A 58-year-old man with a 10-year history of type 2 diabetes mellitus presents to the emergency department with confusion and lethargy. His wife reports he has been increasingly drowsy over the past 3 days. On examination, he is oriented to person only, blood pressure is 128/82 mmHg, heart rate 88/min, and there are no focal neurological signs. Laboratory investigations reveal: serum sodium 118 mEq/L, serum osmolality 252 mOsm/kg, urine osmolality 580 mOsm/kg, urine sodium 45 mEq/L. His blood glucose is 156 mg/dL, HbA1c 8.2%, and renal function is normal. Chest X-ray shows no infiltrates. What is the most likely diagnosis?

## Clinical Diagnosis: SIADH ### Key Diagnostic Features **Key Point:** SIADH is diagnosed when hyponatremia occurs in the setting of inappropriately concentrated urine (high urine osmolality) despite low serum osmolality, with euvolemia and normal renal/adrenal function. In this patient: - Serum sodium 118 mEq/L (hyponatremia) - Serum osmolality 252 mOsm/kg (hypoosmolal) - Urine osmolality 580 mOsm/kg (inappropriately concentrated — should be <100 mOsm/kg in hypoosmolal state) - Urine sodium 45 mEq/L (adequate) - Euvolemic on exam (BP normal, no edema, no orthostasis) - Normal renal and adrenal function ### Pathophysiology of SIADH 1. Excessive ADH secretion → increased aquaporin-2 expression in collecting duct 2. Free water reabsorption exceeds solute reabsorption 3. Serum osmolality falls; urine remains concentrated 4. Hyponatremia develops despite normal kidney function ### Diagnostic Criteria for SIADH | Criterion | Finding | |-----------|----------| | Serum osmolality | <275 mOsm/kg | | Serum sodium | <135 mEq/L | | Urine osmolality | >100 mOsm/kg (often >200) | | Urine sodium | >40 mEq/L | | Volume status | Euvolemic | | TSH, cortisol | Normal | | Renal/adrenal function | Normal | **High-Yield:** The key discriminator is **inappropriately concentrated urine in the face of low serum osmolality** — the kidney is not responding appropriately to the osmotic stimulus to dilute urine. ### Common Causes in This Patient - Malignancy (lung, pancreas, bladder, prostate) - Pulmonary disease (pneumonia, TB, positive pressure ventilation) - CNS disease (meningitis, encephalitis, head trauma) - Medications (SSRIs, carbamazepine, vincristine, desmopressin) - Post-operative state **Clinical Pearl:** In a diabetic patient presenting with acute hyponatremia and altered mental status, always exclude SIADH before attributing symptoms to hyperglycemia alone. The normal glucose here makes SIADH the leading diagnosis. ### Management Approach ```mermaid flowchart TD A[Hyponatremia + Low Serum Osmolality]:::outcome --> B[Check Volume Status]:::decision B -->|Euvolemic| C[Check Urine Osmolality]:::decision C -->|High >100| D[SIADH likely]:::outcome D --> E[Assess TSH, Cortisol]:::action E --> F[Check for Malignancy/Infection]:::action F --> G[Fluid Restriction 800-1000 mL/day]:::action G --> H[Correct slowly: 6-8 mEq/L per 24 hrs]:::action ``` **Warning:** Rapid correction of chronic hyponatremia (>6–8 mEq/L in 24 hours) risks osmotic demyelination syndrome (ODS) — a devastating complication with permanent neurological sequelae.