Situation: A 64-year-old man who has smoked 1 pack of cigare… | 마이메르시 MyMerci
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Nursing Practice III — Care of Clients with Problems in Surgery, Oxygenation, Fluid and Electrolytes, Infectious, Inflammatory and Immunologic Response, Cellular Aberrations
문제

Situation: A 64-year-old man who has smoked 1 pack of cigarettes a day for 40 years is admitted with newly diagnosed small cell lung cancer. Chemotherapy with cisplatin and etoposide is planned. On admission he is mildly confused. His results are: serum sodium 122 mEq/L (135–145), serum osmolality 258 mOsm/kg (275–295), urine osmolality 640 mOsm/kg, and urine sodium 45 mEq/L. He has no edema, his skin turgor is normal, and his blood pressure is 128/78 mmHg without orthostatic change. Which condition do these findings MOST likely indicate?

해설
He has low serum sodium and osmolality, yet his urine is concentrated with a high urine sodium, and he is neither dry nor edematous. Water retention without volume depletion or edema fits the syndrome of inappropriate antidiuretic hormone secretion, a paraneoplastic effect typical of small cell lung cancer. Each alternative would change at least one of these values or findings.
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심화 해설

Core interpretation
This patient’s laboratory pattern is the classic profile of euvolemic hyponatremia caused by syndrome of inappropriate antidiuretic hormone secretion (SIADH). The serum sodium is 122 mEq/L and serum osmolality is 258 mOsm/kg, which means the plasma is dilute. At the same time, the urine is inappropriately concentrated with a urine osmolality of 640 mOsm/kg and the urine sodium is elevated at 45 mEq/L. The combination of dilute plasma, concentrated urine, high urine sodium, and clinical euvolemia is the hallmark of SIADH.

Why this is SIADH in small cell lung cancer
Small cell lung cancer is well known for producing paraneoplastic SIADH. The tumor can secrete antidiuretic hormone (ADH) or an ADH-like substance independently of the normal osmoregulatory feedback system. As a result, the renal collecting ducts continue to reabsorb free water even though the plasma is already hypo-osmolar. Water is retained without a corresponding sodium deficit, so the hyponatremia represents an excess of plasma water rather than true sodium depletion. This explains why the patient has no signs of volume depletion: normal skin turgor, no edema, and stable blood pressure without orthostatic change.

How the differential diagnoses are excluded
The key is to look at the urine osmolality and urine sodium together with the volume status.

ConditionExpected urine osmolalityExpected urine sodiumExpected volume statusWhy it does not fit this patient
SIADHHigh, inappropriately concentratedUsually >40 mEq/LEuvolemic or mildly hypervolemicMatches all findings
Hypovolemic hyponatremia from poor oral intakeHigh as an appropriate ADH response to volume lossLow, usually

임상 시나리오

SIADH in Small Cell Lung CancerEuvolemic Hyponatremia Recognition and Management

The diagnostic triad for SIADH is low serum osmolality (258 mOsm/kg), inappropriately concentrated urine (640 mOsm/kg), and elevated urine sodium (45 mEq/L) in a clinically euvolemic patient.

Small cell lung cancer is the most common malignancy causing paraneoplastic SIADH. Tumor cells secrete ADH or ADH-like peptides independent of normal osmoregulation, leading to free water retention without sodium deficit.

Volume status is the key discriminator. Normal skin turgor, no edema, and stable blood pressure without orthostatic change confirm euvolemia, ruling out hypovolemic and hypervolemic causes.

Caution

Correct serum sodium slowly in SIADH. Rapid correction beyond 8-10 mEq/L per 24 hours risks osmotic demyelination syndrome. Fluid restriction is first-line, with vasopressin receptor antagonists reserved for refractory cases.

핵심 개념

  • SIADH — Syndrome of inappropriate antidiuretic hormone secretion causing euvolemic hyponatremia with concentrated urine and high urine sodium
  • Euvolemic hyponatremia — Low serum sodium with normal total body water and sodium, no edema or volume depletion
  • Paraneoplastic syndrome — Tumor secretion of hormones or hormone-like substances causing systemic effects distant from the tumor
  • Urine osmolality — Measure of urine solute concentration, elevated in SIADH despite dilute plasma
  • Serum osmolality — Measure of plasma solute concentration, low in hyponatremia from water excess
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