Pituitary Adenoma

Etiology

The pathogenesis of pituitary adenoma remains unknown. Most of the pituitary adenomas are sporadic. In a study from Iceland with 410 pituitary adenomas, 43% were non-functioning adenomas, 40% prolactin-secreting adenomas, 11 % growth hormone (GH) secreting adenomas, and 6% Adrenocorticotropic hormone (ACTH) secreting adenomas. Genetic mutation is rarely a feature of pituitary adenoma.[10] Familial cases of pituitary adenomas represent 5% of all pituitary tumors.[4] Mutation in the following genes has been found to play a role in the development of pituitary adenomas.

Multiple endocrine neoplasia type 1 (MEN1): MEN1 is a tumor suppressor gene. Loss of function mutation in this gene leads to tumor formation in the parathyroid, pancreatic, and pituitary glands.[5]

Multiple endocrine neoplasia type 4 (MEN4): MEN 4 has a mutation in the cyclin-dependent kinase inhibitor 1 B gene (CDKN1B) presenting with pituitary tumors, hyperparathyroidism, testicular, and cervical neuroendocrine tumors.[6]

Carney complex (CNC): In the Carney complex, there is a germline mutation of the tumor suppressor gene PRKAR1A leading to primary pigmented nodular adrenocortical disease (PPNAD), testicular tumors, thyroid nodules, spotty skin hyperpigmentation, and acromegaly.[7]

Familial isolated pituitary adenomas (FIPA): Aryl hydrocarbon receptor-interacting protein (AIP) mutation is reported in adolescence or early adulthood in about 15%of all FIPA. These tumors are usually aggressive and they most commonly secrete growth hormone causing acromegaly. [8]

Epidemiology

Pituitary adenomas are mostly found incidentally on imaging modalities obtained for other reasons. Given the insidious nature of the pituitary adenomas, smaller size, and incidental diagnosis, it is challenging to accurately estimate the prevalence of pituitary adenomas in the general population. The estimated prevalence of pituitary adenomas is extrapolated from autopsy and radiological data. There is a wide range of prevalence among studies and the source of information.[1] In a meta-analysis the pituitary adenomas frequency was 16.7% average; in autopsies 14.4% and in radiology tests 22.5%[9] There are several population-based studies from different geographic areas spanned over many years to describe the epidemiology of the pituitary adenomas. The most popular study from Iceland showed a prevalence of 115 per 100,000 population.[1][10][2][11]

History and Physical

The presentation of pituitary adenoma depends on tumor size and functional status.[1][2][11]

Pituitary microadenoma is usually an incidental finding on MRI head. Patients are asymptomatic unless the tumor is hormonally active.

Pituitary macroadenoma presents with mass effects and potentially hormonal deficiency or hormonal excess.

Pituitary apoplexy is a sudden hemorrhage into pituitary adenoma. It is very rare. It presents with symptoms of a mass effect that includes sudden headaches and vision changes along with hormonal deficiency.

1. Symptoms from Mass Effect:

• Visual impairment: Visual impairment is reported in approximated 40% to 60% of patients. Suprasellar extension of the pituitary adenoma compresses the optic chiasm leading to visual field defects. Bitemporal defect is the most prevalent pattern, followed by homonymous defects. The involvement of the oculomotor nerve can lead to diplopia, while the fourth, fifth, and sixth cranial nerves may also be involved with invasive tumors.[12][13] 
• Headache: Headache is commonly reported in pituitary adenoma; however, it is a non-specific symptom.[13]
• Hormonal deficiency: One or more anterior pituitary hormonal deficiencies can be observed in patients with pituitary macroadenoma.

Gonadotropin deficiency presents as amenorrhea in females and erectile dysfunction in males.

Growth hormone (GH) deficiency in adults leads to fatigue and weight gain.

Thyroid-stimulating hormone (TSH) deficiency symptoms are weight gain, fatigue, cold intolerance, and constipation. 

