A 62-year-old woman with a 35 pack-year smoking history presents with a 2-month history of progressive weakness, muscle pain, and difficulty climbing stairs. She also reports nausea, polyuria, and polydipsia. Physical examination reveals proximal muscle weakness and hyporeflexia. Serum sodium is 118 mEq/L (normal 135–145), serum calcium is 12.8 mg/dL (normal 8.5–10.5), and serum potassium is 2.8 mEq/L (normal 3.5–5.0). Chest X-ray shows a small central lung mass. Bronchoscopy with biopsy confirms small cell lung cancer. Which paraneoplastic syndrome is most likely responsible for the clinical presentation?

Explanation

## Paraneoplastic Syndromes in SCLC **Key Point:** Small cell lung cancer is the **most common malignancy associated with paraneoplastic syndromes**, occurring in 10–15% of patients. SCLC frequently secretes ectopic hormones (ACTH, ADH, calcitonin) and produces autoantibodies against neuronal and muscle antigens. ### Clinical Analysis of This Case | Finding | Interpretation | Mechanism | |---|---|---| | **Hyponatremia (Na 118)** | SIADH | Ectopic ADH secretion by tumor cells | | **Hypercalcemia (Ca 12.8)** | PTHrP secretion or osteolytic metastases | Ectopic parathyroid hormone-related peptide (PTHrP) | | **Hypokalemia (K 2.8)** | Hypokalemic metabolic alkalosis | Ectopic ACTH → cortisol excess → urinary K^+^ wasting | | **Proximal muscle weakness** | Steroid myopathy + hypokalemia | Cortisol excess + K^+^ depletion | | **Polyuria, polydipsia** | Hyperglycemia from cortisol excess | Ectopic ACTH → Cushing syndrome | | **Nausea** | Hypercalcemia + hyponatremia | Both cause GI symptoms | ### Ectopic Hormone Production in SCLC ```mermaid flowchart TD A[SCLC Diagnosis]:::outcome --> B[Neuroendocrine Differentiation]:::action B --> C[Ectopic Hormone Secretion]:::action C --> D1[ACTH]:::action C --> D2[ADH]:::action C --> D3[Calcitonin]:::action D1 --> E1[Cushing Syndrome]:::outcome D2 --> E2[SIADH]:::outcome D3 --> E3[Hypercalcemia]:::outcome E1 --> F1[Hypokalemia, Hyperglycemia, Hypertension]:::outcome E2 --> F2[Hyponatremia, Seizures, Confusion]:::outcome E3 --> F3[Polyuria, Polydipsia, Altered Mental Status]:::outcome ``` **High-Yield:** **SCLC is the most common cause of ectopic ACTH syndrome** (Cushing syndrome of malignancy), accounting for ~50% of all ectopic ACTH cases. The combination of **SIADH + Cushing syndrome** in a SCLC patient is pathognomonic. ### Why This Patient Has Both SIADH and Cushing Syndrome 1. **Ectopic ACTH:** SCLC cells express proopiomelanocortin (POMC), which is cleaved to ACTH. This drives cortisol overproduction → hypokalemia, hyperglycemia, muscle weakness, and hypertension. 2. **Ectopic ADH:** The same neuroendocrine tumor cells also secrete ADH, causing water retention and hyponatremia. 3. **PTHrP secretion:** Some SCLC tumors produce PTHrP, contributing to hypercalcemia. **Clinical Pearl:** The **hypokalemia in this case is severe (2.8 mEq/L) and multifactorial**: - Ectopic ACTH → cortisol excess → renal K^+^ wasting - Hyponatremia → osmotic shifts - Possible concurrent hypomagnesemia (common in Cushing syndrome) This combination explains the proximal muscle weakness and hyporeflexia (steroid myopathy + electrolyte derangement). ### Diagnostic Confirmation | Test | Expected Finding in This Case | |---|---| | **24-hour urinary cortisol** | Markedly elevated (>300 μg/24 h) | | **Plasma ACTH** | Elevated (>200 pg/mL) — ectopic source | | **Low-dose dexamethasone suppression test** | NO suppression (distinguishes ectopic ACTH from pituitary Cushing) | | **Serum osmolality** | Low (<280 mOsm/kg) | | **Urine osmolality** | Inappropriately high (>100 mOsm/kg) despite hyponatremia | ### Treatment Approach 1. **Urgent:** Correct hypokalemia (K^+^ repletion) and hyponatremia (fluid restriction for SIADH). 2. **Cortisol control:** Mitotane or ketoconazole to inhibit 11β-hydroxylase and reduce ACTH-driven cortisol synthesis. 3. **Definitive:** Initiate SCLC-directed chemotherapy (cisplatin + etoposide); tumor response often resolves paraneoplastic syndromes. **Warning:** Do NOT give dexamethasone to suppress ACTH in ectopic ACTH syndrome — it will NOT work (unlike pituitary Cushing). Use steroid synthesis inhibitors instead.