A 58-year-old male smoker (40 pack-years) presents with a 3-month history of progressive dyspnea, cough, and chest pain. He reports recent onset of proximal muscle weakness and diplopia. Chest X-ray shows a central hilar mass with mediastinal widening. CT chest confirms a 4 cm central lung mass. Serum sodium is 118 mEq/L (normal 135–145). Which of the following paraneoplastic syndromes is most likely responsible for his hyponatremia?

Question

Accepted Answer

D. Syndrome of inappropriate antidiuretic hormone secretion (SIADH). ## Clinical Presentation & Pathophysiology

This patient has small cell lung cancer (SCLC) with paraneoplastic syndrome. The combination of **central hilar mass**, **mediastinal involvement**, and **severe hyponatremia (Na 118)** in a heavy smoker is classic for SCLC with SIADH.

**Key Point:** SIADH is the most common paraneoplastic endocrine syndrome in SCLC, occurring in 10–15% of cases. SCLC cells produce and secrete vasopressin (ADH) ectopically, leading to:
- Inappropriate water retention
- Dilutional hyponatremia
- Low serum osmolality with inappropriately concentrated urine

## Differential Diagnosis of Hyponatremia in Lung Cancer

| Syndrome | Mechanism | Serum Osmolality | Urine Osmolality | Urine Na | SCLC Frequency |
|----------|-----------|------------------|------------------|----------|----------------|
| **SIADH** | Ectopic ADH secretion | Low | High (>200) | Normal/High | **Most common** |
| Ectopic ACTH | Cortisol → salt retention | Normal/High | Variable | Variable | 10–15% |
| Nephrotic syndrome | Proteinuria + renal dysfunction | Variable | Variable | Low | Rare |
| Cerebellar syndrome | Neurologic, not endocrine | Normal | Normal | Normal | Rare cause of hyponatremia |

**High-Yield:** SIADH in SCLC is characterized by:
- Hyponatremia with **low serum osmolality** (<280 mOsm/kg)
- **High urine osmolality** (>200 mOsm/kg) — paradoxical concentration despite low serum osmolality
- Normal renal and adrenal function
- Absence of volume depletion or edema

## Clinical Correlation

The patient's **proximal muscle weakness and diplopia** suggest concurrent Lambert–Eaton myasthenic syndrome (LEMS), another paraneoplastic syndrome in SCLC caused by anti-voltage-gated calcium channel (VGCC) antibodies. However, LEMS does not cause hyponatremia — the hyponatremia is attributable to SIADH.

**Clinical Pearl:** SCLC is notorious for **multiple simultaneous paraneoplastic syndromes**. Always screen for SIADH, LEMS, paraneoplastic encephalomyelitis, and ectopic ACTH when SCLC is diagnosed.

**Mnemonic: SIADH in SCLC** — **"Small Cell = SIADH Central"** — SCLC is the classic malignancy for SIADH; look for hyponatremia with low serum osmolality and high urine osmolality.

[cite:Robbins 10e Ch 15]

![Lung Cancer — Small Cell diagram](https://mmcphlazjonnzmdysowq.supabase.co/storage/v1/object/public/blog-images/explanation/3512.webp)

Answer

A. Ectopic adrenocorticotropic hormone (ACTH) production causing secondary hyperaldosteronism

Answer

B. Tumor-induced nephrotic syndrome with renal salt wasting

Answer

C. Paraneoplastic cerebellar degeneration with vasopressin dysregulation

Explanation

Clinical Presentation & Pathophysiology

Differential Diagnosis of Hyponatremia in Lung Cancer

Clinical Correlation

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Syndrome	Mechanism	Serum Osmolality	Urine Osmolality	Urine Na	SCLC Frequency
SIADH	Ectopic ADH secretion	Low	High (>200)	Normal/High	Most common
Ectopic ACTH	Cortisol → salt retention	Normal/High	Variable	Variable	10–15%
Nephrotic syndrome	Proteinuria + renal dysfunction	Variable	Variable	Low	Rare
Cerebellar syndrome	Neurologic, not endocrine	Normal	Normal	Normal	Rare cause of hyponatremia