Cardiotoxicity from orphenadrine intoxication in humans

Clinical outcomes associated with orphenadrine deliberate self-poisoning: a retrospective poisons centre study

INTRODUCTION

Orphenadrine overdoses can cause antimuscarinic toxicity, respiratory failure, refractory seizures and cardiotoxicity. The dose-toxicity relationship is poorly defined. Orphenadrine is marketed as immediate and sustained release formulations, and it is not known how the formulation impacts on toxicity. We determined the clinical toxicity of orphenadrine in patients referred to a regional poisons centre.

METHODS

Retrospective case series of patients in New South Wales with orphenadrine deliberate self-poisoning from January 2016 to April 2022 referred to the New South Wales Poisons Information Centre. Demographics, history of exposure, treatment and outcomes were extracted from clinical databases. Receiver-operating characteristic curves were constructed to determine thresholds predicting toxicity.

RESULTS

Forty-eight patients were identified, with information on clinical outcomes in 46 patients and doses in 41 patients. All patients were older than 12 years. The median orphenadrine dose was 770 mg (range 210-10,000 mg), 59 per cent as the immediate release formulation, and 67 per cent reported coingestants. Doses of sustained release formulations were significantly greater than immediate release formulations, median 2,750 mg versus 595 mg. Common clinical features were drowsiness (59 per cent), sinus tachycardia (37 per cent) and confusion (33 per cent). Three patients had mild hypotension, three were intubated for coma, and two had seizures; no patients suffered ventricular dysrhythmias. All patients survived, with 75 per cent being medically cleared within 24 hours of presentation. A dose-toxicity relationship was observed, but conclusions are limited by the small number of cases with moderate or severe toxicity.

DISCUSSION

All patients survived, and severe cardiac and neurological toxicity were not observed. This contrasts to published case reports noting severe poisoning at similar or lower doses. Formulation may have an impact on outcomes, with lesser toxicity from sustained release products.

CONCLUSIONS

Orphenadrine doses up to 10 g were associated with antimuscarinic toxicity and sedation, but not severe cardiotoxicity. More research exploring the effect of dose and formulation on outcomes is required.

A Bitter Taste in Your Heart

The human genome contains ∼29 bitter taste receptors (T2Rs), which are responsible for detecting thousands of bitter ligands, including toxic and aversive compounds. This sentinel function varies between individuals and is underpinned by naturally occurring T2R polymorphisms, which have also been associated with disease. Recent studies have reported the expression of T2Rs and their downstream signaling components within non-gustatory tissues, including the heart. Though the precise role of T2Rs in the heart remains unclear, evidence points toward a role in cardiac contractility and overall vascular tone. In this review, we summarize the extra-oral expression of T2Rs, focusing on evidence for expression in heart; we speculate on the range of potential ligands that may activate them; we define the possible signaling pathways they activate; and we argue that their discovery in heart predicts an, as yet, unappreciated cardiac physiology.

Reversal of orphenadrine-induced ventricular tachycardia with physostigmine

A 3-year-old boy developed confusion, generalized tonic-clonic seizures, and sustained ventricular tachycardia following ingestion of an unknown quantity of orphenadrine (Norflex). Although refractory to precordial thump, synchronous cardioversion, and lidocaine, the ventricular tachycardia was reversed by intravenous administration of the tertiary acetylcholinesterase inhibitor physostigmine. We discuss the underlying physiology and manifestations of anticholinergic overdose, the specific manifestations of orphenadrine overdose, and the current recommendations regarding the utilization and toxicity of physostigmine in the treatment of anticholinergic syndromes and orphenadrine intoxication.