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 (3–4 litres over 2 hours) after a prolonged gym session. On examination: BP 128/82 mmHg, HR 88/min, RR 20/min, temperature 37.2°C. Serum sodium 118 mEq/L, serum osmolality 245 mOsm/kg, urine osmolality 95 mOsm/kg, urine sodium 32 mEq/L. What is the primary pathophysiological mechanism underlying his clinical presentation?

Explanation

## Clinical Diagnosis: Acute Hyponatraemic Encephalopathy (Acute Water Intoxication) ### Pathophysiology **Key Point:** This patient presents with acute water intoxication following excessive water intake (3–4 litres over 2 hours). The critical question is: what is the primary pathophysiological mechanism? ### Analysis of Laboratory Findings | Parameter | Patient Value | Normal Range | Interpretation | |-----------|---------------|--------------|----------------| | Serum Na⁺ | 118 mEq/L | 135–145 mEq/L | Severe hyponatraemia | | Serum osmolality | 245 mOsm/kg | 280–295 mOsm/kg | Hypoosmolar | | Urine osmolality | 95 mOsm/kg | 50–1200 mOsm/kg | Maximally dilute urine | | Urine Na⁺ | 32 mEq/L | Variable | Reflects volume expansion | **High-Yield:** The urine osmolality of **95 mOsm/kg** is the pivotal finding. This near-maximally dilute urine proves that ADH is appropriately **suppressed** in response to low serum osmolality. The osmoregulatory axis is intact and functioning correctly. ### Why Option C is Correct **Clinical Pearl:** This is classic **primary polydipsia with overwhelmed renal excretory capacity**. The maximum renal free water excretion is approximately **0.8–1.0 L/hour**. When a person ingests 3–4 litres over 2 hours (~1.5–2 L/hour), this rate exceeds the kidney's maximum capacity to excrete free water, even with complete ADH suppression and maximally dilute urine. The result is progressive dilutional hyponatraemia despite intact osmoregulation. This is the mechanism described in Ganong's Review of Medical Physiology and Harrison's Principles of Internal Medicine. ### Why Other Options Are Incorrect - **Option A (Excessive ADH secretion):** Excessive ADH would produce **concentrated urine** (osmolality >600 mOsm/kg). This patient has dilute urine (95 mOsm/kg), ruling out ADH excess. - **Option B (Failure of ADH suppression):** This directly contradicts the laboratory data. Urine osmolality of 95 mOsm/kg proves ADH IS suppressed. "Failure of ADH suppression" describes SIADH, which would show urine osmolality >100 mOsm/kg in the setting of hypo-osmolality. Option B is factually incorrect for this clinical scenario. - **Option D (Nephrogenic diabetes insipidus):** NDI causes **hypernatraemia** and **polyuria with dilute urine** due to inability to concentrate urine. It does not cause hyponatraemia. ### Clinical Presentation Explained 1. **Neurological symptoms** (headache, confusion, nausea) → acute cerebral oedema from rapid fall in serum osmolality 2. **Vital signs relatively preserved** → acute process without significant volume depletion 3. **Dilute urine** → confirms intact ADH suppression; kidneys are responding appropriately but cannot keep pace with intake ### Management Principle **Key Point:** Symptomatic acute hyponatraemia (Na⁺ <120 mEq/L with neurological features) requires **hypertonic saline (3%)** to raise serum sodium at a controlled rate (target: raise by 1–2 mEq/L/hour initially, not exceeding 8–10 mEq/L in 24 hours) to avoid osmotic demyelination syndrome (Harrison's, 21st edition). $$\text{Sodium deficit} = 0.6 \times \text{body weight (kg)} \times (\text{desired Na}^+ - \text{current Na}^+)$$ For a 70 kg man: deficit ≈ 0.6 × 70 × (125 – 118) = **294 mEq** → requires controlled hypertonic saline infusion with close monitoring.