Hypopituitarismrelatedeffects:Hypopituitarismiscaused by deficiency of trophic hormones and results from a variety of destructive processes that may damage the pituitary, including ischemic injury, surgery, radiation, and inflammatory reactions. In addition, nonfunctional pituitary adenomas may encroach upon and destroy the normal anterior pituitary, causing hypopituitarism.
Local mass effects: Among the earliest changes associated with pituitary neoplasms are radiographic abnormalities of the sella turcica, including sellar expansion, bony erosion, and disruption of the diaphragma sellae. Because of the close proximity of the optic nerves and chiasm to the sella, expanding pituitary lesions often compress decussating fibers in the optic chiasm. This can give rise to visual field abnormalities, classically in the form of defects in the lateral (temporal) visual fields—a so-called “bitemporal hemianopsia”. As in the case of any expanding intracranial mass, pituitary tumors may produce signs and symptoms of elevated intracranial pressure, including headache, nausea, and vomiting. Pituitary neoplasms that extend beyond the sella turcica into the base of the brain produce seizures or obstructive hydrocephalus; involvement of cranial nerves can result in cranial nerve palsy. On occasion, acute hemorrhage into a pituitary neoplasm is associated with rapid enlargement of the lesion and loss of consciousness, a situation termed pituitary apoplexy. Acute pituitary apoplexy may be rapidly fatal and is therefore considered to be a neurosurgical emergency.
The most common cause of anterior pituitary disorders are pituitary tumors, most of which are benign adenomas. Some of these produce hormones and result in endocrine abnormalities, and others are nonfunctional and produce mass effects. We discuss first the general features of pituitary adenomas, then specific tumors.
PituitaryAdenomas:GeneralFeatures
The most common cause of hyperpituitarism is a hormone-producing adenoma arising in the anterior lobe. Other, less common, causes include hyperplasia and carcinomas of the anterior pituitary, secretion of hormones by some extrapituitary tumors, and certain hypothalamic disorders. Some salient features of pituitary adenomas are as follows: • Pituitary adenomas are classified on the basis of hormone(s) produced by the neoplastic cells, which are detected by immunohistochemical stains performed on tissue sections (
Table 20.1 http://ebooksmedicine.net
Fig.20.2Thehypothalamic/pituitaryaxis.Thehypothalamusregulatesthesecretionofhormonesfromtheadenohypophysis(anteriorpituitarygland)byreleasingstimulatory(corticotropin-releasinghormone,CRH;growthhormone–releasinghormone,GHRH;gonadotropin-releasinghormone,GnRH;thyrotropin-releasinghormone(TRH)andinhibitoryfactors(growthhormoneinhibitoryhormone,GIHorsomatostatin;prolactininhibitoryfactor,PIFordopamine).Theseinturnmodulatethereleaseofsixhormonesfromtheanteriorpituitary:adrenocorticotropichormone(ACTHorcorticotropin);follicle-stimulatinghormone(FSH);growthhormone(GH,orsomatotropin);luteinizinghormone(LH);prolactin(PRL);andthyroid-stimulatinghormone(TSH,orthyrotropin).