Adrenal corticotropic hormone (ACTH) deficiency presents with fatigue, arthralgia, weight loss, low blood pressure, dizziness, nausea, vomiting, and abdominal pain. 

      2. Functioning or secreting adenomas: The clinical presentation depends on the hormone secreted as described below:

• Prolactin-secreting adenoma: Elevated prolactin suppresses the gonadotrophin levels leading to infertility, decrease libido, and osteoporosis in both sexes. Females present with amenorrhea and galactorrhea while men present with erectile dysfunction and gynecomastia.
• GH secreting adenoma (acromegaly): Presentation includes headaches, vision changes, an increase in ring or shoe size, arthritis, carpal tunnel syndrome, and excessive sweating. Clinically patients have coarse facial features, frontal bossing, enlarged nose, prognathism, enlarged tongue, and skin tags. Other comorbidities like hypertension, cardiomyopathy, obstructive sleep apnea, and multiple colonic polyps may be present at the time of diagnosis.
• ACTH-secreting adenoma (Cushing disease): Presents with weight gain, muscle weakness, mood disorders, easy bruising, and multiple fractures. Clinical features include a round face, facial plethora, supra-clavicular fat, ecchymoses, and purple striae on the abdominal area and armpits.
• TSH secreting adenoma: Patients have symptoms of palpitations, arrhythmias, and weight loss. On exam, they may have tremors and a goiter.

Evaluation

Most pituitary adenomas are detected incidentally on routine CT imaging. An MRI with gadolinium is necessary for the distinction of a mass from an aneurysm and to assess for hemorrhage into the mass. Additionally, screening for hypopituitarism and assessing for hypersecretion is warranted. The Endocrine Society clinical practice guidelines recommend a complete biochemical assessment, even in asymptomatic patients. This evaluation includes the measurement of various hormones such as prolactin, TSH, free T4, follicle-stimulating hormone (FSH),  IGF-1, GH, ACTH, estradiol, testosterone, BMP, and fasting early morning cortisol.[1][2][11][12][14]

• Prolactin: The prolactin level usually correlates with the size of the adenoma. Serum prolactin level of less than 200 ng/ml is present in microadenoma, due to the "stalk effect " in macroadenoma or medications. A serum prolactin level of greater than 200 ng/ml suggests prolactin-secreting macroadenoma. Prolactin becomes elevated in pregnancy, breastfeeding, nipple stimulation, chest wall trauma, hypothyroidism, renal failure, and certain categories of medications, including antipsychotics, anti-depressants, opiates, and antiemetics. Macroprolactin, also known as "big -prolactin," is the inactive form of prolactin. It is essential to diagnose macroprolactin to avoid unnecessary medical treatment. The diagnosis of macroprolactin involves precipitation of the serum with polyethylene glycol (PEG). The presence of free prolactin over 60% confirms prolactinoma.

• IGF-1/GH: Serum IGF-1 is a screening test for the diagnosis of acromegaly and GH deficiency. Conditions like poorly controlled diabetes, malnutrition,  sepsis, hypothyroidism, hepatic, and renal failure can impact the IGF-1 and GH levels. In patients with acromegaly, where the IGF-1 level is not found to be high or is equivocal, the serum GH level can be assessed after a 75 gm oral glucose challenge test. A non-suppressed GH level of greater than 1 ng/dl with hyperglycemia confirms acromegaly. In adults, isolated GH deficiency requires further evaluation with provocative testing like Insulin-induced hypoglycemia test, glucagon stimulation test, or the newly developed macimorelin stimulation test. 

• Cortisol: Early morning fasting cortisol level can be useful in assessing the hypothalamic-pituitary-adrenal (HPA) axis insufficiency. AM cortisol level of greater than 14 mcg/dl is suggestive of normal HPA axis. If morning cortisol is equivocal or low, the clinician should obtain a cosyntropin stim test. Random cortisol level measurement does not help in identifying patients with cortisol excess.

