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Difficulty: HardPharmacologic Interventions

During a routine outpatient visit, a 4242-year-old man reports a 44-week history of unquenchable thirst and excessive urination. He estimates drinking over 6 L6 \text{ L} of water daily and wakes up 33 to 44 times each night to void. His only past medical history is bipolar I disorder. He has been maintained on lithium carbonate for the past 33 years, with excellent mood stability and no recent psychiatric hospitalizations. Vital signs are within normal limits.

Laboratory results are as follows:
Serum sodium: 148 mEq/L148 \text{ mEq/L}
Serum potassium: 4.1 mEq/L4.1 \text{ mEq/L}
Serum creatinine: 0.9 mg/dL0.9 \text{ mg/dL}
Serum lithium: 0.8 mEq/L0.8 \text{ mEq/L} (therapeutic range: 0.6−1.2 mEq/L0.6-1.2 \text{ mEq/L})
Urine osmolality: 135 mOsm/kg H2O135 \text{ mOsm/kg H}_2\text{O}

Following the administration of exogenous desmopressin, his urine osmolality remains largely unchanged at 140 mOsm/kg H2O140 \text{ mOsm/kg H}_2\text{O}. The patient's psychiatrist strongly advises against altering the bipolar treatment regimen, citing a history of severe, treatment-resistant mania prior to starting this specific medication. Which of the following is the most appropriate pharmacologic intervention to manage this patient's polyuria?

  1. AmilorideAnswer
  2. B
    Hydrochlorothiazide
  3. C
    Indomethacin
  4. D
    Desmopressin
  5. E
    Tolvaptan

Answer

Amiloride is the preferred pharmacologic treatment for lithium-induced nephrogenic diabetes insipidus because it blocks lithium's entry into the collecting duct cells without significantly altering systemic lithium clearance.
The correct answer is amiloride. Lithium causes nephrogenic diabetes insipidus by entering the principal cells of the collecting duct via epithelial sodium channels (ENaC) and interfering with the cAMP pathway, preventing aquaporin-2 insertion into the luminal membrane. Amiloride is an ENaC inhibitor that blocks lithium's entry into these cells, directly counteracting its toxic cellular effect. Crucially, amiloride is preferred over other typical NDI treatments (like thiazides or NSAIDs) because it does not significantly decrease the renal clearance of lithium, thereby maintaining stable and therapeutic serum lithium levels.

Step-by-Step Solution

1
Interpret the clinical presentation and laboratory findings.
The patient has polyuria, polydipsia, mild hypernatremia (148 mEq/L148 \text{ mEq/L}), and inappropriately dilute urine (135 mOsm/kg H2O135 \text{ mOsm/kg H}_2\text{O}) that does not respond to exogenous desmopressin.
This establishes the diagnosis of nephrogenic diabetes insipidus (NDI), which is a well-known adverse effect of chronic lithium therapy.
2
Determine the optimal management strategy given the constraint.
The psychiatrist advises continuing lithium. Therefore, a pharmacologic intervention that treats NDI without causing lithium toxicity must be selected.
Discontinuing the offending agent is the general primary treatment for drug-induced NDI, but the risk of manic relapse necessitates medical management of the side effect instead.
3
Evaluate the mechanism of available NDI treatments regarding lithium pharmacokinetics.
Amiloride is chosen over hydrochlorothiazide and indomethacin.
Lithium enters the collecting duct principal cells via the epithelial sodium channel (ENaC). Amiloride directly blocks ENaC, preventing lithium accumulation in these cells. Unlike thiazides and NSAIDs, amiloride does not significantly decrease lithium clearance, thus avoiding systemic lithium toxicity.

Key Concept

Management of lithium-induced nephrogenic diabetes insipidus involves using amiloride to block ENaC channels, preventing lithium entry into principal cells while avoiding the decreased lithium clearance seen with thiazides and NSAIDs.
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Pharmacologic Interventions

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