A pituitary adenoma is a common, noncancerous growth that develops on the pituitary gland, a small organ located at the base of the brain behind the nose. This gland is often called the “master gland” because it regulates the body’s hormonal systems, controlling functions like growth, metabolism, and reproduction. Adenomas are generally slow-growing tumors, but they can disrupt the body’s delicate balance by either producing excessive hormones or by simply occupying space.
Defining Microadenomas and Their Types
A microadenoma measures less than 10 millimeters (mm) in diameter, roughly the size of a pea. Pituitary adenomas 10 mm or larger are classified as macroadenomas. This size distinction is important because microadenomas rarely grow large enough to cause symptoms by pressing on surrounding structures, such as the optic nerves.
Microadenomas are further categorized based on their hormonal activity: functional or non-functional. Functional microadenomas actively secrete an excess amount of one or more hormones, leading to recognizable clinical syndromes. Non-functional microadenomas do not produce excess hormones and are often discovered incidentally during imaging tests performed for unrelated reasons. Prolactin-secreting tumors are the most frequent functional type encountered in clinical practice.
Clinical Manifestations and Hormonal Effects
Symptoms associated with a microadenoma are tied to the type of hormone it overproduces, as the tumor’s small size prevents pressure-related issues. Prolactin-secreting microadenomas, known as prolactinomas, are the most common functional type and cause hyperprolactinemia. In women, high prolactin levels often lead to irregular or absent menstrual periods and may cause galactorrhea, which is milky nipple discharge unrelated to pregnancy or nursing.
In men, hyperprolactinemia often presents as hypogonadism, resulting in reduced libido, erectile dysfunction, and sometimes gynecomastia (enlargement of breast tissue). Microadenomas that secrete Adrenocorticotropic Hormone (ACTH) cause Cushing’s disease, characterized by the overproduction of cortisol by the adrenal glands. This excess cortisol can lead to symptoms like weight gain, particularly in the upper body and face, high blood pressure, and fragile skin.
Growth Hormone (GH)-secreting microadenomas cause acromegaly in adults, a slow-onset condition where excessive GH leads to the gradual enlargement of the hands, feet, and facial features. If this type of tumor occurs before puberty, it causes gigantism, resulting in excessive height. Non-functional microadenomas are often asymptomatic because they do not secrete hormones and are too small to compress nearby tissues. If they cause symptoms, they are usually mild and non-specific, such as a general headache.
Diagnostic Procedures
Diagnosis of a microadenoma starts when a patient presents with symptoms suggestive of a hormonal imbalance. Biochemical testing, including blood and sometimes urine panels, measures hormone levels. For instance, an elevated prolactin level suggests a prolactinoma, while testing for elevated cortisol or Insulin-like Growth Factor-1 (IGF-1) helps diagnose ACTH- and GH-secreting tumors.
Once a hormonal abnormality is identified, imaging is used to locate and characterize the tumor. Magnetic Resonance Imaging (MRI) of the pituitary gland is the gold standard for visualizing these small lesions. High-resolution MRI, often with dynamic contrast enhancement, is necessary because the microadenoma can be difficult to distinguish from the surrounding healthy pituitary tissue. Imaging confirms the exact size and location, which is necessary to classify the lesion and inform the treatment strategy.
Management and Treatment Pathways
Management of a pituitary microadenoma is tailored to its type, size, and whether it is causing symptoms. For many non-functional or asymptomatic microadenomas, the approach is observation. This involves regular monitoring with periodic MRI scans and hormone checks to ensure the tumor is not growing and that no hormonal deficiencies are developing.
Medical management is the first-line treatment for specific functional tumors, most notably prolactinomas. Dopamine agonist medications, such as cabergoline or bromocriptine, are highly effective in normalizing prolactin levels and often cause significant tumor shrinkage. For other functional microadenomas, like those secreting GH or ACTH, surgery is typically the preferred initial treatment.
Surgical intervention usually involves transsphenoidal surgery, a minimally invasive procedure where the adenoma is removed through the nose and sphenoid sinus. This approach allows for tumor removal while avoiding a craniotomy and provides excellent cure rates for microadenomas, especially when performed by an experienced surgeon. Surgery is also reserved for functional tumors that do not respond adequately to medical therapy or for non-functional tumors that begin to show signs of growth or mass effect.