The screening tests for Cushing disease include checking late-night salivary cortisol (around midnight), 24-hour urine free cortisol, or dexamethasone suppression test (DST). Late-night salivary cortisol has a greater than 90% sensitivity and specificity if done accurately. DST includes checking early morning cortisol after taking 1 mg of dexamethasone at 11:00 pm to midnight. A cortisol level of 1.8 mcg/dl or higher suggests hypercortisolemia. Urine free cortisol requires accurate 24-hour urine collection. Interpretation of the screening test for cortisol excess should proceed cautiously as many factors like exogenous steroids, depression, excess alcohol intake, and oral contraceptives can affect cortisol levels.

Once hypercortisolemia is confirmed biochemically, the next step is to find the etiology by checking ACTH. Hypercortisolemia with elevated ACTH suggests a corticotroph adenoma. ACTH producing adenomas are frequently small, and an MRI of the head can be normal in 50% of the patients. In the setting of normal MRI head or pituitary microadenoma less the 0.6 cm, inferior petrosal sinus sampling (IPSS) is recommended to differentiate between ectopic vs. pituitary Cushing. IPSS is an invasive procedure where catheters are inserted into the petrosal sinuses bilaterally. ACTH level measurement takes place before and after stimulation with the corticotropin-releasing hormone (CRH). A threefold increase in ACTH levels suggests a pituitary source.

• TSH/FreeT4: Low free T4 with normal or low TSH stipulates secondary (central) hypothyroidism. TSH-producing adenoma will present with elevated T4 and T3 with inappropriately normal or elevated TSH. 

• Gonadotropin levels (LH/FSH): Low estradiol or testosterone levels with normal or low LH/FSH levels suggest hypogonadotropic hypogonadism. The sex hormones can not be interpreted accurately in females if they are on oral contraceptives. Post-menopausal females will physiologically have elevated FSH levels.

Treatment / Management

The management of pituitary adenomas requires an endocrinologist and a neurosurgeon to work closely together and develop an "individualized patient-centric" approach.[1][2][11][15][16][12][17][14](A1)

Treatment of Non-Functioning Adenomas

Transsphenoidal resection is recommended in patients with macroadenomas and the following scenarios:

• Visual field deficit due to tumor 
• Other visual abnormalities as ophthalmoplegia 
• Compression of the optic nerves or chiasm on imaging
• Pituitary apoplexy with visual disturbance
• Loss of endocrine function
• Significant growth of pituitary tumor over time

After surgery there is usually an improvement in visual symptoms and hormonal dysfunction in most patients. Radiotherapy is a consideration in patients with persistent residual or recurrent tumor.

In nonfunctional adenomas that do not require surgical management, annual follow-up with endocrinology is important to assess for tumoral growth and development of hypopituitarism. MRI of head is obtained annually for three years and if stable, thereafter less frequently.

Treatment of Individual Functioning Tumors

• Prolactin-Secreting Adenoma:  The goal of treatment is to restore gonadal function and decrease tumor size. Observation with periodic monitoring of prolactin levels can be an option in patients who are asymptomatic with microadenoma.  

Medical Therapy

Dopamine agonists (DA) are the first-line treatment for prolactin-secreting tumors. The currently available DAs are cabergoline and bromocriptine. Cabergoline is more than 90% effective in normalizing prolactin levels and decreasing tumor size. The adverse effects of DA are dizziness due to postural hypotension,  valvular heart abnormalities, and the development of compulsive behavior or mood changes. DA could be discontinued after two years of treatment if the MRI of head did not show a visible tumor. Monitoring serum prolactin levels annually is needed in these patients as there is some risk of recurrence or growth after stopping DA.

Surgery

Transsphenoidal surgery is often reserved for prolactin-secreting tumors which are resistant to medical treatment, patients who develop adverse effects to dopamine agonists and, in patients desiring pregnancy with tumor size of more than one centimeter.

Radiation Therapy

Radiotherapy is seldom used in cases of aggressive prolactinomas, where frequent surgeries and medical therapy have failed to control the size of the adenoma. 

