Prolactinomas are the most common hormone-secreting pituitary tumors. Based on its size, a prolactinoma can be classified as a microprolactinoma (< 10 mm diameter) or a macroprolactinoma (>10 mm diameter).
Tumor formation is due to neoplastic transformation of anterior pituitary lactotrophs, resulting in excess synthesis and secretion of prolactin (PRL). Linkage to aryl hydrocarbon-interacting protein gene (AIP) mutation has been identified in some families with prolactinoma and in childhood-onset pituitary adenomas.
Physiologically, PRL, a polypeptide hormone consisting of 199 amino acids, is regulated by hypothalamic factors. These include prolactin-releasing factors (PRFs) and prolactin-inhibitory factors (PIFs).
Dopamine (DA) is the principal PIF, and thyrotropin-releasing hormone (TRH), vasoactive intestinal peptide, and peptide histidine methionine are the putative PRFs. The physiologic role of these PRFs is not established. A delicate balance between the PRFs and PIFs normally keeps the serum PRL level within a physiologic range. Moreover, the interplay of various neurohormonal factors results in a pulsatile secretion of PRL from the pituitary.
Prolactinoma is one of the several causes of pathologic hyperprolactinemia (see Other Problems to Be Considered).[2, 3, 4, 5, 6]
The exact frequency with which prolactinomas occur in the general population is not clearly established. In nonselected surgical series, this tumor accounts for approximately 25-30% of all pituitary adenomas. Some growth hormone (GH) – producing tumors also cosecrete PRL. Microprolactinomas are much more common than macroprolactinomas.
In a study of 81,449 inhabitants of Banbury, Oxfordshire, in the United Kingdom, Fernandez et al determined the incidence of pituitary adenomas there to be 77.6 cases per 100,000 population, with the majority of cases (57%, or 44.4 persons per 100,000 population) being prolactinomas. It was also determined that prolactinomas accounted for most pituitary adenomas in persons up to age 60 years, the incidence being 75% of pituitary adenomas occurring in persons up to age 20 years, and 61% of pituitary adenomas in persons between the ages of 20 and 60 years. Moreover, prolactinomas accounted for 76% of pituitary adenomas in females, although in males, the majority of pituitary adenomas (57%) were nonfunctioning lesions.
The incidences of nonprolactinoma pituitary adenomas were as follows: nonfunctioning pituitary adenomas, 28%; adenomas associated with acromegaly, 11%; corticotroph adenomas, 2%; and adenomas of unknown functional status, 2%.
Among patients with prolactinomas, as many as 60% of the males present with macroprolactinomas, while 90% of the females present with microprolactinomas. This may partially be due to the fact that the male patients often present much later (for clinical evaluation of hypogonadism) than do the female patients (for clinical evaluation of amenorrhea).
Prolactinomas can cause symptoms secondary to the hormonal effects of excess PRL and to the space-occupying effects of the tumor itself.
The clinical features of sustained hyperprolactinemia (which vary with the duration and degree of the condition, as well as with the age and sex of the patient) are as follows[2, 6] :
Correlating with the size of the tumor, the space-occupying effects of prolactinoma are as follows:
In a male presenting with symptoms of hypogonadism, measure serum testosterone or bioavailable testosterone levels.
In a patient with a history suggestive of adrenal insufficiency, measure basal and cosyntropin-stimulated cortisol levels.
In a person with features consistent with acromegaly, measure serum insulinlike growth factor-1 levels.
Exclude other possible systemic causes for hyperprolactinemia, such as chronic renal failure or cirrhosis, using appropriate laboratory tests as warranted.
After performing biochemical testing, order a magnetic resonance imaging (MRI) scan of the pituitary hypothalamic area (with gadolinium enhancement) or a computed tomography (CT) scan of the region (with contrast) to determine if a mass lesion is present.
MRI is better for soft-tissue delineation and for the identification of a small lesion. CT scanning is better for the identification of any bone distortion or destruction. Special attention is given to the size of the tumor and its encroachment on surrounding structures (eg, the optic chiasm, other cranial nerves) and resulting effects (eg, bony destruction).
