# Situation: A 66-year-old man with small cell lung cancer is admitted with confusion, nausea, and muscle cramps. He has lost 9 kg over 3 months and eats poorly. His skin turgor and blood pressure are normal, and he has no edema. His results: serum sodium 116 mEq/L (135–145 mEq/L), serum potassium 3.1 mEq/L (3.5–5.0 mEq/L), serum osmolality 244 mOsm/kg (275–295 mOsm/kg), urine osmolality 540 mOsm/kg, and urine sodium 58 mEq/L. His thyroid and adrenal function are normal. Which explanation of his low serum sodium is correct?

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> 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 66-year-old man with small cell lung cancer is admitted with confusion, nausea, and muscle cramps. He has lost 9 kg over 3 months and eats poorly. His skin turgor and blood pressure are normal, and he has no edema. His results: serum sodium 116 mEq/L (135–145 mEq/L), serum potassium 3.1 mEq/L (3.5–5.0 mEq/L), serum osmolality 244 mOsm/kg (275–295 mOsm/kg), urine osmolality 540 mOsm/kg, and urine sodium 58 mEq/L. His thyroid and adrenal function are normal.

Which explanation of his low serum sodium is correct?

## 보기

1. Diuretic-like sodium loss from the kidneys is shrinking his fluid volume
2. Poor intake has depleted his total body sodium and his fluid volume
3. Water intake has exceeded what his kidneys are able to excrete
4. Excess antidiuretic hormone is making his kidneys retain free water **✔ 정답**

**정답: 4**

## 해설

He has low serum sodium and osmolality with inappropriately concentrated urine (540 mOsm/kg), high urine sodium, normal volume status, and normal thyroid and adrenal function: the picture of the syndrome of inappropriate antidiuretic hormone (SIADH), a common paraneoplastic effect of small cell lung cancer. Excess antidiuretic hormone makes the kidneys retain free water, which dilutes the serum sodium.

## 심화 해설

Core interpretation

This patient has euvolemic hypotonic hyponatremia caused by paraneoplastic SIADH from small cell lung cancer. The combination of low serum sodium (116 mEq/L), low serum osmolality (244 mOsm/kg), inappropriately concentrated urine (540 mOsm/kg), and high urine sodium (58 mEq/L) with normal volume status and normal thyroid/adrenal function is the classic diagnostic pattern of syndrome of inappropriate antidiuretic hormone secretion. Excess ADH makes the collecting ducts reabsorb free water even though the serum is already dilute, so the retained water expands total body water and dilutes serum sodium without causing edema.

Why the other options do not fit

Option 1 describes diuretic-like renal sodium loss causing volume depletion. However, this patient has normal skin turgor and blood pressure, no edema, and a urine sodium that is high but not because of a sodium-wasting state — the high urine sodium reflects ADH-driven water retention with ongoing dietary sodium intake, not true volume contraction. Option 2 suggests poor intake depleted total body sodium. Poor intake can contribute to hyponatremia, but it would typically produce a hypovolemic picture with low urine sodium as the kidneys conserve sodium. Here the urine sodium is high, which argues against simple depletion. Option 3 states water intake exceeded renal excretory capacity. That is partially true as a mechanism, but it does not explain why the kidneys cannot excrete the water. The defining problem is not the amount of water ingested; it is that ADH is preventing the kidneys from clearing free water.

Pathophysiology of SIADH in small cell lung cancer

Small cell lung cancer cells can ectopically produce ADH or ADH-like peptides. This paraneoplastic secretion is autonomous — it is not suppressed by low serum osmolality, which is the normal feedback signal. The result is sustained water retention in the collecting ducts through aquaporin-2 insertion. The retained water dilutes serum sodium, but because the water distributes across total body water, the patient remains euvolemic rather than edematous. Key point! Euvolemia plus concentrated urine plus high urine sodium is the triad that separates SIADH from hypovolemic or hypervolemic hyponatremia.

