Nephrogenic Diabetes Insipidus (Nephrogenic DI)

A 46 year old male with bipolar disorder presents with excessive thirst and passage of large volumes of dilute urine for several months. He reports nocturia and drinks water continuously throughout the day. Medications include lithium therapy for many years. On examination, mucous membranes are dry. Laboratory investigations reveal serum sodium of 149 mEq/L and serum osmolality of 308 mOsm/kg. Urine osmolality remains low despite water deprivation and shows minimal increase after desmopressin administration.

Diagnosis?

Diagnosis is nephrogenic diabetes insipidus.

1. Definition

Nephrogenic diabetes insipidus is a disorder characterized by renal unresponsiveness to antidiuretic hormone (ADH), leading to impaired water reabsorption and excretion of large volumes of dilute urine.

2. Pathophysiology

1. ADH secretion is normal or elevated
2. Renal collecting ducts fail to respond appropriately to ADH
3. Impaired insertion or function of aquaporin-2 water channels reduces water reabsorption
4. Large volumes of dilute urine are excreted
5. Free water loss may lead to hypernatremia and hyperosmolality

3. Etiology

3.1 Genetic Causes

AVPR2 Mutation

1. Mutation of vasopressin V2 receptor gene
2. Most common hereditary form
3. Usually X-linked recessive

AQP2 Mutation

1. Mutation involving aquaporin-2 water channels
2. May be autosomal dominant or recessive

3.2 Acquired Causes

Drugs

1. Lithium (most common acquired cause)
2. Demeclocycline
3. Amphotericin B
4. Cisplatin

Carbamazepine more commonly causes SIADH rather than nephrogenic DI

Electrolyte Disorders

1. Hypercalcemia
2. Hypokalemia

Renal Disease

1. Chronic kidney disease
2. Obstructive uropathy
3. Tubulointerstitial disease

4. Clinical Features

1. Polyuria
2. Polydipsia
3. Nocturia
4. Preference for cold water
5. Dehydration
6. Hypernatremia in severe cases

Patients with intact thirst mechanisms may maintain near-normal serum sodium levels

Infants May Present With

1. Irritability
2. Failure to thrive
3. Recurrent fever
4. Vomiting

5. Diagnosis

5.1 Initial Laboratory Findings

1. Elevated serum osmolality
2. Hypernatremia may be present
3. Low urine osmolality
4. Low urine specific gravity

5.2 Water Deprivation Test

Findings

1. Plasma osmolality rises appropriately during water deprivation
2. Urine remains inappropriately dilute despite dehydration
3. Urine osmolality fails to rise adequately

5.3 Desmopressin Stimulation Test

Performed after water deprivation test

Findings in Nephrogenic DI

1. Minimal or no increase in urine osmolality after desmopressin administration
2. Partial nephrogenic DI may show a small increase in urine osmolality
3. Indicates renal resistance to ADH

5.4 Diagnostic Interpretation

Central DI

• Significant increase in urine osmolality after desmopressin

Nephrogenic DI

• Minimal or absent response to desmopressin

6. Differential Diagnosis

1. Central diabetes insipidus
2. Primary polydipsia
3. Osmotic diuresis:
• Diabetes mellitus
• Mannitol use
4. Chronic kidney disease

7. Management

7.1 Treat Underlying Cause

1. Discontinue offending drugs if possible
2. Correct hypercalcemia
3. Correct hypokalemia
4. Manage renal disease

7.2 Dietary Measures

1. Sodium restriction
2. Low-solute diet
3. Adequate free water intake

7.3 Thiazide Diuretics

1. Reduce polyuria by inducing mild volume depletion
2. Increase proximal tubular sodium and water reabsorption
3. Decrease distal tubular flow and urine volume

7.4 ENaC Blockers

Amiloride

1. Particularly useful in lithium-induced nephrogenic DI
2. Blocks lithium entry into collecting duct cells

Triamterene

• Alternative ENaC blocker

7.5 NSAIDs

Indomethacin

1. May reduce urine output by decreasing renal prostaglandin synthesis
2. Reduces prostaglandin-mediated antagonism of ADH action
3. Used in selected refractory cases

8. Complications

1. Severe dehydration
2. Hypernatremia
3. Neurologic dysfunction
4. Growth retardation in children
5. Chronic urinary tract dilation

9. Prognosis

1. Depends on underlying cause
2. Drug-induced nephrogenic DI may partially improve after stopping offending agent
3. Hereditary disease is usually chronic

10. Key Clinical Insight

Patient with persistent polyuria, hypernatremia, low urine osmolality, and failure to respond appropriately to desmopressin strongly suggests nephrogenic diabetes insipidus

11. Key Exam Points

1. Nephrogenic DI is caused by renal resistance to ADH
2. Lithium is the most common acquired cause
3. AVPR2 mutation is the most common hereditary cause
4. Urine remains dilute despite water deprivation
5. Minimal or no response to desmopressin distinguishes nephrogenic DI from central DI
6. Partial nephrogenic DI may show a small response to desmopressin
7. Hypercalcemia and hypokalemia are important reversible causes
8. Thiazide diuretics paradoxically reduce polyuria
9. Amiloride is particularly useful in lithium-induced nephrogenic DI
10. Indomethacin may be used in refractory cases