Good correlation exists between the size of the prolactinoma and the degree of elevation of the serum PRL. A serum PRL value of 200 ng/mL or greater in the presence of a macroadenoma (>10 mm) is virtually diagnostic of prolactinoma. However, if the serum PRL value is less than 200 ng/mL in the presence of a large pituitary mass lesion, this is more suggestive of hyperprolactinemia occurring secondary to stalk compression by the lesion; it could also indicate the occurrence of the hook effect, which is an artifact in the lab method used to measure prolactin. Immunoassays used to measure PRL, such as immunoradiometric assay (IRMA), enzyme immunoassay (EIA), and immunochemiluminometric assay (ICMA), employ a "sandwich" (2-antibody) technique. When the PRL level is markedly elevated, the excess antigen (PRL) is washed off in the liquid phase of the sandwich assay and thereby results in gross underestimation of antigen levels in the specimen. When this is suspected, serial dilutions of theserumsamplewill reveal the actual hormone level.
Repeat scans are obtained postoperatively and during follow-up examinations thereafter, or they are obtained after medical treatment to help determine if the tumor has progressed or regressed. The frequency with which repeat imaging scans are performed is individualized to the patient. For instance, in patients with microprolactinoma, pituitary MRI could be performed 1 year after treatment start and then every few years thereafter, less frequently than such scanning would be performed in patients undergoing treatment for macroprolactinoma.
When the tumor is large and is in close vicinity of the optic chiasm, formal visual-field (VF) testing by an ophthalmologist is performed prior to any therapy. The same testing is repeated after treatment is begun, to monitor the patient's response to treatment.
If a pituitary/hypothalamic lesion other than a prolactinoma (such as lymphocytic hypophysitis or a granulomatous condition) is considered, biopsy of the lesion by a neurosurgeon is indicated.
Treatment is indicated if mass effects from the tumor and/or significant effects from hyperprolactinemia are present.
Although the natural history of prolactinomas is unclear, most microprolactinomas (up to 95%) do not progress to macroadenomas, as determined after a 4- to 6-year observation period. Hence, if a patient with a microprolactinoma has minimal symptoms, the patient can be monitored closely with serial estimations of serum PRL levels combined with imaging studies at yearly intervals. However, if a patient with a microprolactinoma has significant effects from the hyperprolactinemia, treatment is indicated.
Any patient with macroprolactinoma needs treatment, because the tumor has already shown a propensity to grow. Such treatment includes the following:
Bromocriptine (BEC) is generally considered to be the agent of choice in the treatment of prolactinoma because of its long track record and safety. As a DA agonist, it decreases the synthesis and secretion of PRL. It also decreases the rate of tumor cell division and the growth of individual cells.
Typically, BEC is administered at an initial dose of 1.25 mg nightly with food and is gradually increased to 2.5 mg bid in 1-2 weeks, as tolerated. Doses larger than 7.5 mg/d are seldom needed except in the treatment of macroadenomas.
Common adverse effects include nausea, nasal stuffiness, and dizziness associated with orthostatic hypotension. Others include vasospasm in the peripheral circulation and exacerbation or unmasking of depression and psychosis.
In patients who are intolerant to even small doses of BEC, one alternative is to administer the same daily dose intravaginally, a method that has almost equal efficacy.
Normalization of PRL levels occurs in 85-90% of all patients with prolactinomas.
In microprolactinomas, PRL levels return to normal within days to a few weeks of starting treatment in almost all patients who can tolerate appropriate doses of BEC. If PRL levels normalize, gonadal function also typically has a near-total recovery. Menses return to normal within a few months. Sometimes, pregnancy can occur before the resumption of menstruation; therefore, the couple must be advised to use barrier methods of contraception until normal menses have returned.
In macroprolactinomas, BEC treatment results in some reduction of tumor size in up to 80-85% of the patients. Significant VF improvements have been noted to occur in as few as 1-3 days, and significant changes on imaging findings occur as soon as 2 weeks after starting treatment.
In contrast to patients with microadenomas, resolution of hyperprolactinemia is often incomplete in patients with macroadenomas. However, the extent of reduction in tumor size is not well correlated with the changes in serum PRL levels. Nevertheless, reductions in PRL levels always precede tumor shrinkage, and patients who do not show a drop in PRL do not have any tumor shrinkage.
A reduction in tumor size is often accompanied by improvement in pituitary function. Examples include improved serum testosterone levels and an increased sperm count.
Once normalization of PRL levels is achieved and sustained, the dose of BEC is gradually tapered to approximately 2.5 mg/d. If PRL levels and tumor size are stable on the above dose, consider tapering BEC to the lowest dose possible. The patient should be evaluated periodically with monitoring of symptoms, PRL levels, and radiological changes.
