By definition, a microadenoma (seen in the image below) is a tumor less than 10 mm in diameter. Pituitary adenomas may secrete hormones, but most are clinically inactive. Many pituitary lesions are discovered while investigating other neurologic problems; these lesions are called incidentalomas. With the use of MRI increasing, the discovery of such incidental microadenomas will become more of a clinical problem.
The microadenoma may be discovered during an investigation for the cause of a clinically diagnosed hypersecretory syndrome of hyperprolactinemia, acromegaly, or Cushing syndrome. A nonsecreting microadenoma is a nonfunctioning pituitary adenoma in more than 90% of cases, although a variety of other cystic, vascular, neoplastic, hyperplastic, or inflammatory processes may present in a similar manner.
Workup and management
In the absence of symptoms or signs suggesting excess of specific hormones, the most cost-effective strategy is simply measurement of the prolactin level. 
If clinical suspicion of Cushing syndrome, acromegaly, or other hormone excess exists, order appropriate tests. Because excess growth hormone secretion may not produce the clinical phenotype in all cases, especially if early in the course, a serum insulinlike growth factor-1 (IGF-1) level is recommended in all cases.
Magnetic resonance imaging (MRI) studies have shown sensitivity and specificity of about 90% for secretory tumors. Enhancement with gadolinium diethylenetriaminepentaacetic acid (DTPA) improves the detection rate. Sensitivity for detection of corticotropin-secreting adenomas is much less (60-75%); diagnosis may require specialized tests such as petrosal sinus sampling.
For prolactinomas, therapy with a dopaminergic drug is the treatment of choice (see Hyperprolactinemia). The most common are bromocriptine and cabergoline. Cabergoline is the primary dopamine agonist used, or it is used if there is bromocriptine intolerance or resistance. Nonsecreting microadenomas only need observation.
For prolactin-secreting microadenomas, surgical removal is followed by recurrence in 10-50% of patients. Therefore, medical therapy is preferred. Secretory tumors are best removed by the transsphenoidal approach.
Most pituitary tumors are sporadic. Some are part of genetic syndromes such as multiple endocrine neoplasia type 1 (MEN1), McCune-Albright syndrome, or Carney complex. Clonal analysis shows almost all are monoclonal in origin from a genetically mutated single cell. The cause of sporadic tumors is unknown.
Of the secretory tumors, the most common are prolactinomas. Other secretory tumors may secrete (1) corticotropin, causing Cushing disease; (2) growth hormone, causing acromegaly; (3) gonadotropins with clinical presentations reflective of severity and sex (rare); (4) or thyroid-stimulating hormone (TSH), causing hyperthyroidism (rare). Most clinically nonsecreting adenomas are gonadotropin in origin and secrete fragments of beta or alpha subunits of gonadotropin peptide. Such clinically inactive microadenomas are of little clinical consequence. 
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. 
Autopsy studies find pituitary adenomas, almost all microadenomas, in 10-14%. Meta-analysis of autopsy studies showed microadenomas in 22% and 14% on imaging studies. [4, 5] They occur in persons of all ages, with no sex predisposition.
Although clinically silent during life, the most frequent did immunostain for the presence of prolactin. The use of MRI now may identify these previously unsuspected presumed pituitary microadenomas.
The frequency of microadenomas and the scarcity of macroadenomas at autopsy indicate that microadenomas only rarely progress to macroadenomas and that most macroadenomas clinically present during life.
A study of 3048 autopsies found one or more adenomas in 316 pituitary glands (10%), the majority being less than 3 mm. Immunostaining for prolactin was positive in 40%.  There does not appear to be any difference in incidence from worldwide studies.
Microadenomas do not cause excess mortality. These tumors generally are too small to cause pain, diplopia, or pressure on the optic chiasm. Otherwise-normal anterior and posterior pituitary function remains intact. Any morbidity is caused by excessive hormone secretion  or patient anxiety.
Secretory microadenomas are almost all medically or surgically treated.
Nonsecretory microadenomas are best clinically followed without treatment. Their natural history is that tumor enlargement is uncommon and is rarely clinically significant. There is tumor enlargement in 10%, decreased size in 10%, and no change in 80% of microadenomas.  Acute changes from hemorrhage may rarely occur, but this is less common than with macroadenomas.
There are no controlled studies, so the clinical approach is based on expert opinion or retrospective analysis. A clinical practice guideline is presented by The Endocrine Society. 
A retrospective study by Machado et al found that in patients with Cushing disease, those with microadenomas had a higher cortisol/adrenocorticotropic hormone (ACTH) ratio than did those with macroadenomas. The study also found that hirsutism, facial plethora, and muscular weakness and atrophy occurred more often in the patients with microadenomas, while nephrolithiasis, osteopenia, hyperprolactinemia, and galactorrhea occurred at a higher rate in those with macroadenomas. 
A study by Harbeck et al indicated that in patients with hypothalamic-pituitary disorders, those with microadenomas suffer from headache and depression more frequently than do those with macroadenomas. 
No race predilection exists.
Microadenomas may occur in either sex. Prolactinomas, the most common secretory microadenoma, are diagnosed more frequently in women, possibly because of the more striking presenting features such as amenorrhea and/or galactorrhea.
Microadenomas may occur at any age, but prevalence appears to increase with advancing age.
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