Case Presentation: A 63-year-old female with history of end-stage renal disease, atrial fibrillation (AFib), cirrhosis, and Parkinson’s disease was admitted to the hospital after an unwitnessed fall with persistent encephalopathy and altered mental status. Upon admission the patient was found to have wide QRS complexes with an Atrial fibrillation, clinical findings of heart failure, and a BNP of 16,568. The patient was intubated due to worsening encephalopathy and inability to protect her airway, without improvement of oxygen requirements. CT and MRI were negative for stroke. The patient was ultimately diagnosed with amantadine toxicity due to unintentional overdose. Two weeks before admission the patient increased their intake of amantadine to 100mg three times a day. Hospital care was complicated by pneumonia and sepsis with septic shock requiring vasopressors and treatment with antibiotics. The patient’s workup for encephalopathy included EEG, negative for seizures, repeated head CT, and a lumbar puncture that was negative for infection. Due to continued deterioration, the family decided to proceed with comfort measures only. The patient died twenty-three days after admission.
Discussion: This case reviews an occurrence of amantadine overdose in the treatment of Parkinson’s disease. Amantadine intoxication can cause altered mental status, seizures, anticholinergic effects, acute respiratory failure, heart failure, and QRS widening. In this patient we saw heart failure, QRS widening, respiratory failure, and altered mental status secondary to encephalopathy as a result of amantadine intoxication. The risk factors for amantadine toxicity in this case were advanced age, renal impairment, and increased dose without renal adjustment. Amantadine is 90% excreted by the kidneys, and even though the patient was on hemodialysis before and during hospitalization, amantadine is poorly excreted in dialysis and only a small fraction of the total body store is removed. To complicate matters, there is no antidote for amantadine overdose, and treatment is mostly supportive.
Conclusions: The therapeutic window of amantadine is poorly defined, but toxicity often manifests as altered mental status and cardiotoxicity. Patients on amantadine treatment should be monitored for toxic symptoms, especially if the patient has a history of moderate to severe renal dysfunction. Physicians should be able to identify and treat amantadine overdose to prevent adverse outcomes.
