Anterior pituitary gland

Anterior pituitary gland

1. Physiology. The anterior pituitary regulates the thyroid and adrenal glands, the ovaries and testes, growth, and lactation by producing TSH, ACTH, follicle-stimulating hormone, luteinizing hormone, GH, and prolactin. A negative feedback system mediated by peripheral hormones tightly controls anterior pituitary secretion. Anterior pituitary adenomas may lead to hormone excess or hypopituitarism. Macroadenomas (>1 cm in diameter) can compress adjacent structures producing visual disturbances, seizures, or increased intracranial pressure.
2. Anterior pituitary hyperfunction. Pituitary adenoma is the most common cause of anterior pituitary hyperfunction. Prolactinomas generally do not affect anesthetic management. The hyperthyroidism of a TSH-secreting adenoma and the hyperadrenalism of an ACTH-secreting adenoma are treated as described above. The anatomic and physiologic changes seen with GH-secreting tumors warrant careful consideration.
3. Clinical features. GH stimulates bone, cartilage, and soft tissue growth leading to prognathism, subglottic narrowing of the trachea, and soft tissue overgrowth of the lips, tongue, epiglottis, and vocal cords. Connective tissue overgrowth can cause recurrent laryngeal nerve paralysis, carpal tunnel syndrome, and other peripheral neuropathies. These patients often develop glucose intolerance, arthritis, osteoporosis, OSA, HTN, cardiomyopathy generally manifesting as diastolic dysfunction, arrhythmias, and heart failure. They also have an increased incidence of valvular heart disease, coronary artery disease, and colon carcinoma. The primary treatment is surgical removal of the tumor. Medical management for persistent disease after surgery includes dopamine agonists (bromocriptine and cabergoline), somatostatin analogues (octreotide and lanreotide), GH receptor antagonists (pegvisomant), and radiation therapy. Excision of GH-secreting pituitary adenomas is often performed via the transsphenoidal approach.
4. Anesthetic considerations.
   1. Blood pressure should be measured directly. The need for additional invasive hemodynamic monitoring depends on the patient’s medical status.
   2. Patients should be evaluated for other endocrinopathies and cardiac disease preoperatively.
   3. Patients with OSA are at high risk for airway obstruction.
   4. Conventional mask airways are often difficult to achieve, and endotracheal intubation can be challenging. Advanced airway and tracheostomy equipment should be available, and awake fiberoptic intubation should be considered. Small-diameter endotracheal tubes may be required.
   5. Serum glucose levels should be carefully monitored and muscle relaxants titrated using a peripheral nerve stimulator.
   6. Patients may be osteoporotic and have an increased susceptibility to peripheral neuropathies. Careful positioning is necessary.
5. Anterior pituitary hypofunction
   1. Etiologies. Pituitary adenoma is the most common cause of anterior pituitary hypofunction. Other causes of pituitary failure include trauma, radiation, pituitary apoplexy, tumors, infiltrative disease, and surgical hypophysectomy. Sheehan syndrome is a condition of pituitary failure in which hemorrhagic shock causes vasospasm and subsequent pituitary necrosis in postpartum women.
   2. Anesthetic considerations. Adrenal insufficiency develops over 4 to 14 days after destruction of the pituitary gland. Perioperative glucocorticoid supplementation may be necessary. Because the half-life of thyroid hormone is 7 to 10 days, symptomatic hypothyroidism does not occur until 3 to 4 weeks after pituitary surgery or apoplexy.