Pituitary Adenoma

Pituitary Adenoma

Signs and Symptoms
Pituitary adenomas typically present during early adulthood, equally affecting males and females. There are no outstanding reported risk factors. The clinical presentation of pituitary adenoma varies depending on the location and severity of the tumor.

Prolactinomas, the most common form of pituitary tumor, cause amenorrhea (the loss of menstruation), galactorrhea (the spontaneous flow of milk from the breast), and infertility in females. They cause hypogonadism, decreased libido and impotence in males. Tumors that secrete excess growth hormone cause gigantism in children and acromegaly in adults. Adrenocorticotropic hormone (ACTH) secreting adenomas produce Cushing’s disease (hyperadrenalism). Symptomatic pituitary adenomas comprise 12 to 15 percent of all intracranial tumors, and must be differentiated from non-neoplastic mass lesions.

Visual symptoms vary and include bitemporal visual field loss more dense from the superior to inferior, color desaturation, diplopia as well as ophthalmoplegia tumors expand into the cavernous sinus. The funduscopic sign of long-standing chiasmal compression is primary optic atrophy (secondary to retrograde axonal degeneration). Severe optic atrophy indicates a poor prognosis for visual recovery following surgical decompression. (For visual field printout, see "Understanding Visual Fields and the Visual Pathways" page 56.)

Pathophysiology
The pituitary gland is situated within the sella turcica of the sphenoid bone, at the base of the skull. The anterior lobe of the pituitary gland secretes six hormones: thyroid stimulating hormone (TSH), ACTH, follicle stimulating hormone (FSH), leutenizing hormone, growth hormone (GH), and prolactin. The posterior pituitary gland secretes vasopressin and oxytocin.

Pituitary adenomas typically are slow growing, benign neoplasms of epithelial origin. In most circumstances they arise from the adenohypophysis (the anterior lobe of the pituitary gland) and are capable of producing both systemic and visual signs. Approximately 8 mm to 13 mm above the pituitary gland is the optic chiasm. The nasal retinal fibers of each eye (temporal visual field) cross at this point, proceeding into the contralateral optic tract.

Upwardly growing pituitary tumors, which reach appropriate sizes, impinge on the anterior notch of the chiasm at its lowest lying aspect. This produces the bitemporal hemianopsia with increased density superiorly. Since tumor growth is usually asymmetrical, the field loss between two eyes is also typically asymmetrical.

Diseases affecting the pituitary gland produce two types of disturbances: mechanical and hormonal. Mechanical disturbances occur whenever a tumor compresses adjacent structures. Hormonal manifestations result when hyper-or hyposecretions occur. Pituitary adenomas are further differentiated by size. Microadenomas have little impact on the visual system or gland function and are defined as intrasellar adenomas that measure up to one centimeter in diameter without sellar enlargement. Macroadenomas present with mass effect symptoms such as headaches and measure larger than one centimeter with generalized sellar enlargement. Macroadenomas include (in order of most common to least common): non-secreting adenomas, prolactin secreting (chromaphobe) adenomas, growth hormone secreting (acidophil) adenomas, ACTH secreting (basophil) adenomas and FSH or TSH secreting adenomas.

Secreting tumors are usually diagnosed by general physicians and endocrinologists. Non-secreting tumors are often diagnosed by eye care practitioners because they produce visual symptoms in the absence of systemic signs.

Management
Surgical therapy, medicinal therapy or radiotherapy are the three treatment options for pituitary adenomas. Since the late 1970s, the transphenoidal surgical approach has been the preferred procedure for removal of tumors. Surgery is indicated if there is evidence of tumor enlargement, especially when growth is accompanied by compression of the optic chiasm, cavernous sinus invasion, or the development of pituitary hormone deficiencies. Visual improvement following treatment is often dramatic, with the greatest degree of improvement occurring within the first few months. Perimetry and MRI occupy an important role in the post treatment monitoring of patients. Medicinal treatment is limited to prolactinomas. Bromocriptine, a dopamine agonist, is useful in shrinking the sizes of these tumors. Low dose therapy of Bromocriptine is maintained for life. If the drug therapy is discontinued, there is often regrowth and enlargement of the adenoma. Conventional radiotherapy is usually added adjunctively to prevent tumor regrowth.

Clinical Pearls

In pregnant women, bitemporal visual field loss and headache may signal pituitary apoplexy (rapid degeneration with hemorrhagic necrosis of the pituitary gland). Pituitary apoplexy is a potentially life-threatening condition. Any woman with sudden onset of headache and suspicious visual symptoms (confirmed by perimetry) should be referred for a MRI with or without lumbar puncture to rule out subarachnoid hemorrhage from the this type of tumor. Women with pituitary adenomas and MRI evidence of subarachnoid bleeding should deliver by cesarean section to avoid risk of apoplexy during delivery. Postpartum hemorrhage can cause infarction of the pituitary gland leading to hypopituitarism (Sheehan’s syndrome).

Differential diagnoses of pituitary tumor includes craniopharyngioma (slow growing tumors arising from vestigial remnants of Rathke’s pouch), meningiomas and gliomas.
Masqueraders include chronic retrobulbar optic neuritis, nutritional amblyopia, uncorrected refractive error, normal tension glaucoma and age-related maculopathy. Misdiagnosis is often attributable to inadequate history, failure to correlate systemic signs and visual symptoms, or failure to perform adequate testing or follow up.
Bilateral tilted disc syndrome can result in a superior bi-temporal field defect similar to that seen in pituitary adenoma. However, the field defect in tilted disc syndrome is unchanging and does not respect the vertical midline, whereas the field defects in chiasmal compressive lesions do respect the vertical midline and are progressive.

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