# 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?

> source: MyMerci (mymerci.kr)  
> url: https://mymerci.kr/pages/nclex_q.php?qn_id=629742  
> language: ko  
> subject: 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?

## 보기

1. Hypovolemic hyponatremia from poor oral intake
2. Dilutional hyponatremia from early heart failure
3. Inappropriate antidiuretic hormone secretion **✔ 정답**
4. Primary polydipsia from excessive water drinking

**정답: 3**

## 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.

## 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.

| Condition | Expected urine osmolality | Expected urine sodium | Expected volume status | Why it does not fit this patient |
| --- | --- | --- | --- | --- |
| SIADH | High, inappropriately concentrated | Usually >40 mEq/L | Euvolemic or mildly hypervolemic | Matches all findings |
| Hypovolemic hyponatremia from poor oral intake | High as an appropriate ADH response to volume loss | Low, usually | Dry mucous membranes, poor turgor, orthostasis | Urine sodium is high and volume status is normal |
| Dilutional hyponatremia from early heart failure | High due to reduced effective circulating volume | Low, usually | Edema, elevated JVP, possible pulmonary congestion | No edema and urine sodium is high |
| Primary polydipsia | Low, maximally dilute urine | Variable but often low | Euvolemic | Urine osmolality should be very low, not 640 mOsm/kg |

Key point! In hypovolemic hyponatremia and heart failure, the kidney is trying to conserve sodium, so the urine sodium is typically low. In SIADH, the kidney is receiving a constant ADH signal and the patient is euvolemic, so sodium continues to be excreted in the urine despite the low serum sodium.

Clinical significance of the confusion

The mild confusion is an important finding because it reflects the neurologic effect of hypo-osmolality. As the serum sodium falls, water shifts into brain cells, causing cerebral edema and symptoms such as confusion, lethargy, or seizures. The speed of onset matters more than the absolute sodium value. A serum sodium of 122 mEq/L can produce neurologic symptoms if the drop occurred relatively quickly, and the presence of confusion means the hyponatremia should be corrected cautiously and monitored closely.

Watch out! Rapid correction of chronic hyponatremia can cause osmotic demyelination syndrome. In SIADH, treatment focuses on fluid restriction and addressing the underlying cause, not on aggressive sodium replacement. The planned cisplatin and etoposide chemotherapy is relevant because tumor lysis during initial chemotherapy can transiently worsen SIADH by releasing additional ADH from dying tumor cells.

## 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.

CautionCorrect 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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