Disorders of Water and Salt Metabolism Associated with Pituitary Disease

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Disorders of water and sodium homeostasis are very common problems encountered in clinical medicine. Disorders of water metabolism are divided into hyperosmolar and hypoosmolar states, with hyperosmolar disorders characterized by a deficit of body water in relation to body solute and hypoosmolar disorders characterized by an excess of body water in relation to total body solute. This article briefly reviews the physiology of hyperosmolar and hypoosmolar syndromes, then focuses on a discussion of the pathophysiology, evaluation, and treatment of specific pre- and postoperative disorders of water metabolism in patients with pituitary lesions.

Section snippets

Overview of normal water metabolism

Water metabolism is controlled primarily by arginine vasopressin (AVP), a nonapeptide that is synthesized in the neurohypophyseal magnocellular neurons of the supraoptic and paraventricular nuclei of the hypothalamus. The newly synthesized AVP prohormone is packaged into neurosecretory granules and then transported down the supraopticohypophyseal tract to the posterior pituitary, during which it is enzymatically cleaved into AVP, neurophysin, and a C-terminal glycopeptide. When release is

Hyperosmolality and hypernatremia

Hyperosmolality indicates a deficiency of water relative to solute in the extracellular fluid. Because water moves freely between the extracellular (ECF) and intracellular (ICF) fluid, this also indicates a deficiency of total body water relative to total body solute. In evaluating hypernatremia, it is helpful to classify it broadly by etiology: hypervolemic, inadequate water intake, and increased free water losses.

Diabetes insipidus

Diabetes insipidus (DI) is the most well-known manifestation of a deficiency in AVP secretion or abnormal renal response to AVP. Central DI is caused by a variety of acquired or congenital anatomic lesions that disrupt the hypothalamic-posterior pituitary axis, and include some types of tumors, trauma, hemorrhage, thrombosis, infarction, granulomatous disease, and pituitary surgery. It is unusual for pituitary adenomas to present with DI. This is because synthesis of AVP occurs in the

Hypoosmolality and hyponatremia

Hyponatremia is the most common electrolyte disorder in hospitalized adult patients. The reported incidence of hyponatremia varies depending on the age of hospitalized patients studied, and the definition of hyponatremia used. When hyponatremia is defined as a serum [Na+] less than 135 mmol/L, incidences of 6% to 22% have been reported [1]. However, when hyponatremia is defined by more stringent criteria of serum [Na+] less than 130 mmol/L, incidences fall to 1% to 4%. This cutoff likely

Hypovolemic hyponatremia

Hypovolemic hyponatremia occurs when there are simultaneous losses of body water and sodium, resulting in ECF volume depletion. The decrease in blood volume and pressure results in secondary stimulated AVP secretion, and ultimately decreased free water excretion by the kidney. Retention of water from ingested or infused fluids can then lead to the development of hyponatremia. Primary solute depletion, from either renal losses (eg, diuretics, mineralocorticoid deficiency, and various

Hypervolemic hyponatremia

In hypervolemic hyponatremia, there is an excess of total body water and total body sodium, resulting in clinically evident hypervolemia manifested by edema or ascites. Hyponatremia occurs because the increase in total body water is usually in excess of the increase in total body sodium as a result of potent AVP secretion in response to a decreased effective arterial blood volume (EABV) [32]. Hypoosmolality in these patients suggests a relatively decreased intravascular volume, leading to water

Euvolemic hyponatremia

Euvolemic hyponatremia can be caused by virtually any disorder causing hypoosmolality. The pathogenesis of euvolemic hyponatremia is typically excessive water retention, caused by either impaired water excretion from advanced renal failure, or more likely from increased secretion of AVP. Occasionally, hyponatremia can occur from over-ingestion of water, where the gastrointestinal tract absorbs water faster than the kidney's ability to excrete it. Because clinical assessments of volume status

Vasopressin receptor antagonists

A new class of agents, AVP receptor antagonists, have been recently introduced as a method of correcting hyponatremia by blocking the binding of AVP to V2 receptors in the kidney. AVP receptor antagonists are highly effective in producing a safe and predictable increased excretion of free water that increases the serum [Na+] in hyponatremic patients. Because these agents induce excretion of free water without accompanying natriuresis or kaliuresis, this effect has been termed “aquaresis,” to

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