Question:
A 55-year-old woman presents to the emergency department with confusion, nausea, and generalized weakness. She has a history of chronic obstructive pulmonary disease (COPD), for which she uses inhalers as prescribed. Physical examination reveals a thin woman with a normal blood pressure and heart rate. Laboratory findings are as follows:

• Serum sodium: 118 mEq/L (Normal: 135-145 mEq/L)

• Serum osmolality: 260 mOsm/kg (Normal: 275-295 mOsm/kg)

• Urine sodium: 30 mEq/L

• Urine osmolality: 480 mOsm/kg

Which of the following is the most likely underlying cause of her symptoms?

A) Psychogenic polydipsia
B) SIADH
C) Addison’s disease
D) Primary polydipsia
E) Congestive heart failure

Correct Answer B) SIADH

Chronic Illness (COPD) or Hypoxia
⬇️
🧠Trigger inappropriate ADH secretion due to stress on the body
⬇️
↑ ADH (Antidiuretic Hormone) Secretion
⬇️
ADH released even when plasma osmolality is low
🧠 ADH conserves water to counter stress but is inappropriate here
⬇️
↑ Water Retention in Kidneys (Collecting Ducts)
⬇️
ADH increases water permeability in collecting ducts, leading to water retention
🧠 Free water reabsorbed, diluting plasma sodium without affecting sodium content directly
⬇️
Dilutional Hyponatremia Develops

⬇️
Excess water dilutes serum sodium, causing hyponatremia (⬇️ serum Na⁺)
⬇️
Plasma Becomes Hypotonic (↓ Serum Osmolality)
⬇️
Plasma hypotonicity due to excess water retention
⬇️
Water Shifts into Cells
🧠 Hypotonicity causes water to move into cells, leading to cellular swelling (especially in the brain)
⬇️
Cerebral Edema
⬇️
Brain cells swell due to intracellular water shift
🧠 Cellular swelling in the brain can lead to confusion, nausea, headache, weakness, and in severe cases, seizures and coma
⬇️
Symptoms of Hyponatremia Appear
⬇️
Confusion, nausea, generalized weakness
⬇️
Despite Low Serum Sodium, ADH Continues Concentrating Urine
⬇️
High urine osmolality and sodium due to ADH effects on kidneys
🧠 Urine sodium > 20 mEq/L and urine osmolality remain high, which is typical in SIADH

Why Other Options Are Incorrect:

Psychogenic Polydipsia
Excessive water intake → Dilutional hyponatremia
⬇️ Serum Na and ⬇️ Serum Osmolality
⬇️ Urine Osmolality (diluted due to excess water intake)

🧠 Explanation: Serum and urine osmolality would both be low if this were the cause
Result: Incorrect due to high urine osmolality in the patient

Addison’s Disease (Adrenal Insufficiency)
⬇️ Aldosterone → ⬇️ Na reabsorption in kidneys
⬇️ Serum Na and ⬆️ Serum K
⬇️ Serum Osmolality, but typically with low blood pressure

🧠 Explanation: Addison’s often presents with hypotension and hyperkalemia, which are absent here
Result: Unlikely due to normal BP and lack of hyperkalemia

Primary Polydipsia
↑ Water intake → ⬇️ Serum Na (dilutional hyponatremia)
⬇️ Serum Osmolality
⬇️ Urine Osmolality (due to water excretion)

🧠 Explanation: Would show low urine osmolality, not high as seen in patient
Result: Incorrect due to high urine osmolality

Congestive Heart Failure (CHF)
⬇️ Effective blood volume → Activation of RAAS and ADH
Water and Na retention → ⬇️ Serum Na
⬇️ Serum Osmolality

🧠 Explanation: Typically presents with peripheral edema and high blood pressure
Result: Incorrect due to lack of physical CHF signs

Flashcards

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