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Difficulty: HardPharmacologic Interventions

A 4242-year-old woman comes to the clinic reporting excessive thirst and frequent urination for the past two months. She states she drinks up to 6 L6\text{ L} of water daily and wakes up several times during the night to urinate. Her medical history is significant for bipolar I disorder, for which she has taken lithium for the past 88 years. She has a history of severe manic episodes with psychotic features when she previously attempted to switch to alternative mood stabilizers. Vital signs are within normal limits.

Laboratory studies reveal:

TestResultReference Range
Serum sodium147 mEq/L147\text{ mEq/L}136−145 mEq/L136-145\text{ mEq/L}
Serum potassium4.0 mEq/L4.0\text{ mEq/L}3.5−5.0 mEq/L3.5-5.0\text{ mEq/L}
Serum creatinine1.0 mg/dL1.0\text{ mg/dL}0.6−1.2 mg/dL0.6-1.2\text{ mg/dL}
Serum lithium0.8 mEq/L0.8\text{ mEq/L}0.6−1.2 mEq/L0.6-1.2\text{ mEq/L}
Urine osmolality140 mOsm/kg H2O140\text{ mOsm/kg H}_2\text{O}

A water deprivation test is performed. After 66 hours of fluid restriction, her urine osmolality is 155 mOsm/kg H2O155\text{ mOsm/kg H}_2\text{O}. Subcutaneous administration of desmopressin is given, and 22 hours later, her urine osmolality is 160 mOsm/kg H2O160\text{ mOsm/kg H}_2\text{O}. The patient's psychiatrist is consulted and strongly advises continuing the current lithium regimen due to the high risk of psychiatric decompensation.

Which of the following is the most appropriate pharmacologic intervention to manage this patient's polyuria?

  1. AmilorideAnswer
  2. B
    Desmopressin
  3. C
    Demeclocycline
  4. D
    Lisinopril
  5. E
    Tolvaptan

Answer

Amiloride
The correct intervention is amiloride. This patient has developed nephrogenic diabetes insipidus (NDI) secondary to chronic lithium therapy, which is confirmed by the low urine osmolality that fails to correct with water deprivation or exogenous desmopressin. Lithium enters the principal cells of the renal collecting duct through the epithelial sodium channels (ENaC), where it interferes with the cyclic AMP signaling cascade required for the insertion of aquaporin-2 water channels into the apical membrane. Because the patient cannot safely discontinue lithium due to the high risk of a severe bipolar relapse, amiloride is the treatment of choice. Amiloride is a potassium-sparing diuretic that specifically blocks the ENaC channels on the luminal surface of the principal cells. This blockade prevents lithium from entering the cells, thereby restoring the ability of antidiuretic hormone to promote water reabsorption. Importantly, amiloride accomplishes this without significantly decreasing systemic lithium clearance, allowing the patient to maintain therapeutic lithium levels for her psychiatric condition.

Step-by-Step Solution

1
Interpret the water deprivation test results.
The lack of significant increase in urine osmolality after water restriction and exogenous desmopressin confirms nephrogenic diabetes insipidus.
Accurate interpretation of laboratory data is essential to establish the correct etiology of the patient's polyuria and rule out central diabetes insipidus or primary polydipsia.
2
Identify the etiology of the nephrogenic diabetes insipidus.
The patient's chronic lithium therapy is identified as the underlying cause of her renal concentrating defect.
Lithium is a classic cause of acquired nephrogenic diabetes insipidus. It enters the principal cells of the collecting duct via ENaC channels and interferes with ADH signaling.
3
Determine the most appropriate pharmacologic intervention.
Amiloride is selected to treat the polyuria while maintaining the necessary lithium therapy.
Amiloride specifically blocks the ENaC channels in the collecting duct, preventing lithium uptake into the principal cells without significantly altering systemic lithium clearance.

Key Concept

Management of lithium-induced nephrogenic diabetes insipidus
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Topics

Pharmacologic Interventions

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