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Congestive heart failure
0%
0/7
Nephrosis
Prerenal acute kidney injury
Mineralocorticoid deficiency
57%
4/7
Lung malignancy
43%
3/7
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A cause of euvolemic hyponatremia is SIADH as a result of a paraneoplastic production of ADH from a small cell lung cancer. Plasma osmolality will be low, and urine osmolality will be inappropriately high. Euvolemic hyponatremia may be caused by SIADH, psychogenic polydipsia, postoperative hyponatremia, hypothyroidism, or iatrogenic administration of excess free water. The inappropriate release of ADH may be caused by factors that either stimulate the release of ADH from the hypothalamus or decrease the sensitivity to ADH of the collecting duct in the kidneys. Neoplastic conditions, such as those encountered with small cell carcinoma of the lung, result in ectopic ADH production. Other common causes of SIADH are drugs, including: NSAIDS, antidepressants (such as fluoxetine), chemotherapy, clofibrate, morphine, and carbamazepine. Other causes of SIADH, less commonly tested by the USMLE, are CNS disturbances, pulmonary disease, hypothyroidism, and Conn's syndrome. Important causes of hypovolemic hyponatremia to remember are diarrhea, vomiting, burns, and pancreatitis. In contrast, important causes of hypervolemic hyponatremia to remember are heart failure, nephrotic syndrome, and liver disease. Goh discusses the management of hyponatremia. Differentiating between euvolemia and hypovolemia can be clinically difficult, but a useful investigative aid is measurement of plasma osmolality. Hyponatremia with a high plasma osmolality is caused by hyperglycemia, while a normal plasma osmolality indicates pseudohyponatremia or the post-transurethral prostatic resection syndrome. Castillo et al. discuss the diagnosis and management of hyponatremia in cancer patients. Small cell lung cancer is the most common malignancy to cause SIADH in this cohort. Other causes of hyponatremia include hypovolemic hyponatremia secondary to diarrhea and vomiting caused by chemotherapy. Illustration A depicts the mechanism of ADH production in a small cell lung cancer. Incorrect Answers: Answers 1 and 2: Congestive heart failure and nephrosis cause hypervolemic hyponatremia, which would result in Urine Na < 10, Fractional Excretion Na < 1%. Answer 3: Prerenal acute kidney injury is a cause of hypovolemic hyponatremia. One would expect Urine Na < 20, Fractional Excretion Na < 1%. Answer 4: Mineralocorticoid deficiency causes hypovolemic hyponatremia. One would expect Urine Na > 20 and Fractional Excretion Na > 1%.
3.7
(6)
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