Diagnostic criteria applied to this case

| Diagnostic feature | Expected in SIADH | This patient |
| --- | --- | --- |
| Serum sodium | Low, hypotonic | 116 mEq/L |
| Serum osmolality | Low | 244 mOsm/kg |
| Urine osmolality | Inappropriately high, usually above 100 mOsm/kg | 540 mOsm/kg |
| Urine sodium | Usually above 40 mEq/L with normal sodium intake | 58 mEq/L |
| Volume status | Euvolemic | Normal turgor, normal BP, no edema |
| Thyroid and adrenal function | Normal | Normal |

Why the urine is concentrated despite low serum osmolality

In a healthy person, a serum osmolality of 244 mOsm/kg would suppress ADH to near zero, and the urine would be maximally dilute, often below 100 mOsm/kg. Here the urine osmolality is 540 mOsm/kg, which means ADH activity is present when it should be absent. Watch out! The term “inappropriate” in SIADH refers specifically to this mismatch: ADH is acting despite hypotonic plasma. This is the single most important concept for recognizing SIADH on a licensure exam.

Clinical correlation with the patient’s symptoms

Confusion, nausea, and muscle cramps are neurologic and neuromuscular manifestations of acute or subacute hypotonic hyponatremia. As serum sodium falls, water shifts into brain cells, causing cerebral edema and neurologic dysfunction. The muscle cramps are also aggravated by the mild hypokalemia (3.1 mEq/L), which can accompany SIADH because increased distal tubular flow from water retention enhances potassium secretion. The weight loss and poor intake are consistent with advanced small cell lung cancer, but they are not the primary cause of the sodium abnormality. The paraneoplastic ADH secretion is the dominant driver of the hyponatremia, even in a patient who is eating poorly.

Why this matters for nursing care

SIADH-related hyponatremia in small cell lung cancer is common. One review notes hyponatremia occurs in approximately 25% of patients with small cell lung cancer, and SIADH accounts for about 60% of those cases [3]. Another review emphasizes that hyponatremia is the most common electrolyte complication of solid tumors, with an incidence up to 18.9% in small cell lung cancer, and that it negatively affects quality of life and prognosis [2]. For the bedside nurse, the priorities are strict intake and output monitoring, daily weights, neurologic checks, and safe correction of sodium. Fluid restriction is often the first-line intervention, and vasopressin receptor antagonists such as tolvaptan may be used in refractory cases [1]. Key point! Rapid overcorrection of chronic hyponatremia can cause osmotic demyelination syndrome, so sodium correction must follow protocol-based limits regardless of the underlying cause.References (research sources)

- [1]Tolvaptan for paraneoplastic SIADH in small cell lung cancer: a scoping review.Research articleCammann VL, Sheryl M, Fürst T, Jehle AW. (2026) · DOI: 10.1016/j.ctarc.2026.101233

- [2]A narrative review of progress in diagnosis and treatment of small cell lung cancer patients with hyponatremia.Research articleWu R, Li C, Wang Z, Fan H, Song Y, Liu H (2020) · DOI: 10.21037/tlcr-20-1147

- [3]Hyponatraemia--SIADH in lung cancer diagnostic and treatment algorithms.Research articleGrohé C, Berardi R, Burst V (2015) · DOI: 10.1016/j.critrevonc.2015.04.005

## 임상 시나리오

Euvolemic Hypotonic Hyponatremia in Small Cell Lung CancerRecognizing Paraneoplastic SIADH
In small cell lung cancer, hyponatremia with low serum osmolality, urine osmolality >100 mOsm/kg, and urine sodium >40 mEq/L in a euvolemic patient strongly suggests SIADH from ectopic ADH production.

Excess ADH causes the collecting ducts to retain free water despite an already dilute serum, expanding total body water and diluting serum sodium without causing edema.

CautionDo not mistake high urine sodium for diuretic-induced volume depletion; normal skin turgor and blood pressure, plus concentrated urine, point to ADH-driven water retention rather than true sodium wasting or poor intake.

## 핵심 개념

- **SIADH** — Syndrome of inappropriate antidiuretic hormone secretion; excess ADH causes renal free water retention, leading to euvolemic hypotonic hyponatremia with concentrated urine and high urine sodium.
- **Euvolemic hypotonic hyponatremia** — Low serum sodium and osmolality with normal volume status, no edema, and no signs of dehydration; hallmark of SIADH.
- **Urine osmolality** — In SIADH, urine is inappropriately concentrated (>100 mOsm/kg) despite low serum osmolality, reflecting ongoing water reabsorption.
- **Urine sodium** — Typically >40 mEq/L in SIADH due to ADH-driven water retention and continued dietary sodium intake, distinguishing it from hypovolemic states.
- **Paraneoplastic syndrome** — Tumor-mediated hormone or cytokine effects; small cell lung cancer commonly causes SIADH through ectopic ADH production.

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