A 42-year-old woman with chronic kidney disease (eGFR 25 mL/min) presents with polyuria and polydipsia. Her serum sodium is 152 mEq/L, serum osmolality is 310 mOsm/kg, and urine osmolality is 180 mOsm/kg despite elevated serum osmolality. Which is the most common cause of her diabetes insipidus?

Explanation

## Nephrogenic Diabetes Insipidus in CKD **Key Point:** In a patient with advanced CKD and the laboratory pattern of high serum osmolality + low urine osmolality (inability to concentrate urine), nephrogenic diabetes insipidus (NDI) is the most common cause. ### Pathophysiology of NDI in CKD Nephrogenic DI occurs when the kidney collecting duct is unable to respond to ADH (vasopressin), despite normal or elevated ADH levels. In CKD: 1. Loss of renal medullary osmotic gradient (reduced urea recycling, impaired countercurrent multiplier) 2. Downregulation of aquaporin-2 water channels in collecting duct 3. Chronic interstitial fibrosis and tubular atrophy 4. Uremia-induced ADH resistance ### Diagnostic Features of DI | Feature | Central DI | Nephrogenic DI | Primary Polydipsia | |---------|-----------|----------------|--------------------| | Serum osmolality | High (>295) | High (>295) | Normal (280–295) | | Urine osmolality | Low (<200) | Low (<200) | Low (<200) | | ADH level | Low/absent | High/elevated | Suppressed | | Response to desmopressin | Improves | No improvement | No improvement | | Thirst mechanism | Intact | Intact | Exaggerated | **High-Yield:** CKD is one of the most common causes of acquired nephrogenic DI in adults. Other common causes include lithium therapy, hypercalcemia, and post-obstructive diuresis. ### Why NDI is Most Common in CKD - Prevalence increases as eGFR declines (especially <30 mL/min) - Multifactorial: loss of medullary gradient + tubular dysfunction + uremia - Occurs in ~50% of patients with advanced CKD - Central DI from pituitary disease is much rarer in the general population **Clinical Pearl:** The key discriminator is the **inability to concentrate urine despite elevated ADH levels**. In central DI, desmopressin (synthetic ADH) improves urine osmolality; in nephrogenic DI, it does not. **Mnemonic: Causes of Nephrogenic DI** — **LITHIUM** (Lithium, Hypercalcemia, Hypokalemia, Interstitial kidney disease, Ureteral obstruction/post-obstructive, Medullary cystic disease, Amyloidosis).