Sodium (Na) is distributed in extra-cellular fluid by approximately 90%. As there is less movement of Na across cell membranes, the serum Na level is dependent upon Na content and circulatory blood volume. Urinary concentration ability contributes to homeostasis of body water, that is, renal medullary hypertonicity and release and the renal action of arginine vasopressin (AVP). Also drinking behavior is involved in water homeostasis. Disorders of water metabolism causes hyper-and hypo volemic states, which are related to hyponatremia and hypernatremia, respectively.
Serum Na levels range from 136 to 145 mmol/l, and hyponatremia defined as serum Na levels are less than 135 mmol/l. Hyponatremia is classified into 3 groups; hypovolemic, euvolemic and hypervolemic hyponatremia. Hypovolemic hyponatremia is primarily caused by renal and extra-renal Na loss, including Addison's disease, Na-losing nephritis, renal tubular acidosis, diuretic abuse, diarrhea, vomiting and so on. Exaggerated release of AVP produces euvolemic hyponatremia, such as the syndrome of inappropriate secretion of antidiuretic hormone (SIADH), hypopituitarism and hypothyroidism. Renal sodium handling disturbance in elderly subjects causes mineralocorticoid-responsive hyponatremia in the elderly (MRHE). Hypervolemic hyponatremia is found in edematous disorders, including congestive heart failure, liver cirrhosis and nephrotic syndrome. The symptoms of hyponatremia are nausea, vomiting, appetite loss, conscious disturbance and convulsions. Severe hyponatremia below 120 mmol/l should be corrected to maintain more than 125 mmol/l in any groups of hyponatremia. After keeping serum Na greater than 125 mmol/l, diagnosis and specific treatment must be performed.
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