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Difficulty: HardFormulating a Differential Diagnosis

A 54-year-old man presents to the emergency department with profound weakness and difficulty standing up from a seated position. Over the past 3 months, he has experienced progressive fatigue, a 10-kg10\text{-kg} (22-lb22\text{-lb}) weight loss, and worsening of his previously well-controlled hypertension. He has a 40-pack-year smoking history. Vital signs show a blood pressure of 170/100 mmHg170/100 \text{ mmHg} and a heart rate of 88/min88\text{/min}. Physical examination is notable for generalized muscle weakness, particularly in the proximal muscles of the lower extremities, and hyperpigmentation of the palmar creases. Laboratory evaluation reveals the following:

Laboratory TestResultReference Range
Serum sodium145 mEq/L145 \text{ mEq/L}136−145 mEq/L136-145 \text{ mEq/L}
Serum potassium2.2 mEq/L2.2 \text{ mEq/L}3.5−5.0 mEq/L3.5-5.0 \text{ mEq/L}
Serum chloride98 mEq/L98 \text{ mEq/L}98−106 mEq/L98-106 \text{ mEq/L}
Serum bicarbonate34 mEq/L34 \text{ mEq/L}22−28 mEq/L22-28 \text{ mEq/L}
Blood glucose210 mg/dL210 \text{ mg/dL}70−99 mg/dL70-99 \text{ mg/dL}

Which of the following is the most likely underlying diagnosis?

  1. Ectopic adrenocorticotropic hormone secretionAnswer
  2. B
    Primary hyperaldosteronism
  3. C
    Pituitary adrenocorticotropic hormone-producing adenoma
  4. D
    Primary adrenal insufficiency
  5. E
    Adrenocortical carcinoma

Answer

Ectopic adrenocorticotropic hormone (ACTH) secretion is the most likely diagnosis.
The patient's presentation of worsening hypertension, profound hypokalemia, metabolic alkalosis, hyperglycemia, and proximal muscle weakness is highly indicative of severe hypercortisolism. The presence of hyperpigmentation indicates that the hypercortisolism is ACTH-dependent, as ACTH and its precursor pro-opiomelanocortin (POMC) share sequence homology with melanocyte-stimulating hormone. In a patient with a significant smoking history and weight loss (rather than typical Cushingoid weight gain), the most likely diagnosis is ectopic ACTH secretion, often from a small cell lung cancer. The massive amounts of cortisol produced overwhelm the renal 11β11\beta-hydroxysteroid dehydrogenase type 2 enzyme, allowing cortisol to activate mineralocorticoid receptors, leading to the profound hypokalemia and alkalosis seen in this patient.

Step-by-Step Solution

1
Analyze the patient's vital signs and laboratory results.
The patient has severe hypertension, profound hypokalemia (2.2 mEq/L2.2 \text{ mEq/L}), metabolic alkalosis (bicarbonate 34 mEq/L34 \text{ mEq/L}), and hyperglycemia (210 mg/dL210 \text{ mg/dL}).
Identifying the primary metabolic derangement helps narrow the differential to conditions causing mineralocorticoid and glucocorticoid excess.
2
Synthesize the laboratory data with the physical examination findings.
The combination of mineralocorticoid effects (hypertension, hypokalemia) and glucocorticoid effects (hyperglycemia, proximal muscle weakness), along with hyperpigmentation, points to an ACTH-dependent cause of hypercortisolism.
Hyperpigmentation is caused by elevated ACTH and melanocyte-stimulating hormone (MSH) levels, ruling out ACTH-independent causes like an adrenal tumor.
3
Differentiate between the ACTH-dependent causes (Pituitary vs. Ectopic).
The patient's significant weight loss, heavy smoking history, and extreme severity of hypokalemia strongly favor an ectopic ACTH source (e.g., small cell lung cancer) over a pituitary adenoma.
Pituitary adenomas (Cushing disease) typically present with weight gain and milder metabolic derangements, whereas ectopic ACTH secretion often presents with weight loss (due to malignancy) and severe hypokalemia from overwhelming cortisol production.

Key Concept

Patients with ectopic ACTH secretion (often due to small cell lung cancer) typically present with severe hypokalemic metabolic alkalosis, hypertension, hyperpigmentation, and weight loss, distinguishing it from pituitary Cushing disease which classically presents with weight gain and milder electrolyte abnormalities.
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Formulating a Differential Diagnosis

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