General Medical Information

Pituitary

David M. Barrs

The otolaryngologist often is involved in the removal of pituitary adenomas and other skull-base surgery. If a complete hormonal evaluation has not been performed, the otolaryngologist should be aware of possible adrenal and thyroid insufficiency. More common, however, is perioperative diabetes insipidus and the syndrome of inappropriate antidiuretic hormone (ADH) secretion.

Diabetes insipidus is caused by a decreased level of ADH, which impairs free-water reabsorption, leading to increased thirst and large volumes of dilute urine in the face of inappropriate hyperosmolarity of the serum. In effect, the patient has a free-water deficit. Other conditions may mimic diabetes insipidus. The stress of surgery normally increases ADH secretion and free-water retention, resulting later in a water diuresis that may be mistaken for diabetes insipidus. Excessive i.v. fluid replacement also may result in large urinary volume secondary to the input. Mannitol given during neurologic and skull-base surgery may produce an osmotic diuresis. A diagnosis of diabetes insipidus should be restricted to patients with urinary outputs greater than 200 mL/h with associated hypernatremia, serum hyperosmolarity, low urinary specific gravity, osmolarity, and sodium. Urine osmolarity is usually less than 200 mOsm and urine specific gravity less than 1.005. The patient should be monitored with frequent intake and output, body weights, urine-specific gravity, and serum sodium osmolarity. In mild cases, simple oral intake of water to satisfy thirst may provide adequate free water. In more severe cases or in the patient not taking water orally, i.v. 5% dextrose in water with only enough sodium to cover obligate sodium requirements is given. The volume should equal the urinary loss plus insensible losses. For high urinary volumes (i.e., more than 4 L daily), pharmacologic replacement should be considered. Overtreatment should be avoided, especially in neurosurgical cases, to lessen the chance of water intoxication and brain edema. Desmopressin acetate is a long-acting vasopressin analog given intranasally at a dose of 5 to 10 mg once or twice daily, subcutaneously 1 to 2 mg daily or i.v. 0.3 µg/kg at 12- to 24-hour intervals. It must be used with caution in patients with coronary artery disease.
Aqueous pitressin has the drawbacks of short duration of action and a vasoconstrictor effect, which may precipitate angina. The dose is 5 U intramuscularly every 3 to 6 hours. Usually, the condition will resolve over 3 to 5 days with conservative treatment of water replacement and judicious use of ADH compounds.

The syndrome of inappropriate ADH secretion is characterized by hyponatremia with a urine osmolality that is inappropriately concentrated compared with serum osmolality. Because surgery and trauma cause ADH release, excessive administration of hypotonic i.v. solutions should be avoided. Treatment usually is accomplished by fluid restriction or use of demeclocycline, which inhibits the renal effects of ADH.