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Difficulty: MediumPsychopharmacology and Psychological Therapies

A 52-year-old man is brought to the emergency department by his wife due to worsening confusion, unsteadiness, and a prominent tremor over the past three days. He has a history of bipolar I disorder that has been well controlled on lithium carbonate for the past five years. Two weeks ago, he was diagnosed with essential hypertension, and a new daily medication was added to his regimen. His temperature is 37.1 C (98.8 F), blood pressure is 135/85 mm Hg, and pulse is 88/min. On examination, the patient is lethargic but arousable. Neurological examination reveals dysarthria, an unsteady, broad-based gait, and a coarse resting tremor in both hands. Laboratory studies show a serum creatinine of 1.2 mg/dL (baseline 0.9 mg/dL) and a serum lithium level of 2.1 mEq/L (therapeutic range: 0.6-1.2 mEq/L). Which of the following best describes the mechanism by which the new antihypertensive medication precipitated this patient's condition?

  1. Decreased renal clearance due to volume depletion and increased proximal tubule reabsorptionAnswer
  2. B
    Inhibition of hepatic cytochrome P450 enzyme metabolism
  3. C
    Displacement of the psychiatric medication from plasma carrier proteins
  4. D
    Pharmacodynamic synergism at central postsynaptic dopamine receptors
  5. E
    Increased gastrointestinal absorption due to prolonged transit time

Answer

Decreased renal clearance due to volume depletion and increased proximal tubule reabsorption
The patient presents with acute lithium toxicity, characterized by neurologic symptoms (confusion, ataxia, dysarthria, and a coarse tremor) and an elevated serum lithium level. The most likely precipitant is the recent addition of a thiazide diuretic (e.g., hydrochlorothiazide) for essential hypertension. Thiazide diuretics act at the distal convoluted tubule to increase sodium and water excretion, leading to mild systemic volume depletion. This hypovolemia triggers the renin-angiotensin-aldosterone system and a compensatory increase in sodium reabsorption at the proximal convoluted tubule (PCT). Because lithium and sodium share similar transport mechanisms in the kidney, lithium reabsorption is concomitantly increased at the PCT. This results in decreased renal clearance of lithium, rapid accumulation, and subsequent toxicity. Other common drugs that decrease lithium clearance include NSAIDs (via decreased afferent arteriolar blood flow) and ACE inhibitors/ARBs.

Step-by-Step Solution

1
Evaluate the clinical presentation and laboratory results.
The patient exhibits confusion, ataxia, dysarthria, a coarse tremor, and a supratherapeutic serum lithium level (2.1 mEq/L), consistent with acute lithium toxicity.
Recognizing the toxic syndrome is the necessary first step to determine its underlying etiology.
2
Identify the precipitating event.
The recent initiation of a new antihypertensive medication is the likely trigger for the toxicity.
Several common drug classes interact dangerously with lithium, most notably certain antihypertensives and analgesics.
3
Determine the pharmacokinetic interaction between the likely antihypertensive (a thiazide diuretic) and lithium.
Thiazides induce volume depletion, which triggers a compensatory increase in the reabsorption of sodium and lithium in the proximal convoluted tubule, decreasing overall renal clearance.
Lithium shares renal transport mechanisms with sodium, making its clearance highly dependent on the patient's volume status and concurrent diuretic use.

Key Concept

Pharmacokinetic interactions of Lithium
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Topics

Psychopharmacology and Psychological Therapies

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