Pituitary adenomas are noncancerous tumors that occur in the pituitary gland. Pituitary adenomas are generally divided into three categories dependent upon their biological functioning: benign adenoma, invasive adenoma or carcinomas, with carcinomas accounting for 0.1% to 0.2%, approximately 35% being invasive adenomas and most being benign adenomas. Pituitary adenomas represent from 10% to 25% of all intracranial neoplasms and the estimated prevalence rate in the general population is approximately 17%.
Invasive adenomas may invade the dura mater, cranial bone, or sphenoid bone. Although previously believed that clinically active pituitary adenomas were rare, recent studies have suggested that they may affect approximately one in 1000 of the general population.

Classification

• Anatomically pituitary tumors are classified by their size based on radiological findings; either microadenomas (less than <10 mm) or macroadenomas (equal or greater than ≥I0 mm).

Classification based on radioanatomical findings places adenomas into 1 of 4 grades (I–IV):

Stage I: microadenomas (<1 cm) without sella expansion.

Stage II: macroadenomas (≥1 cm) and may extend above the sella.

Stage III: macroadenomas with enlargement and invasion of the floor or suprasellar extension.

Stage IV is destruction of the sella.

• Histological classification utilizes an immunohistological characterization of the tumors in terms of their hormone production. Historically they were classed as either basophilic, acidophilic, or chromophobic on the basis of whether or not they took up the tinctorial stains hematoxylin and eosin. This classification has fallen into disuse, in favor of a classification based on what type of hormone is secreted by the tumor. Approximately 20-25% of adenomas do not secrete any readily identifiable active hormones ('non-functioning tumors') yet they are still sometimes referred to as 'chromophobic'.

• Functional classification is based upon the tumors endocrine activity as determined by serum hormone levels detected via immunohistochemical staining. The "Percentage of hormone production cases" values are the fractions of adenomas producing each related hormone of each tumor type as compared to all cases of pituitary tumors, and does not directly correlate to the percentages of each tumor type because of smaller or greater incidences of absence of secretion of the expected hormone. Thus, nonsecretive adenomas may be either null cell adenomas or a more specific adenoma that, however, remains nonsecretive.

Symptoms

Hormone secreting pituitary adenomas cause one of several forms of hyperpituitarism. The specifics depend on the type of hormone. Some tumors secrete more than one hormone, the most common combination being GH and prolactin.

A pituitary adenoma may present with visual field defects, classically bitemporal hemianopia. It arises from the compression of the optic nerve by the tumor. The specific area of the visual pathway at which compression by these tumours occurs is at the optic chiasma.
Various psychiatric manifestations have been associated with pituitary disorders including pituitary adenomas. Psychiatric symptoms such as depression, anxiety apathy, emotional instability, easy irritability and hostility have been noted.
Complications
- Acromegaly is a syndrome that results when the anterior pituitary gland produces excess growth hormone (GH).The disease which is often also associated with gigantism, is difficult to diagnose in the early stages and is frequently missed for many years, until changes in external features, especially of the face, become noticeable with the median time from the development of initial symptoms to diagnosis being twelve years.
- Cushing's syndrome is a hormonal disorder that causes hypercortisolism, which is elevated levels of cortisol in the blood. Cushing's disease (CD) is the most frequent cause of Cushing's syndrome, responsible for approximately 70% of cases.
- Hyperpituitarism is a disease of the anterior lobe of the pituitary gland which is usually caused by a functional pituitary adenoma and results in hypersecretion of adenohypophyseal hormones such as growth hormone; prolactin; thyrotropin; luteinizing hormone; follicle stimulating hormone; and adrenocorticotropic hormone.
- Pituitary apoplexy is a condition that occurs when pituitary adenomas suddenly hemorrhage internally, causing a rapid increase in size or when the tumor outgrows its blood supply which causes tissue necrosis and subsequent swelling of the dead tissue. Pituitary apoplexy often presents with visual loss and sudden onset headache and requires timely treatment often with corticosteroids and if necessary surgical intervention.
- Central diabetes insipidus is caused by diminished production of the antidiuretic hormone vasopressin that causes severe thirst and excessive production of very dilute urine (polyuria) which can lead to dehydration.

Treatment

• Prolactinomas are most often treated with bromocriptine or more recently, cabergoline or quinagolide which decrease tumor size as well as alleviates symptoms, both dopamine agonists, and followed by serial imaging to detect any increase in size. Treatment where the tumor is large can be with radiation therapy or surgery, and patients generally respond well. Efforts have been made to use a progesterone antagonist for the treatment of prolactinomas, but so far have not proved successful.
• Somatotrophic adenomas respond to octreotide, a long-acting somatostatin analog, in many but not all cases according to a review of the medical literature. Unlike prolactinomas, thyrotrophic adenomas characteristically respond poorly to dopamine agonist treatment.
• Surgery is a common treatment for pituitary tumors. Trans-sphenoidal adenectomy surgery can often remove the tumor without affecting other parts of the brain. Endoscopic surgery has become common recently.