Polyuria

Definition

Two Fundamental Categories

1. Water Diuresis

A. Central Diabetes Insipidus (CDI)

• Idiopathic (selective destruction of AVP-secreting neurons, often autoimmune)
• Hypophysectomy, pituitary surgery, or trauma
• Neoplastic, inflammatory, vascular, or infectious hypothalamic/pituitary disease (trauma, tumor, sarcoidosis, histiocytosis, Sheehan's syndrome, Guillain-Barré, fat embolus, empty sella)
• Congenital lesions

B. Nephrogenic Diabetes Insipidus (NDI)

• Congenital: Mutations in vasopressin V2 receptor gene (X-linked) or AQP2 gene
• Acquired tubular diseases: Pyelonephritis, analgesic nephropathy, multiple myeloma, amyloidosis, obstruction, sarcoidosis, hypercalcemia, hypokalemia, Sjögren's syndrome, sickle cell anemia
• Drugs/toxins: Lithium (most common drug cause - blocks AQP2 insertion), demeclocycline, methoxyflurane, ethanol, diphenylhydantoin, amphotericin B

C. Primary (Psychogenic) Polydipsia

• Psychiatric disorders (most common - schizophrenia, bipolar disorder)
• Hypothalamic disease
• Drugs causing dry mouth (thioridazine, chlorpromazine, anticholinergics)
• Peripheral disorders causing elevated renin/angiotensin II

2. Solute (Osmotic) Diuresis

• Glucosuria from uncontrolled diabetes mellitus - the most common cause of solute diuresis; causes hypertonic volume depletion
• Mannitol administration (iatrogenic)
• Radiocontrast media
• High-protein enteral/parenteral nutrition - increased urea production and excretion
• Post-obstructive diuresis - retained urea as primary osmotic agent
• Resolving ATN - natriuresis and polyuria from tubular damage
• Salt-wasting disorders (Bartter's syndrome, cystic kidney disease) - direct impairment of sodium reabsorption
• Renal failure - early sign is loss of concentrating ability (isosthenuria), presenting as nocturia and polyuria

Diagnostic Approach

Figure 55-4 from Harrison's Principles of Internal Medicine 22E - Approach to the patient with polyuria

Copeptin-Based Testing (Modern Approach)

• Copeptin ≥21.4 pmol/L: confirms partial or complete nephrogenic DI
• Copeptin <2.6 pmol/L: identifies complete central DI
• Intermediate levels: need water deprivation test OR hypertonic saline stimulation test
• Goldman-Cecil Medicine, p. 2422

Water Deprivation Test

• Fluids withheld under controlled supervision; urine osmolality and body weight measured sequentially
• Endpoint: Two consecutive urine samples differing <10% in osmolality AND patient has lost ≥2% body weight
• 2 μg desmopressin (DDAVP) IV or SC is then given; urine osmolality checked at 2 hours

• Goldman-Cecil Medicine, p. 2422

Treatment

Central DI

• Desmopressin (DDAVP) is the drug of choice - a synthetic AVP analogue with prolonged action and minimal vasopressor effects
  • Oral tablets: start with half of a 0.1 mg tablet; typical schedule every 8-12 hours
  • Intranasal spray: 10 μg fixed dose per spray
  • IV/SC: 1-2 μg every 8-12 hours in hospitalized patients
• Monitor serum sodium regularly to prevent hyponatremia from over-treatment
• Post-traumatic/post-surgical CDI: may recover within the first year - "hold" one dose weekly to assess ongoing need

Nephrogenic DI

• Remove the offending drug if drug-induced (e.g., lithium)
• Amiloride 5-10 mg/day for lithium-induced NDI - blocks lithium entry into distal tubule cells
• Thiazide diuretics (e.g., hydrochlorothiazide): induce mild hypovolemia, increasing proximal tubular reabsorption of water and reducing distal flow
• NSAIDs occasionally used to reduce prostaglandin-mediated inhibition of AVP action
• Congenital NDI: high-dose DDAVP (10-20 μg SC or 100-200 μg intranasal) may work partially; thiazides + amiloride are the mainstay

Primary Polydipsia

• Treat the underlying psychiatric disorder
• Preliminary evidence: GLP-1 agonist dulaglutide 1.5 mg weekly may reduce thirst and fluid intake - Goldman-Cecil Medicine, p. 2424

Osmotic Diuresis

• Address the underlying cause (optimize glycemic control in DM, discontinue mannitol, manage post-obstructive state)
• Replace fluid and electrolyte losses

Gestational DI

• Desmopressin is the drug of choice - unaffected by placental vasopressinase (which degrades native AVP)
• Often resolves after delivery

