Introduction: Diphenhydramine is an antihistamine commonlyimplicated in overdose. It has many pharmacologic effects, including sodium-channel blockade. Overdosesin adults have been associated with the typical manifestationsof sodium-channel blockade, including wide complex tachycardia (WCT) and Brugada pattern ECG changes. No cases of isolated diphenhydramine ingestion causing WCT in children exist in the literature. We report such a case. Case report: An otherwise healthy 13 month-old was brought in by EMS for a witnessed tonic-clonic seizure. Two hours prior to arrival the child had been found with an open bottle of 25 mg diphenhydramine tabs, 24 of which were missing. Maximum possible ingestion was 50 mg/kg. Midazolam was administered en route with seizure resolution. Exam revealed 4 mm pupils, nystagmus, warm, dry, flushed skin, and altered mental status. ECG revealed a WCT at a rate of 180 with a measured QRS duration of 130 ms. The child was treated with a bolus of 1 mEq/kg of hypertonic sodium bicarbonate. Ninety-eight minutes later a repeat ECG revealed sinus tachycardia at a rate of 188 with a narrow QRS complex. The home was searched and no other medications were found. The child’s symptoms resolved and she was discharged home the following day with no sequelae.
Discussion: Diphenhydramine toxicity is a common poisoning in children. Toxicity typically presents with signs and symptoms of the anti-muscarinic
toxidrome. Diphenhydramine also has sodium-channel blocking properties, and therefore it is not surprising it may also cause WCT via fast cardiac sodium channel blockade. The QRS prolongation in a child this age is of particular note because of the normally short QRS of roughly 80 ms. One case of WCT in a child from dimenhydrinate exists in the literature, however dimenhydrinate contains, in addition to diphenhydramine, 9-chlorotheophylline, a methylxanthine. Methylxanthine toxicity has been described to cause WCT. Thus the above case is unique in that it is the first case of WCT in a child from isolated diphenhydramine ingestion.
Conclusion: Wide-complex tachycardia should be recognized as a complication of pediatric diphenhydramine overdose, and it appears responsive to hypertonic
sodium bicarbonate.