Diagnosis (Fig. 46-1) Hyponatremia is not a disease but a manifestation of a variety of disorders. The underlying cause can often be ascertained from an accurate history and physical examination, including an assessment of ECF volume status and effective circulating arterial volume. The differential diagnosis of hyponatremia, an expanded ECF volume, and decreased effective circulating volume includes congestive heart failure, hepatic cirrhosis, and the nephrotic syndrome. Hypothyroidism and adrenal insufficiency tend to present with a near-normal ECF volume and decreased effective circulating arterial volume. All of these diseases have characteristic signs and symptoms. Patients with SIADH are usually euvolemic. Figure 46-1 Algorithm. | Chapter 046. Sodium and Water Part 7 Diagnosis Fig. 46-1 Hyponatremia is not a disease but a manifestation of a variety of disorders. The underlying cause can often be ascertained from an accurate history and physical examination including an assessment of ECF volume status and effective circulating arterial volume. The differential diagnosis of hyponatremia an expanded ECF volume and decreased effective circulating volume includes congestive heart failure hepatic cirrhosis and the nephrotic syndrome. Hypothyroidism and adrenal insufficiency tend to present with a near-normal ECF volume and decreased effective circulating arterial volume. All of these diseases have characteristic signs and symptoms. Patients with SIADH are usually euvolemic. Figure 46-1 Algorithm depicting clinical approach to hyponatremia. ECF extracellular fluid SIADH syndrome of inappropriate antidiuretic hormone secretion. Four laboratory findings often provide useful information and can narrow the differential diagnosis of hyponatremia 1 the plasma osmolality 2 the urine osmolality 3 the urine Na concentration and 4 the urine K concentration. Since ECF tonicity is determined primarily by the Na concentration most patients with hyponatremia have a decreased plasma osmolality. The appropriate renal response to hypoosmolality is to excrete the maximum volume of dilute urine . urine osmolality and specific gravity of 100 mosmol kg and respectively. This occurs in patients with primary polydipsia. If this is not present it suggests impaired free-water excretion due to the action of AVP on the kidney. The secretion of AVP may be a physiologic response to hemodynamic stimuli or it may be inappropriate in the presence of hyponatremia and euvolemia. Since Na is the major ECF cation and is largely restricted to this compartment ECF volume contraction represents a deficit in total body Na content. Therefore volume depletion in patients with normal underlying renal function results in enhanced .
