Review Question - QID 100380

This patient has the syndrome of inappropriate antidiuretic hormone secretion (SIADH). Of the diagnoses listed, lung cancer (specifically small cell carcinoma) is most likely to cause SIADH.

SIADH describes the inappropriate release of antidiuretic hormone due to (1) increased ADH release by the hypothalamus, (2) increased sensitivity of the kidney to ADH, or (3) ectopic release of ADH by a neoplasm. Small cell carcinomas of the lung can cause a paraneoplastic syndrome of SIADH. SIADH presents with hyponatremia, urine osmolality > 300, and random urine Na > 40.

Goh reviews the differential diagnosis of hyponatremia. They emphasize the urinary sodium concentration helps in diagnosing patients with low plasma osmolality. High urinary sodium concentration in the presence of low plasma osmolality can be caused by renal disorders, endocrine deficiencies, reset osmostat syndrome, SIADH, and medications. Low urinary sodium concentration is caused by severe burns, gastrointestinal losses, and acute water overload.

Castillo et al. describe hyponatremia in the oncology setting which is associated with a poor prognosis based on a systematic analysis of published studies. Most commonly, hyponatremia in the cancer patient is caused by SIADH, which is seen most frequently with small cell lung cancer.

Illustration A shows a cytology specimen of SCLC.

Incorrect Answers:
Answer 1: Aspirin overdose presents classically with a metabolic acidosis (Anion gap) and is sometimes seen with a concomitant respiratory alkalosis.
Answer 2: Diarrhea may present with hypernatremia, eunatremia, or hyponatremia but would also be hypokalemic.
Answer 3: DI presents with hypernatremia not hyponatremia.
Answer 4: Primary polydipsia presents with dilute urine (low osmolality) in the setting of hyponatremia.