Other medical treatments are available for patients who do not respond to BEC or for those who cannot tolerate the drug.
A long-acting, nonergot DA agonist, cabergoline is available in the United States. It is usually better tolerated than BEC, and its efficacy profiles are somewhat superior to those of BEC. It offers the convenience of twice-a-week administration, with a usual starting dose of 0.25 mg biweekly to a maximum dose of 1 mg biweekly. Some studies have shown efficacy even with once-a-week dosing. Cabergoline appears to be more effective in lowering prolactin levels and restoring ovulation. Up to 70% of patients who do not respond to BEC respond to cabergoline. The only problem is cost. Side effects are somewhat fewer than with BEC and include headache, nausea, postural hypotension, and fatigue.
Quinagolide is a nonergot DA agonist that has a long duration of action but is not yet available for use in the United States. It can be administered once daily. Efficacy and tolerance are comparable to that of BEC.
A drug approved for the treatment of Parkinson disease, pergolide is a DA agonist with a long duration of action. Daily doses of 50-150 mcg are nearly as effective as treatment with BEC. Tolerance is comparable to that with BEC. Some patients who do not respond to BEC tend to respond to pergolide, and vice versa. Rare cases of valvular heart disease have been reported in users of pergolide, much like the lesions noted with the use of fenfluramine or in patients with carcinoid tumors. The use of pergolide is therefore not generally recommended in patients with prolactinoma for this reason.
Pergolide was withdrawn from the US market on March 29, 2007, because of heart valve damage resulting in cardiac valve regurgitation. It is important not to abruptly stop pergolide. Health care professionals should assess patients’ need for DA agonist therapy and consider alternative treatment. If continued treatment with a DA agonist is needed, another such agent should be substituted for pergolide. For more information, see FDA MedWatch Product Safety Alert and Medscape Alerts: Pergolide Withdrawn From US Market.
A study of patients with prolactinoma demonstrated that attempting dopamine agonist withdrawal in patients who have been treated for 2 years is practical and safe if normalization of prolactin levels and evidence of tumor reduction are observed.
Pharmacologic resistance to DA agonists refers to a failure to respond to such agents in terms of a normalization of PRL levels and a reduction in the size of prolactinoma by at least 50%. DA agonist resistance results primarily from a reduction in D2 receptors on tumor cells.
In some patients with prolactinoma who have adequately responded to medical treatment, withdrawing medical treatment after about 24 months may be possible. Although no clear predictive criteria for such successful withdrawal exist, evidence indicates that the recurrence of hyperprolactinemia is generally lower in patients with microadenomas than in those with macroadenomas. Hyperprolactinemia is more likely to recur in patients with tumor remnant on pituitary MRI than it is in patients with no such remnant. In very large adenomas, regrowth of tumor is often seen after the withdrawal of medical treatment. In any case, if medical treatment is withdrawn, close clinical, biochemical, and radiologic monitoring is warranted to look for evidence of tumor recurrence.
Most prolactinomas are medically responsive to dopamine agonist therapy. Slightly less than 10% of patients with prolactinomas do not respond to such treatment. Dopamine agonist- resistance consists of failure to achieve normal prolactin level on maximally tolerated doses of dopamine agonist along with a failure to achieve a 50% reduction in tumor size. These patients harbor tumors that are more likely to be invasive macroadenomas, more proliferative, more angiogenic, and more likely to exhibit cellular atypia. A few of these patients have malignant prolactinomas.
In those patients with DARPs, having persistent hyperprolactinemia despite surgical debulking, with or without radiotherapy, temozolomide, a chemotherapeutic alkylating agent has been recommended. Several case reports have shown temozolomide to reduce prolactin level and control tumor growth.[22, 23] Despite this, treatment of malignant prolactinomas is difficult, and survival is approximately one year.
Radiation treatment (XRT) has a limited role in the treatment of prolactinomas. Following conventional XRT, PRL levels normalize in only approximately 25% of patients. The major complication of this treatment is hypopituitarism, which occurs in 12.5-80% of patients. Other complications include optic nerve damage and neurologic dysfunction (see Complications).
Because of the excellent results produced by medical treatment, with or without surgery, XRT is seldom used. XRT is considered only in select cases, ie, when there is rapid tumor regrowth despite medical and surgical treatment.
Pregnancy and prolactinoma
During pregnancy, a physiologic doubling occurs in the volume of a normal pituitary gland. Moreover, prolactin levels increase by 10-fold during this period.
