Definition and Overview
The pituitary gland is a small gland that sits on the base of the brain. Although small, it is responsible for releasing a number of hormones, which are necessary for growth, regulation, and other body functions.
One of the most frequently encountered medical conditions affecting the pituitary gland is the development of a mass or tumor, which are typically benign and called an adenoma. Of all the tumors occurring in the brain, pituitary adenomas account for approximately 15%. In autopsy reports, the incidence of pituitary adenomas has been found to be as high as 25%.
A tumor in the pituitary gland may result in increased secretion of normal hormones produced by the gland. A prolactinoma, for instance, is a kind of pituitary adenoma that secretes the hormone prolactin. This is the most common type of pituitary adenoma, occurring in about 30% of patients. Mixed tumors, which secrete multiple hormones, can also develop.
Patients who have microprolactinomas (prolactinomas that are less than 1cm in diameter) have good prognoses. No evidence of growth is observed in the majority of these cases over several years. Macroprolactinomas, or prolactinomas that are bigger than 1cm in diameter, however, grow progressively. Because of this, they need to be managed more aggressively than smaller lesions. However, only a small percentage (5%) of microadenomas grow to become macroadenomas.
Cause of Condition
Pituitary adenomas arise from a specific pituitary cell type. For example, prolactinomas arise from pituitary lactotrophs. The development of pituitary adenomas has been associated with specific genetic mutations and certain familial syndromes, such as multiple endocrine neoplasia (MEN). These mutations are usually sporadic, which means that they occur spontaneously and are not inherited. Other causes of prolactinomas and pituitary masses remain unknown.
The symptoms of a pituitary tumor may be due to either the mass effect or increased hormone secretion. Although pituitary masses are usually benign, they can produce significant, debilitating symptoms, primarily because of their location.
The symptoms of a pituitary tumor may be attributed to the mass effect of the lesion. These symptoms are due to the expansion of the tumor, resulting in the compression of adjacent structures. The symptoms may vary, depending on the direction of the tumor extension. Pituitary adenomas typically present with headaches, even in small tumors. They can also present with non-specific symptoms, such as vertigo and nausea.
The compression of the pituitary gland may result in hypothyroidism and growth problems. With continued expansion, masses in the pituitary gland usually progress and extend to the optic chiasm. The involvement of this area results in visual symptoms, such as visual field defects and bitemporal hemianopia. In advanced cases, this can lead to irreversible loss of vision and blindness. Pituitary lesions may also extend to the cavernous sinus. This can lead to cranial nerve deficits, resulting in symptoms like facial numbness, diplopia, and ophthalmoplegia.
Aside from local mass effect, pituitary adenomas may present with symptoms related to the hypersecretion of specific hormones. Masses that are hormonally active independently release hormones, with decreased responses to the body's normal inhibitory mechanisms. It is important to note, however, that not all adenomas are actively functioning.
In prolactinomas, the tumor secretes the hormone prolactin. Increased prolactin levels can result in various symptoms. In females, symptoms can include menstrual irregularities such as amenorrhea or absence of one’s menstrual period; loss of genital hair; galactorrhea or unexplained production of milk; and small-sized reproductive organs. In males, a prolactinoma can present with gynecomastia or breast enlargement; erectile dysfunction and impotence; and rarely, milk production. Aside from these, increased prolactin levels in the body can lead to decreased production of reproductive hormones (estrogen and testosterone). This, in turn, can lead to decreased bone strength and osteoporosis in the long-term.
Who to See and Types of Treatments Available
In patients with hyperprolactinemia or elevated prolactin levels, a cranial MRI should be requested to determine whether a pituitary mass is present. In general, the management of a pituitary tumor involves removing as much of the mass as possible, normalizing hormone levels, and restoring the function of the pituitary gland. Some microadenomas may require no treatment at all although lifelong follow-up and surveillance are recommended in these cases.
The medical treatment of prolactinomas is targeted towards the inhibition of prolactin secretion. Drug therapy is also used to decrease the size of the tumor. Dopamine agonists, such as cabergoline and bromocriptine, are typically used. These medications are effective in decreasing prolactin levels to within normal range by as much as 80% of cases. Possible side effects of dopamine agonists include dry mouth, constipation, and insomnia. Psychiatric symptoms, such as rapid changes in mood and delusions, have also been reported. An endocrinologist can properly titrate these drugs to minimize complications and prevent rebound elevation.
The definitive treatment of a prolactinoma is surgery, which is performed by a neurosurgeon. This form of treatment is reserved for patients who do not respond well to medical therapy. Preoperative symptoms, including defects in one's vision, may be resolved with surgery if the symptoms have not been present for a long time and as long as the damage incurred is not permanent. The tumor may recur in about 20 to 50% of patients, depending on the tumor size and the completeness of the resection.
Transsphenoidal surgery is the preferred approach for the removal of pituitary masses. In these operations, a microscope is typically used to ensure precise tumor removal. The crucial aspect in surgery of the pituitary gland is ensuring that maximal excision of the mass is performed while the function of the gland is preserved. This is especially important in children and young adults who have not completed their growth and reproductive phases.
Radiation therapy is typically used as an adjunctive treatment to medical or surgical therapy in patients with prolactinoma. In recent years, stereotactic and gamma knife radiosurgery are proving to be useful in the management of these cases. Radiation is usually used to target residual disease after surgical intervention, particularly in patients with large, aggressive tumors. Selective focused radiotherapy using a linear accelerator is guided by MRI localization.
- Melmed S, Kleinberg D. Pituitary masses and tumors. In: Kronenberg HM, Melmed S, Polonsky KS, Larsen PR, eds. Williams Textbook of Endocrinology. 12th ed. Philadelphia, Pa: Elsevier Saunders; 2011:chap 9.