A 40-year-old woman is brought to the emergency department due to ataxia and tremor. The patient's symptoms started gradually 2 weeks ago and have worsened acutely over the past 2 days. She is having difficulty walking and almost fell while getting up from a chair. The patient's medical problems include bipolar disorder and recently diagnosed hypertension. Her medications include a stable dose of lithium for many years and a new blood pressure medication that was started several weeks ago. Vital signs are within normal limits. Neurologic examination reveals a resting tremor and difficulty with balance. Physical examination is otherwise normal. A drug interaction involving which of the following medications is most likely causing this patient's condition?

Question

A 40-year-old woman is brought to the emergency department due to ataxia and tremor.  The patient's symptoms started gradually 2 weeks ago and have worsened acutely over the past 2 days.  She is having difficulty walking and almost fell while getting up from a chair.  The patient's medical problems include bipolar disorder and recently diagnosed hypertension.  Her medications include a stable dose of lithium for many years and a new blood pressure medication that was started several weeks ago.  Vital signs are within normal limits.  Neurologic examination reveals a resting tremor and difficulty with balance.  Physical examination is otherwise normal.  A drug interaction involving which of the following medications is most likely causing this patient's condition?

Quizplus · Accepted Answer

11ec160b_3c53_839d_9249_9569fefb65b7_MD0001_00 Lithium has a very narrow therapeutic index. Patients with acute toxicity initially develop symptoms of gastrointestinal upset (eg, vomiting, diarrhea) with later development of neurologic findings (eg, neuromuscular excitability, delirium) as the drug penetrates the central nervous system. However, chronic lithium toxicity generally presents with gradual onset of neurologic symptoms, such as this patient's worsening ataxia and tremor. Lithium is similar to sodium in its properties and is almost exclusively excreted by the kidneys. Like sodium, it is filtered and reabsorbed mostly in the proximal tubules (>60%). Any cause of decreased glomerular filtration (eg, volume depletion, congestive heart failure, cirrhosis) can increase proximal sodium/lithium reabsorption and lead to increased lithium retention. Thiazide diuretics limit sodium reabsorption in the distal tubule, causing mild volume depletion (and potential hyponatremia) that stimulates proximal tubular sodium/lithium reabsorption, leading to lithium toxicity over time. ACE inhibitors and nonsteroidal anti-inflammatory drugs (NSAIDs) can also impair lithium clearance and lead to toxicity. (Choice A) Amiloride is a potassium-sparing diuretic that works primarily on the distal tubule and collecting duct by inhibiting sodium reabsorption. Potassium-sparing diuretics often decrease serum lithium levels. (Choices B, C, and E) Clonidine, hydralazine, and metoprolol do not alter renal ion exchange or cause volume depletion and therefore do not cause lithium toxicity. Educational objective: Chronic lithium toxicity (eg, confusion, ataxia, neuromuscular excitability) can be precipitated by volume depletion and drug interactions with thiazide diuretics, ACE inhibitors, and nonsteroidal anti-inflammatory drugs. __________ References: Drug interactions with lithium: an update. (http://www.ncbi.nlm.nih.gov/pubmed/26936045)

A 40-Year-Old Woman Is Brought to the Emergency Department Due