When a woman with prolactinoma presents with infertility and is proceeding with medical treatment for hyperprolactinemia, the patient is advised to use mechanical (barrier) methods of contraception until her menstrual cycles resume and the first few cycles have occurred, so that accurate dating of pregnancy can be performed. BEC is the preferred medical treatment in this situation because of its long safety record. The drug can be discontinued after the first skipped period; despite BEC's safety record, this precaution is taken to prevent unwanted fetal exposure to the agent. To date, however, increased rates of spontaneous abortion, ectopic pregnancy, or teratogenic effects have not been reported with BEC therapy.
Clinical experience with cabergoline-induced pregnancies in approximately 600 patients suggests no excess risk of miscarriage or fetal malformation.
Most women with microprolactinomas do not show significant increases in tumor size during pregnancy. Tumor progression rates of 1-5% have been reported in these patients. In contrast, women with macroadenomas show significant tumor enlargement (15-35%) during gestation, secondary to the hormonal stimulation of lactotrophs.
The treatment of pregnant women with prolactinomas must be tailored to the individual patient. In women with microadenomas, as well as in the subgroup of women who have intrasellar macroadenomas without significant suprasellar or parasellar extension, BEC can usually be safely discontinued upon conception, and the patient can be monitored clinically for symptoms of tumor enlargement. Periodic monitoring of PRL levels and VFs is not usually required in these patients.
In women with larger macroadenomas, a definitive, individualized plan is made only after thorough discussions with the patient. Options include the following:
Transsphenoidal pituitary adenomectomy is the preferred surgical treatment in patients with microprolactinoma and in most patients with macroprolactinoma. A transcranial approach is used only in patients with large extrapituitary extension. A transcranial pituitary tumor resection is more hazardous, being associated with higher mortality and morbidity rates.
A combination of surgery followed by postoperative medical treatment with BEC or one of the other agents is used in patients with incomplete resolution of elevated PRL levels and in persons with residual tumors seen on follow-up imaging studies.
In surgical series of patients with microadenomas, normalization of PRL levels is reported in approximately 70-75% of patients. Recurrence rates of approximately 17% are reported. This surgery is associated with low mortality and morbidity rates (approximately 0.3% and 0.4%, respectively).
Data from a tertiary center indicate an initial cure rate of approximately 90% and a recurrence rate of 16% for microprolactinomas. However, results vary with the experience of the neurosurgeon and the duration of follow-up. Complications include hypopituitarism, bleeding, cerebrospinal fluid rhinorrhea, and diabetes insipidus (see Complications).
In patients with macroprolactinomas, normalization of the PRL level occurs initially in approximately 30% of patients, and the recurrence rate is about 15-20%. Mortality and morbidity rates are less than 1% and 6%, respectively.
The drugs that are effective in the treatment of hyperprolactinemia are DA agonists. DA is the primary physiologic inhibitor of PRL secretion; however, DA is not used for treatment, because it does not cross the blood-brain barrier. Therefore, drugs that mimic the action of DA on the lactotrophs are used in the medical management of prolactinoma.
Clinical Context: DOC for prolactinoma. Bromocriptine is the DA-receptor agonist with the longest record of use for hyperprolactinemia.
Clinical Context: Now available in the United States for use in prolactinoma. Cabergoline is a long-acting DA agonist with efficacy and adverse effects that are similar to those of BEC.
Clinical Context: Pergolide withdrawn from US market. Potent DA-receptor agonist at D1 and D2 receptor sites. Pergolide is approximately 10-1000 times more potent than BEC on a mg-per-mg basis.
This agent inhibits the secretion of PRL; it causes a transient rise in serum concentrations of GH and decreases serum concentrations of luteinizing hormone.
Clinical Context: Specific DA-receptor (type 2) agonist with a relatively long duration of action. Quinagolide is not available in the United States.
Salutary effects on inhibition of PRL synthesis and secretion. These agents are also effective for reducing tumor size.
For patients receiving medical treatment, attempts should be made to continue maintenance treatment at the lowest effective dose to maintain PRL in the normal range. If a decision is made to withdraw medical treatment, especially in microprolactinoma patients, PRL levels and radiologic imaging with MRI or CT scanning should be periodically performed to monitor for recurrence and growth of prolactinoma. Medical treatment may be withdrawn cautiously after menopause in microprolactinoma patients, followed by biochemical and radiologic monitoring.