Key Complications

• Hypernatremia and hyperosmolarity: develops if thirst mechanism is also impaired (dipsogenic failure + DI is particularly dangerous) - repeated episodes cause irreversible brain damage
• Bladder and upper tract dilatation: chronic massive polyuria from childhood can cause hydronephrosis and megabladder
• Hyponatremia: from over-treatment with desmopressin or primary polydipsia
• Volume depletion: particularly in solute diuresis (e.g., DM, post-obstructive)

Summary Table

• Harrison's Principles of Internal Medicine 22E (2025), Chapter 55
• Goldman-Cecil Medicine International Edition, Chapters 102 & 206
• Brenner and Rector's The Kidney, Chapter 24
• Comprehensive Clinical Nephrology, 7th Edition

Why Hypercalcemia Causes Polyuria

Mechanism 1: CaSR Activation in the Thick Ascending Limb (TAL) - Destroys the Medullary Gradient

• High tubular fluid Ca²⁺ activates the CaSR on the basolateral surface of TAL cells
• This inhibits the NKCC2 cotransporter (Na⁺-K⁺-2Cl⁻), which is the principal NaCl reabsorber in the TAL
• It also inhibits ROMK channels (K⁺ recycling, needed to keep NKCC2 running) and directly inhibits paracellular NaCl permeability

"Stimulation of the calcium receptor in the basolateral membrane of the thick ascending limb... reduces transcellular solute flux by inhibiting NKCC2 and ROMK channels and also by direct inhibition of paracellular permeability... Overall, this reduces the medullary osmotic gradient for water reabsorption." - Comprehensive Clinical Nephrology, 7th Ed.

"Calcium signals directly through the calcium-sensing receptor to downregulate Na⁺, K⁺, and Cl⁻ transport by the TALH and water transport in principal cells, thus reducing renal concentrating ability in hypercalcemia." - Harrison's Principles of Internal Medicine 22E, p. 388

Mechanism 2: CaSR Activation in the Collecting Duct Principal Cell - Blocks AQP2 Insertion

• Apical CaSR activation prevents AVP-stimulated insertion of AQP2 water channels into the luminal membrane
• Without apical AQP2, the principal cell becomes water-impermeable regardless of AVP signaling
• Water cannot cross from the tubular lumen into the cell and onward into the interstitium

"With hypercalcemia, an elevated concentration of Ca²⁺ in tubular fluid activates the calcium-sensing receptor in the apical membrane of the principal cell, preventing insertion of AQP2. This is thought to be a protective mechanism to hinder kidney stone formation." - Comprehensive Clinical Nephrology, 7th Ed.

Mechanism 3: Downregulation of AQP2 Expression

• Downregulation of AQP2 gene expression in collecting duct cells (similar to what is seen in hypokalemia)
• This reduces the total cellular AQP2 pool available for membrane insertion

"Hypercalcemia has also been found to be associated with downregulation of AQP2." - Brenner and Rector's The Kidney

Summary Diagram of Mechanisms

HIGH SERUM Ca²⁺
       |
       ├──► CaSR activation (TALH basolateral)
       │         ↓
       │    ↓ NKCC2, ↓ ROMK, ↓ paracellular NaCl
       │         ↓
       │    ↓ Medullary osmotic gradient
       │         ↓
       │    Even with AQP2 open → ↓ water reabsorption
       │
       ├──► CaSR activation (Collecting duct apical)
       │         ↓
       │    Blocks AQP2 insertion into apical membrane
       │         ↓
       │    Collecting duct becomes water-impermeable
       │
       └──► Chronic: ↓ AQP2 gene expression
                 ↓
            Fewer AQP2 channels available

                ALL THREE → DILUTE, HIGH-VOLUME URINE (NDI)

The Vicious Cycle

1. NDI → large volume diuresis → volume depletion
2. Volume depletion → ↑ proximal tubule Na⁺ and Ca²⁺ reabsorption (solvent drag)
3. Less Ca²⁺ reaches the urine → hypercalciuria falls → calcium not excreted
4. Volume depletion → ↑ renin-angiotensin → further proximal Ca²⁺ retention
5. Can also precipitate acute kidney injury → even less Ca²⁺ excretion

"The resulting diuresis worsens the hypercalcemia, because volume depletion limits the protective hypercalciuria and exacerbates the volume-dependent proximal tubule reabsorption of calcium." - NKF Primer on Kidney Diseases, 8th Ed.

• Comprehensive Clinical Nephrology, 7th Ed. - detailed CaSR/AQP2 mechanism
• Harrison's Principles of Internal Medicine 22E (2025) - TALH CaSR signaling
• Brenner and Rector's The Kidney - AQP2 downregulation
• NKF Primer on Kidney Diseases, 8th Ed. - vicious cycle and clinical consequences