Pituitary adenomas are tumors that occur in the pituitary gland divided in three categories: benign adenoma, invasive adenoma, or carcinomas.
Pituitary adenomas are tumors that occur in the pituitary gland. They 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%. Non-invasive and non-secreting pituitary adenomas are considered to be benign in the literal as well as the clinical sense; however a recent meta-analysis of available research has shown there are to date scant studies – of poor quality – to either support or refute this assumption.
Pituitary Adenomas which exceed 10 millimetres in size are defined as macroadenomas, with those smaller than 10 mm referred to as microadenomas. Most pituitary adenomas are microadenomas, and have an estimated prevalence of 16.7%. A majority of pituitary microadenomas often remain undiagnosed and those that are diagnosed are often found as an incidental finding, and are referred to as incidentalomas. Pituitary macroadenomas are the most common cause of hypopituitarism, and in the majority of cases they are non-secreting adenomas. 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. Overview The pituitary gland or hypophysis is often referred to as the “master gland” of the human body. Part of the hypothalamic-pituitary axis, it controls most of the body’s endocrine functions via the secretion of various hormones into the circulatory system.
The pituitary gland is located below the brain in a depression of the sphenoid bone known as the sella turcica. Although anatomically and functionally connected to the brain, the pituitary gland sits outside the blood–brain barrier. It is separated from the subarachnoid space by the diaphragma sella, therefore the arachnoid mater and thus cerebral spinal fluid cannot enter the sella turcica. The pituitary gland is divided into two lobes, the anterior lobe, and the posterior lobe separated by the pars intermedia. The posterior lobe of the pituitary gland is not, despite its name, a true gland. The posterior lobe contains axons of neurons that extend from the hypothalamus to which it is connected via the pituitary stalk.
The hormones vasopressin and oxytocin, produced by the neurons of the supraoptic and paraventricular nuclei of the hypothalamus, are stored in the posterior lobe and released from axon endings within the lobe. The pituitary gland’s anterior lobe is a true gland which produces and secretes six different hormones: thyroid-stimulating hormone, adrenocorticotropic hormone, follicle-stimulating hormone, luteinizing hormone, growth hormone, and prolactin. Classification Pituitary adenomas are classified based on upon anatomical, histological and functional criteria. Anatomically pituitary tumors are classified by their size based on radiological findings; either microadenomas or macroadenomas. Classification based on radioanatomical findings places adenomas into 1 of 4 grades: Stage I: microadenomas without sella expansion.
Stage II: macroadenomas 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 yet they are still sometimes referred to as ‘chromophobic’. Functional classification is based upon the tumors endocrine activity as determined by serum hormone levels and pituitary tissue cellular hormone secretion 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. Pituitary incidentalomas Pituitary incidentalomas are pituitary tumors that are characterized as an incidental finding.
They are often discovered by computed tomography or magnetic resonance imaging, performed in the evaluation of unrelated medical conditions such as suspected head trauma, in cancer staging or in the evaluation of nonspecific symptoms such as dizziness and headache. It is not uncommon for them to be discovered at autopsy. In a meta-analysis, adenomas were found in an average of 16.7% in postmortem studies, with most being microadenomas; macrodenomas accounted for only 0.16% to 0.2% of the decedents. While non-secreting, noninvasive pituitary microadenomas are generally considered to be literally as well as clinically benign, there are to date scant studies of low quality to support this assertion.
It has been recommended in the current Clinical Practice Guidelines by the Endocrine Society – a professional, international medical organization in the field of endocrinology and metabolism – that all patients with pituitary incidentalomas undergo a complete medical history and physical examination, laboratory evaluations to screen for hormone hypersecretion and for hypopituitarism. If the lesion is in close proximity to the optic nerves or optic chiasm, a visual field examination should be performed.
For those with incidentalomas which do not require surgical removal, follow up clinical assessments and neuroimaging should be performed as well follow-up visual field examinations for incidentalomas that abut or compress the optic nerve and chiasm and follow-up endocrine testing for macroincidentalomas. Ectopic pituitary adenoma An ectopic pituitary adenoma is a rare type of tumor which occurs outside of the sella turcica, most often in the sphenoid sinus, suprasellar region, nasopharynx and the cavernous sinuses. Metastases to the pituitary gland Carcinomas that metastasize into the pituitary gland are uncommon and typically seen in the elderly, with lung and breast cancers being the most prevalent, In breast cancer patients, metastases to the pituitary gland occur in approximately 6-8% of cases.
Symptomatic pituitary metastases account for only 7% of reported cases. In those who are symptomatic Diabetes insipidus often occurs with rates approximately 29-71%. Other commonly reported symptoms include anterior pituitary dysfunction, visual field defects, headache/pain, and ophthalmoplegia. Risk factors Multiple endocrine neoplasia Adenomas of the anterior pituitary gland are a major clinical feature of multiple endocrine neoplasia type 1, a rare inherited endocrine syndrome that affects 1 person in every 30,000. MEN causes various combinations of benign or malignant tumors in various glands in the endocrine system or may cause the glands to become enlarged without forming tumors. It often affects the parathyroid glands, pancreatic islet cells, and anterior lobe of the pituitary gland. MEN1 may also cause non-endocrine tumors such as facial angiofibromas, collagenomas, lipomas, meningiomas, ependymomas, and leiomyomas. Approximately 25 percent of patients with MEN1 develop pituitary adenomas. Carney complex Carney complex, also known as LAMB syndrome and NAME syndrome is an autosomal dominant condition comprising myxomas of the heart and skin, hyperpigmentation of the skin, and endocrine overactivity and is distinct from Carney’s triad. Approximately 7% of all cardiac myxomas are associated with Carney complex.
Patients with CNC develop growth hormone-producing pituitary tumors and in some instances these same tumors also secrete prolactin. There are however no isolated prolactinomas or any other type of pituitary tumor. In some patients with CNC, the pituitary gland is characterized by hyperplastic areas with the hyperplasia most likely preceding the formation of GH-producing adenomas. Familial isolated pituitary adenoma Familial isolated pituitary adenoma is a term that used to identify a condition that displays an autosomal dominant inheritance and is characterised by the presence of two or more related patients affected by adenomas of the pituitary gland only, with no other associated symptoms that occur in Multiple endocrine neoplasia Type 1 or Carney complex.
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