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Disorders of Water Homeostasis: Hyponatremia and Hypernatremia


Acknowledgment

The authors would like to thank Drs. J. Thomas Murphy and John L. Atlee for their contributions to the previous edition of this chapter.

Hyponatremia

Case Synopsis 1

A 68-year-old man with a medical history of hypertension, chronic obstructive pulmonary disease, chronic renal insufficiency, and insulin-dependent diabetes mellitus presents for right total-knee replacement. His laboratory values are as follows: serum sodium, 130 mEq/L; serum osmolality, 260 mOsm/kg; urine sodium, 35 mEq/L; and normal glucose, blood urea nitrogen (BUN), and thyroid and adrenal function tests. The patient denies nausea, lethargy, and weakness.

Problem Analysis

Definition

Serum sodium concentration and osmolality are closely regulated by water homeostasis; this is mediated by thirst, arginine vasopressin, and the kidneys. A disruption in water homeostasis is manifested by an abnormal serum sodium concentration—hyponatremia or hypernatremia. The former is defined as a serum sodium concentration of less than 135 mEq/L, with severe hyponatremia occurring at values less than 120 mEq/L. Causes of hypotonic hyponatremia are listed in Box 140.1 ; causes of nonhypotonic hyponatremia (formerly known as pseudohyponatremia) are listed in Box 140.2 .

BOX 140.1
Causes of True Hypotonic Hyponatremia

Decreased Expanded Extracellular Fluid (ECF) Volume

  • Extrarenal sodium loss

    • Gastrointestinal diseases: vomiting, diarrhea

    • Trauma: blood loss

    • Skin: burns, sweating

  • Renal causes

    • Cerebral salt wasting syndrome

    • Diuretics

    • Adrenal insufficiency

    • Kidney disease

Normal ECF Volume

  • Syndrome of inappropriate antidiuretic hormone secretion (SIADH)

  • Diuretics (e.g., thiazide)

  • Endocrine disorders (adrenal insufficiency and hypothyroidism)

  • Primary polydipsia

Increased ECF Volume

  • Congestive heart failure

  • Nephrotic syndrome

  • Cirrhosis

BOX 140.2
Causes of Nonhypotonic Hyponatremia
From Rose BD: Hypoosmolal states: hyponatremia. In Jeffers JD, Navrozov M, editors: Clinical physiology of acid-base and electrolyte disorders. New York, McGraw-Hill, 1994, pp 651–694.

Normal Plasma Osmolarity

  • Hyperlipidemia

  • Hyperproteinemia

  • Transurethral resection of prostate or bladder tumor; hysteroscopy

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