What are the chances?
© 2017 Society of Hospital Medicine
The present patient developed a constellation of neurologic symptoms after starting ertapenem, one of the carbapenem antibiotics, which is a class of medications that can cause CNS ADEs. Carbapenems are renally cleared, and adjustments must be made for acute or chronic changes in kidney function. Carbapenems are associated with increased risk of seizure; the incidence of seizure with ertapenem is 0.2%.7,8 Food and Drug Administration postmarketing reports have noted ertapenem can cause somnolence and dyskinesia,9 and several case reports have described ertapenem-associated CNS side effects, including psychosis and encephalopathy.10-13 Symptoms and examination findings can include confusion, disorientation, garbled speech, dysphagia, hallucinations, miosis, myoclonus, tremor, and agitation.10-13 Although reports of dysmetria and dysdiadochokinesia are lacking, suspicion of an ADE in this case was heightened by the timing of the exposure and the absence of alternative infectious, metabolic, and vascular explanations for bilateral cerebellar dysfunction.
The Naranjo Adverse Drug Reaction (ADR) scale may help clinicians differentiate ADEs from other etiologies of symptoms. It uses 10 weighted questions (Table) to estimate the probability that an adverse clinical event is caused by a drug reaction.14 The present case was assigned 1 point for prior reports of neurologic ADEs associated with ertapenem, 2 for the temporal association, 1 for resolution after medication withdrawal, 2 for lack of alternative causes, and 1 for objective evidence of neurologic dysfunction—for a total of 7 points, indicating ertapenem was probably the cause of the patient’s neurologic symptoms. Of 4 prior cases in which carbapenem toxicity was suspected and the Naranjo scale was used, 3 found a probable relationship, and the fourth a highly probable one.10,12 Confusion, disorientation, hallucinations, tangential thoughts, and garbled speech were reported in the 3 probable cases of ADEs. In the highly probable case, tangential thoughts, garbled speech, and miosis were noted on examination, and these findings returned after re-exposure to ertapenem. Of note, these ADEs occurred in patients with normal and abnormal renal function, and in middle-aged and elderly patients.10,11,13
Most medications have a long list of low-frequency and rarely reported adverse effects. The present case reminds clinicians to consider rare adverse effects, or variants of previously reported adverse effects, in a patient with unexplained symptoms. To estimate the probability that a drug is causing harm to a patient, using a validated tool such as the Naranjo scale helps answer the question, What are the chances?
KEY TEACHING POINTS
Clinicians should include rare adverse effects of common medications in the differential diagnosis.
The Naranjo score is a validated tool that can be used to systematically assess the probability of an adverse drug effect at the bedside.
Disclosure
Nothing to report.
