Background
By definition, a microadenoma (seen in the image below) is a tumor less than 10 mm in diameter. Pituitary adenomas may secrete hormones, or they may be clinically inactive. Many pituitary lesions are discovered while investigating other neurologic problems; these lesions are called incidentalomas.
MRI showing a nonenhancing area in the pituitary consistent with a microadenoma in a patient with hyperprolactinemia. Pathophysiology
Of the secretory tumors, the most common are prolactinomas. Other secretory tumors may secrete (1) adrenocorticotrophic hormone (ACTH), causing Cushing disease; (2) growth hormone, causing acromegaly; (3) gonadotropins with clinical presentations reflective of severity and sex; or, rarely, (4) thyroid-stimulating hormone (TSH), causing hyperthyroidism. Many secrete inactive alpha-subunits of the glycoprotein hormones. Most incidentalomas are inactive and are of no clinical consequence. As with adenomas in other endocrine glands, all these tumors probably derive from a local mutation, with loss of function of the genes controlling cell proliferation.
The role of genetic mutations was highlighted in a report suggesting that patients with pituitary tumors from 4 Irish families share a common mutation with a patient from the 18th century who had pituitary tumor–mediated gigantism.[1]
Epidemiology
Frequency
United States
A high-resolution magnetic resonance imaging (MRI) screen of a normal population has shown a 10% prevalence of pituitary lesions.
International
In studies with a total of 10,370 autopsies, the prevalence of pituitary microadenomas was 11%; however, in a Japanese series of 1000 autopsies, many of the midline pituitary lesions were Rathke cysts, not adenomas.
Mortality/Morbidity
Microadenomas do not cause directly attributable excess mortality. These tumors generally are too small to cause bony erosion or to put pressure on the optic chiasm. Any morbidity is caused by excessive hormone secretion.
Race
No race predilection exists.
Sex
Microadenomas may occur in either sex. Prolactinomas are diagnosed more frequently in women, possibly because of the more striking presenting features such as amenorrhea and/or galactorrhea. In men, the diagnosis of inactive adenomas is often delayed until they have become macroadenomas and cause mass effects.
Age
Microadenomas may occur at any age, but prevalence appears to increase with advancing age.
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