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Difficulty: MediumPsychopharmacology and Psychological Therapies

A 48-year-old woman with a history of bipolar I disorder is brought to the emergency department by her husband due to progressive confusion, a coarse hand tremor, and an unsteady gait over the last three days. Her psychiatric condition has been stable on a fixed dose of lithium carbonate for the past five years. Two weeks ago, she was diagnosed with essential hypertension and started on a new antihypertensive medication by her primary care provider. On physical examination, she appears lethargic, has a coarse resting tremor in both hands, and demonstrates ataxia on finger-to-nose testing. Vital signs are within normal limits except for a blood pressure of 128/82 mmHg. Which of the following antihypertensive medications was most likely prescribed to this patient?

  1. HydrochlorothiazideAnswer
  2. B
    Amlodipine
  3. C
    Metoprolol
  4. D
    Doxazosin
  5. E
    Clonidine

Answer

Hydrochlorothiazide
The correct answer is the medication that directly interacts with lithium clearance. The patient is exhibiting classic signs of lithium toxicity (coarse tremor, ataxia, confusion, lethargy) shortly after starting a new antihypertensive drug. Lithium is exclusively excreted by the kidneys, and its clearance is highly dependent on sodium handling in the proximal tubule. Thiazide diuretics cause mild volume depletion, which stimulates a compensatory increase in the reabsorption of both sodium and lithium in the proximal tubule. This significantly reduces lithium clearance, leading to elevated serum levels and neurologic toxicity.

Step-by-Step Solution

1
Analyze the patient's acute presentation.
The patient exhibits progressive confusion, a coarse hand tremor, and ataxia.
Identifying the clinical syndrome is essential. These are classic neurologic signs of lithium toxicity.
2
Identify potential triggers for the acute change in the patient's condition.
The patient was recently started on a new antihypertensive medication, which correlates temporally with the onset of her toxicity symptoms.
Lithium has a narrow therapeutic index, and its levels are easily altered by concurrent medications.
3
Determine which antihypertensive classes interact with lithium.
Medications that decrease glomerular filtration rate or increase proximal tubular reabsorption (e.g., thiazide diuretics, NSAIDs, ACE inhibitors) significantly elevate serum lithium levels.
Hydrochlorothiazide is a thiazide diuretic that induces mild volume depletion, prompting the kidneys to reabsorb more sodium and lithium, precipitating toxicity.

Key Concept

Lithium is cleared by the kidneys, and its reabsorption is tied to sodium. Thiazide diuretics, ACE inhibitors, and NSAIDs decrease lithium clearance and can rapidly precipitate lithium toxicity.
Estimated Time:1m 15s
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Topics

Psychopharmacology and Psychological Therapies

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