A 52-year-old man undergoes spinal anesthesia for transurethral resection of prostate. Intraoperatively, he develops sudden onset of restlessness, confusion, and seizures 45 minutes after the procedure begins. Vital signs show hyponatremia (Na+ 118 mEq/L), bradycardia, and hypertension. Which investigation is most appropriate to confirm the suspected diagnosis?

Explanation

## Clinical Context: TURP Syndrome **Key Point:** The clinical presentation of acute hyponatremia (Na+ 118 mEq/L), restlessness, confusion, seizures, and hypertension occurring during/after spinal anesthesia for TURP is pathognomonic for TURP syndrome — a form of hypervolemic hyponatremia caused by absorption of hypotonic irrigating fluid. ## Diagnostic Approach ### Why Serum Osmolality & Urine Osmolality? This paired measurement is the **gold standard** for diagnosing and classifying hyponatremia: | Parameter | TURP Syndrome (Hypervolemic) | Expected Value | |-----------|------------------------------|----------------| | Serum Na+ | <120 mEq/L | Low | | Serum osmolality | <260 mOsm/kg | Low | | Urine osmolality | >200 mOsm/kg | Inappropriately high | | Urine Na+ | >40 mEq/L | Elevated | **Clinical Pearl:** In TURP syndrome, the kidneys are responding to volume expansion by attempting to conserve sodium (high urine osmolality), but the dilutional hyponatremia is so severe that CNS symptoms dominate. ### Mechanism of Hyponatremia in TURP 1. Absorption of hypotonic irrigant (glycine, sorbitol, or water) through opened venous sinuses 2. Acute volume expansion → dilutional hyponatremia 3. Hypervolemia triggers ANP release → further sodium wasting 4. Result: **low serum osmolality with inappropriately concentrated urine** **High-Yield:** Serum osmolality <260 mOsm/kg + urine osmolality >200 mOsm/kg in the setting of hyponatremia = TURP syndrome until proven otherwise. ## Why Other Investigations Are Insufficient **Arterial blood gas:** Provides pH and oxygenation status but does NOT diagnose the cause of hyponatremia or confirm fluid overload. **Cerebrospinal fluid analysis:** Unnecessary and invasive; CSF findings are non-specific in TURP syndrome (may show mild pleocytosis from cerebral edema, but diagnosis is biochemical, not CSF-based). **Electroencephalography:** Shows non-specific diffuse slowing but does NOT confirm hyponatremia or guide acute management. **Mnemonic:** **OSMO** = Osmolality & urine Sodium/osmolality Measurement for Osmotic emergencies. ## Clinical Management Correlation Once osmolality confirms hyponatremia with high urine osmolality: - **Acute symptomatic:** 3% hypertonic saline 4–6 mL/kg IV over 10–20 min - **Rate of correction:** ≤10 mEq/L in first 24 hours (avoid osmotic demyelination) - **Fluid restriction** if chronic/asymptomatic [cite:Gupta & Singh Textbook of Anesthesia 3e Ch 32]