Clinical Focus Box 241


Hyponatremia, defined as a plasma [Na+] < 135 mEq/L, is the most common disorder of body fluid and electrolyte balance in hospitalized patients. Most often it reflects too much water, not too little Na+, in the plasma. Since Na+ is the major solute in the plasma, it is not surprising that hy-ponatremia is usually associated with hypoosmolality. Hy-ponatremia, however, may also occur with a normal or even elevated plasma osmolality.

Drinking large quantities of water (20 L/day) rarely causes frank hyponatremia because of the large capacity of the kidneys to excrete dilute urine. If, however, plasma AVP is not decreased when plasma osmolality is decreased or if the ability of the kidneys to dilute the urine is impaired, hyponatremia may develop even with a normal water intake.

Hyponatremia with hypoosmolality can occur in the presence of a decreased, normal, or even increased total body Na+. Hyponatremia and decreased body Na+ content may be seen with increased Na+ loss, such as with vomiting, diarrhea, and diuretic therapy. In these instances, the decrease in ECF volume stimulates thirst and AVP release. More water is ingested, but the kidneys form osmotically concentrated urine and plasma hypoosmolality and hyponatremia result. Hyponatremia and a normal body Na+ content are seen in hypothyroidism, cortisol deficiency, and the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). SIADH occurs with neurological disease, severe pain, certain drugs (such as hypoglycemic agents), and with some tumors. For example, a bronchogenic tumor may secrete AVP without control by plasma osmolality. The result is renal conservation of water. Hyponatremia and increased total body Na+ are seen in edematous states, such as congestive heart failure, hepatic cirrhosis, and nephrotic syndrome. The decrease in effective arterial blood volume stimulates thirst and AVP release. Excretion of a dilute urine may also be impaired because of decreased delivery of fluid to diluting sites along the nephron and collecting ducts. Although Na+ and water are retained by the kidneys in the edematous states, relatively more water is conserved, leading to a dilutional hyponatremia.

Hyponatremia and hypoosmolality can cause a variety of symptoms, including muscle cramps, lethargy, fatigue, disorientation, headache, anorexia, nausea, agitation, hypothermia, seizures, and coma. These symptoms, mainly neurological, are a consequence of the swelling of brain cells as plasma osmolality falls. Excessive brain swelling may be fatal or may cause permanent damage. Treatment requires identifying and treating the underlying cause. If Na+ loss is responsible for the hyponatremia, isotonic or hypertonic saline or NaCl by mouth is usually given. If the blood volume is normal or the patient is edematous, water restriction is recommended. Hyponatremia should be corrected slowly and with constant monitoring because too rapid correction can be harmful.

Hyponatremia in the presence of increased plasma osmolality is seen in hyperglycemic patients with uncontrolled diabetes mellitus. In this condition, the high plasma [glucose] causes the osmotic withdrawal of water from cells, and the extra water in the ECF space leads to hyponatremia. Plasma [Na+] falls by 1.6 mEq/L for each 100 mg/dL rise in plasma glucose.

Hyponatremia and a normal plasma osmolality are seen with so-called pseudohyponatremia. This occurs when plasma lipids or proteins are greatly elevated. These molecules do not significantly elevate plasma osmolality. They do, however, occupy a significant volume of the plasma, and because the Na+ is dissolved only in the plasma water, the [Na+] measured in the entire plasma is low.

for example, inhibit thirst, preventing excessive water intake. For example, if a dog is deprived of water for some time and is then presented with water, it will commence drinking but will stop before all of the ingested water has been absorbed by the small intestine. Monitoring of water intake by

^BGfflHHHWI^ Factors affecting the thirst sensation. A plus ^flBBaB^flH^ sign indicates stimulation of thirst, the minus sign indicates an inhibitory influence.

the mouth and stomach in this situation limits water intake, preventing a dip in plasma osmolality below normal.

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