituitary tumors are identified either incidentally (e.g., on head magnetic resonance imaging [MRI] to evaluate unrelated symptoms) or because of sellar mass-related symptoms (e.g., visual field defect). On the basis of autopsy studies, pituitary microadenomas (10 mm in largest dimension) are relatively common, present in approximately 11% of all pituitary glands examined. However, pituitary macroadenomas (>10 mm in largest dimension) are uncommon. Immunohistochemical studies on resected pituitary adenomas can determine the adenohypophyseal cell of origin. The most frequent type of pituitary macroadenoma is the gonadotroph cell adenoma; most do not hypersecrete gonadotropins; thus, affected patients do not present with a hormone excess syndrome. The second most common clinically nonfunctioning pituitary macroadenoma is the null cell adenoma that is not basophilic or acidophilic (chromophobe adenoma); this is a benign neoplasm of adenohypophyseal cells that stains negatively for any anterior pituitary hormone on immunohistochemistry. Rarely, lactotroph, somatotroph, and corticotroph pituitary adenomas may be clinically silent.

CLINICALLY NONFUNCTIONING PITUITARY TUMOR
Plate 1-24


The mass-effect symptoms in patients with clinically nonfunctioning pituitary macroadenomas usually prompt evaluation with head MRI. Suprasellar extension of the pituitary adenoma causes compression of the optic chiasm, resulting in the gradual onset of superior bitemporal quadrantopia that may progress to complete bitemporal hemianopsia (see Plate 1-11). Because the onset is gradual, patients may not recognize vision loss until it becomes marked. Additional mass-effect symptoms from an enlarging sellar mass include diplopia (with cavernous sinus extension and oculomotor nerve compression), varying degrees of pituitary insufficiency (related to compression of the normal pituitary gland by the macroadenoma), and headaches.

MRI is the imaging of choice to evaluate the sella and surrounding structures. MRI clearly shows the degree of suprasellar and parasellar extension of pituitary macroadenomas. All patients with pituitary macroadenomas should be assessed for tumoral hyperfunction, compression-related hypopituitarism, and visual field defects. Nonfunctioning pituitary macroadenomas are often associated with mild hyperprolactinemia (e.g., serum prolactin concentration between 30 and 200 ng/mL) because of pituitary stalk compression and prevention of hypothalamic dopamine (prolactin inhibitory factor) from reaching all of the anterior pituitary lactotrophs. Pituitary lactotrophs are the only anterior pituitary cells that are under continuous inhibitory control from the hypothalamus. Additional pituitary-related hormones that should be measured in all patients with pituitary macroadenomas include luteinizing hormone, folliclestimulating hormone,α-subunit of glycoprotein hormones, target gonadal hormone (estrogen in women and testosterone in men), insulinlike growth factor 1, corticotropin, cortisol, thyrotropin, and free thyroxine. Diabetes insipidus is rare in patients with benign tumors of the adenohypophysis.

The goals of treatment are to correct mass-effect symptoms (e.g., vision loss) and to preserve pituitary function. Currently, no effective pharmacologic options are available to treat patients with clinically nonfunctioning pituitary tumors. Observation is a reasonable management approach in elderly patients who have normal visual fields. However, intervention should be considered in all patients with vision loss. Transsphenoidal surgery (see Plate 1-31) can provide prompt resolution of visual field defects and a permanent cure. If present preoperatively, pituitary insufficiency may recover in some patients after operation. Effectiveness of transsphenoidal surgery is assessed by the findings on postoperative MRI (typically performed 3 months after surgery) and by blood levels of adenoma secretory products that may have been increased before surgery (e.g., α-subunit of glycoprotein hormones). Recurrence of the pituitary adenoma after transsphenoidal surgery can be treated with stereotactic Gamma knife radiotherapy.