Hyponatremia SIADH causing dilutional

The most common cause of hyponatremia in hospital patients is SIADH. However, other disorders can cause dilutional hyponatremia and must be differentiated from SIADH. These conditions include (1) congestive heart failure, (2) renal insufficiency, (3) nephrotic syndrome, (4) liver cirrhosis, and (5) hypothyroidism. Excessive administration of hypotonic fluids and treatment with drugs that stimulate AVP (e.g., chlorpropamide, vincristine, clofibrate, carbamazepine, nicotine, phenothiazines, and cyclophosphamide) can cause dilutional hyponatremia as well. Hyponatremia may also occur from renal or extrarenal sodium losses (depietional hyponatremia) as a result of vomiting, diarrhea, excessive sweating, diuretic abuse, saltlosing nephropathy, or mineralocorticoid deficiency. 

Relatedly, malfunction of one of the sodium-water control mechanisms, such as a kidney that normally excretes excess water, can result in fluid retention and dilutional hyponatremia. The pituitary gland and hypothalamus function to release ADH (which controls water reabsorption), and the cortex of the adrenal gland seaetes aldosterone (which controls sodium reabsorption). An alteration in the function of either of these hormone systems will alter the body s regulation of sodium or water and can result in hyponatremia. 2 For example, in the syndrome of inappropriate antidiuretic hormone (SIADH), excessive ADH is produced (usually by a tumor or some pulmonary diseases such as tuberculosis or bacterial pneumonia), and the kidneys reabsorb excessive fluids, resulting in dilutional hyponatremia. Conditions causing decreased aldosterone secretion include... 

The autonomous, sustained production of AVP in the absence of known stimuli for its release is called SIADH. In this syndrome, plasma AVP concentrations are inappropriately increased relative to a low plasma osmolality and to a normal or increased plasma volume. SIADH may be the result of one of several factors production of vasopressin by a malignancy (such as a small cell carcinoma of the lung), the presence of acute and chronic diseases of the central nervous system, pulmonary disorders, or a side effect of certain drug therapies. In addition, as many as 10% of patients undergoing pituitary surgery have a transient SIADH approximately 8 to 9 days after surgery (when the patient is at home), which responds to water restriction (2 to 3 days) and resolves without recurrence. In SIADH, a primary excess of AVP, coupled with unrestricted fluid intake, promotes increased reabsorption of free water by the kidney. The result is a decreased urine volume and an increased urine sodium concentration and urine osmolality. As a consequence of water retention, these patients become modestly volume expanded. The increase in intravascular volume causes hemodilution accompanied by dilutional hyponatremia and a low plasma osmolality. Volume expan-... 

If the cause for mild hyponatremia remains unclear after the above tests are performed, a water-loading test may be performed (Box 50-12), This test is potentially dangerous in patients with severe hyponatremia and should not be performed if the serum sodium concentration is <130 mmol/L. Patients with SIADH have impaired excretion of the water load and fail to dilute their urine. Measurements of vasopressin in plasma are not usually needed to make a diagnosis of SIADH, but basal values would be expected to be inappropriately high relative to plasma hyposmoiality. Interpretations of plasma vasopressin concentrations are sometimes complicated, because values are often within the physiological reference interval or are undetectable. ... 

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