A 58-year-old man from Delhi presents to the emergency department with severe headache, confusion, and nausea for 6 hours. His wife reports he has been drinking excessive water (8–10 L/day) for the past 3 days following a bout of gastroenteritis. On examination: BP 128/82 mmHg, HR 88/min, RR 18/min, temperature 37.2°C. Serum sodium is 118 mEq/L (normal 135–145), serum osmolality 245 mOsm/kg (normal 280–295), and urine osmolality 180 mOsm/kg. Urine sodium is 45 mEq/L. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Primary Polydipsia ### Key Clinical Features **Key Point:** Primary polydipsia is excessive voluntary water intake leading to dilutional hyponatremia, characterized by low serum and urine osmolality with suppressed ADH levels. ### Laboratory Interpretation | Parameter | Patient Value | Normal | Significance | |-----------|---------------|--------|---------------| | Serum Na⁺ | 118 mEq/L | 135–145 | Hyponatremia | | Serum osmolality | 245 mOsm/kg | 280–295 | Hypoosmolality | | Urine osmolality | 180 mOsm/kg | 300–900 | Dilute urine | | Urine Na⁺ | 45 mEq/L | 40–100 | Normal | ### Pathophysiology 1. **Excessive water intake** → plasma dilution 2. **Serum osmolality falls** → ADH suppression (osmoreceptor feedback) 3. **Low ADH** → kidneys cannot concentrate urine 4. **Result:** Dilute urine (low osmolality) despite hyponatremia **High-Yield:** The **key discriminator** is the **low urine osmolality (180 mOsm/kg) in the presence of hyponatremia**. This indicates the kidneys are responding appropriately to suppress ADH — they are *not* retaining water inappropriately. ### Differential Diagnosis Logic ```mermaid flowchart TD A[Hyponatremia + Low Serum Osmolality]:::outcome --> B{Urine Osmolality?}:::decision B -->|Low < 200| C{History of Excessive Water Intake?}:::decision B -->|High > 300| D[SIADH]:::outcome C -->|Yes| E[Primary Polydipsia]:::action C -->|No| F[Central DI]:::outcome B -->|High despite low serum osm| G[Nephrogenic DI]:::urgent ``` ### Why This Is Primary Polydipsia - **Excessive water intake** documented in history (8–10 L/day) - **Low urine osmolality** proves kidneys can dilute urine (ADH is suppressed) - **No polyuria** (urine output not mentioned as excessive) - **Acute onset** (3 days) with identifiable trigger (gastroenteritis → psychogenic drinking) **Clinical Pearl:** In primary polydipsia, patients often have a psychiatric history or recent stressor. The kidneys are normal and respond appropriately by diluting urine when ADH is suppressed. ### Management - **Fluid restriction** (1–1.5 L/day) - Gradual correction of Na⁺ (no faster than 8–10 mEq/L in 24 hours to avoid osmotic demyelination) - Psychiatric evaluation if psychogenic cause suspected - Treatment of underlying gastroenteritis **Warning:** Rapid sodium correction can cause central pontine myelinolysis — a devastating complication.