Carbamazepine Diminishing the Effects of Warfarin and Leading to Mesenteric Embolism in a Patient with Atrial Fibrillation

Abstract

LEARNING OBJECTIVE: 1). To know drugs that decrease warfarin levels CASE: A 66 year old woman, nursing home resident, with history of chronic atrial fibrillation, hemiplegia secondary to a stroke, congestive heart failure, hypertension and seizure disorder, presented to the hospital with worsening nausea and vomiting persisting for 3 weeks. Her medications included digoxin, warfarin and enalapril. The physical exam showed hemiplegia. Standard admission work up revealed an INR of 2.3 and a high serum phenytoin level of 42 mg/dl. The patient had phenytoin toxicity, so phenytoin was stopped and the patient observed. Two days later the patient developed generalized clonic-tonic seizures. At that time the phenytoin level was 22 mg/dl. It was decided to give the patient loading doses of carbamazepine oral suspension in order to prevent further seizures. The following evening the patient suddenly developed acute abdomen. The INR was 1.4. The patient developed mesenteric infarction secondary to embolism, as revealed by urgent exploratory laparotomy. The patient expired the next day. DISCUSSION: The event was brought on by inadequate anticoagulation secondary to a drug-drug interaction between warfarin and carbamazepine that resulted in a drop in INR. This, in our patient with atrial fibrillation, allowed for cardiac thrombi formation and their embolisation to the mesenteric circulation. Drug-drug interactions with warfarin are among the most common interactions seen in clinical practice. The following drugs decrease warfarin level and predispose to thrombosis (remember the acronym DECREASE): Diuretics: spironolactone Estrogen containing oral contraceptives Carbamazepine, Corticosteroides, Rifampin Ethchlorvynol Alcoholism, Aminogluthethimide Sucralfate Etc: barbiturates, griseofulvin, and vitamin K (broccoli and bananas).