A macroprolactinoma is a non-cancerous tumor (adenoma) that develops in the pituitary gland, causing the body to produce excessive amounts of the hormone prolactin. This endocrine disorder arises when the prolactin-producing cells, called lactotrophs, multiply uncontrollably to form a mass. The resulting high prolactin levels disrupt the normal function of the body’s hormonal systems.
Understanding the Condition
The pituitary gland is a small structure situated at the base of the brain. It regulates numerous hormones, including prolactin, which primarily influences breast tissue development and milk production. In men, prolactin also plays a role in testicular function.
Prolactin-secreting tumors are collectively called prolactinomas and are categorized by size. A tumor is designated a macroprolactinoma when its diameter reaches 10 millimeters (1 centimeter) or larger. This is distinct from a microprolactinoma, which is smaller than 10 millimeters.
The size distinction is important because macroprolactinomas often cause symptoms due to their physical presence, known as a mass effect. While microprolactinomas typically only cause problems through hormone overproduction, a macroprolactinoma is large enough to press on adjacent brain structures. Macroprolactinomas are the most common type of pituitary tumor, though they represent a smaller fraction of all prolactinomas.
Recognizing the Signs
The symptoms of a macroprolactinoma fall into two categories: those stemming from excess prolactin (hyperprolactinemia) and those caused by the tumor’s physical size. Hormonal effects are often noticed earlier in women. Mass effects tend to be the initial presentation in men, as male tumors are commonly detected at a larger size.
Hormonal Effects
Excessive prolactin disrupts the balance of other sex hormones, leading to hypogonadism (a reduction in the function of the gonads). In women, common signs include changes in the menstrual cycle, such as irregular periods or the complete cessation of menstruation (amenorrhea). Many women also experience galactorrhea, which is the production of breast milk or a milky discharge from the nipples when not pregnant or breastfeeding.
In men, high prolactin levels frequently result in symptoms of hypogonadism, including low libido and erectile dysfunction. Hyperprolactinemia can also cause infertility in both sexes. Over time, low estrogen in women and low testosterone in men can decrease bone mineral density, increasing the risk of osteopenia and osteoporosis.
Mass Effects
Because a macroprolactinoma is 10 millimeters or larger, its growth can compress nearby nerves and brain structures. The most frequent complaint is chronic headaches, which result from the tumor stretching the pain-sensitive structures around the pituitary gland.
A more specific mass effect is the compression of the optic chiasm, the structure where the optic nerves cross. This pressure leads to visual field disturbances, typically affecting peripheral vision. The specific pattern of vision loss, called bitemporal hemianopsia, involves the loss of outer vision fields in both eyes.
Diagnostic Process
Diagnosis begins with assessing the patient’s symptoms and conducting a physical examination. If hyperprolactinemia is suspected, laboratory tests are the first step to confirm the hormonal imbalance.
Blood Tests
A simple blood test measures the serum prolactin level, a direct measure of the hormone concentration. Prolactin levels are generally proportional to the tumor size. A serum prolactin value of 200 nanograms per milliliter (ng/mL) or greater, alongside a large mass, is considered diagnostic of a prolactinoma. Levels above 500 ng/mL make the diagnosis almost certain.
Other conditions, such as certain medications, hypothyroidism, or kidney disease, can also cause elevated prolactin and must be ruled out. Specialists may also screen for the “hook effect,” where extremely high prolactin levels saturate the lab assay, leading to a falsely low reading. Comprehensive testing involves checking other pituitary hormones to ensure the tumor has not caused additional deficiencies.
Imaging and Visual Assessment
Once hyperprolactinemia is confirmed, an imaging study is required to locate and measure the tumor. Magnetic resonance imaging (MRI) of the pituitary-hypothalamic area is the preferred method. The MRI provides detailed images that confirm the adenoma’s size, establishing it as a macroprolactinoma, and assesses its proximity to sensitive structures like the optic chiasm.
Due to the risk of visual impairment from tumor compression, all patients must undergo a formal visual field test. This specialized eye exam is conducted by an ophthalmologist to accurately map out any potential deficits in peripheral vision. The results of this testing heavily influence the urgency and type of treatment plan.
Therapeutic Approaches
Treatment for macroprolactinomas is highly effective and focuses on shrinking the tumor, normalizing prolactin levels, and reversing symptoms. The standard approach is sequential, starting with medication before considering more invasive options.
Medical Management
Dopamine agonists are the first-line treatment for nearly all macroprolactinomas. These medications, such as cabergoline and bromocriptine, mimic the effects of dopamine, a natural hormone that inhibits prolactin secretion. The drugs bind to receptors on the tumor cells, causing them to decrease prolactin production and shrink.
Cabergoline is often the preferred choice due to its higher efficacy in normalizing prolactin and reducing tumor size, along with a more convenient dosing schedule and better tolerance compared to bromocriptine. These medications are effective, typically achieving prolactin normalization and significant tumor shrinkage in most patients. Tumor shrinkage can be observed quickly, with visual field improvements sometimes noted within days to weeks of starting therapy.
Surgical Intervention
Surgery is generally reserved as a second-line treatment, considered only when medical management is unsuccessful. The primary surgical technique is transsphenoidal adenomectomy, a minimally invasive procedure performed through the nasal passages.
Indications for surgery include intolerance to dopamine agonists, resistance to the medication, or acute visual deterioration that does not improve rapidly with drug treatment. Surgery can also be considered for patients who have a significant cystic component within the tumor. Even after a successful operation, patients often require continued medical therapy to maintain normal prolactin levels and prevent tumor recurrence.
Radiation Therapy
Radiation therapy is considered a third-line option, used infrequently when both medical and surgical treatments have failed to control tumor growth or normalize prolactin levels. This is typically delivered through highly focused methods like stereotactic radiosurgery. Radiation is a slower process than medication or surgery and is reserved for aggressive cases or those resistant to other forms of therapy.