• GH Secreting adenoma: The goal is to decrease growth hormone levels to less than 1ug/L and get IGF-1 levels to the normal age-adjusted range.

Surgery: Trans-sphenoidal surgery is the first-line treatment for GH-secreting tumors. In the hands of an experienced surgeon, normalization of IGF-1 is achieved in 80% to 90% of patients with microadenomas and 40% to 60%  of patients with macroadenomas.

Medical therapy:  Medical treatment is considered in patients with persistently elevated IGF-1 and GH levels at three months after surgery or in non-surgical candidates with invasive tumors. Somatostatin analogs (SSA)  are the first-line treatment for acromegaly. Current available SSAs are octreotide, lanreotide, and pasireotide. The adverse effects of SSA include gallbladder sludge and stones, abdominal cramps, flatulence, diarrhea, and alopecia. Pasireotide can lead to hyperglycemia in 50 to 70% of patients. DA, like cabergoline, is also used for mildly elevated IGF-1 level post-surgery or as an adjunct therapy with SSA. If the GH remains elevated, pegvisomant, a GH receptor blocker, can be used in combination with SSA or alone for the treatment of acromegaly.

Radiotherapy: Radiation treatment may be used as an adjunct in patients with elevated IGF-1 levels after surgery, but it will take several years to be effective.

• ACTH secreting adenoma: The goal of treatment is to decrease cortisol levels rapidly and to reduce the associated complications and mortality.

Surgery: Trans-sphenoidal surgery is the first-line treatment for Cushing's disease. The cure rate is 70% to 90 % in the hands of an expert surgeon with initial and repeated surgeries.

Medical therapy:  DA (cabergoline) and SSA (pasireotide, pasireotide LAR) are the drugs directed at the pituitary to decrease ACTH secretion. Ketoconazole, metyrapone, mitotane, and etomidate also decrease adrenal cortisol production. Ketoconazole can lead to liver toxicity and prolongation of the QT interval. Metyrapone is 50 to 60% effective in reducing the cortisol level. Mitotane is an adrenolytic drug, mostly used in patients with adrenocortical cancer.  Etomidate is given intravenously in critically ill patients with severe hypercortisolemia as a bridge to work on other treatment interventions. The glucocorticoid receptor blocker, Mifepristone, can be used in selected patients with hypercortisolemia and diabetes.

Bilateral adrenalectomy can lead to an immediate cure of hypercortisolemia with resultant adrenal insufficiency requiring lifelong treatment. Nelson syndrome, which is radiological pituitary tumor enlargement, can occur in 50 % of patients after adrenalectomy.

Radiotherapy: Radiation treatment serves as an adjunct after surgery and medical therapy. 

•  TSH secreting adenoma: Trans-sphenoidal surgery is the initial preferred treatment option leading to a cure in 50% to 90% of patients. It is crucial to control hyperthyroidism before surgery to avoid thyroid storm. Presurgical euthyroidism is achieved by using anti-thyroidal medical therapy like methimazole or SSA. Patients who are not cured by surgery can be treated with SSA alone to decrease TSH levels and tumor size, or in combination with radiation treatment. 

Differential Diagnosis

Differential diagnosis includes other sellar masses as:

• Arachnoid cyst
• Basilar artery thrombosis
• Brainstem glioma
• Cavernous sinus syndrome
• Cerebral venous thrombosis
• Craniopharyngioma
• Dermoid cyst
• Ependymoma
• Glioblastoma multiforme
• Leptomeningeal carcinomatosis
• Low-grade astrocytoma
• Meningioma
• Primary CNS lymphoma
• Rathke cleft cyst
• Tuberculous meningitis

Prognosis

The prognosis of pituitary adenomas depends on its being functioning or non-functioning. The non-functioning adenomas and prolactinomas have an excellent prognosis if treated promptly with surgery and/or medical therapy. Functioning adenomas like Cushing's disease and acromegaly are associated with several other co-morbidities and complications. There is increased mortality especially in patients with Cushing's disease with delays in medical or surgical treatment.