The influence of physostigmine on respiratory and circulatory changes caused by overdoses of orphenadrine or imipramine in the rat

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.

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.

Tricyclic antidepressant poisoning. Management of arrhythmias

Deaths from tricyclic antidepressant (TCA) overdose are usually due to arrhythmias and/or hypotension. Tricyclic antidepressant toxicity is due mainly to the quinidine-like actions of these drugs on cardiac tissues. Slowing of phase 0 depolarisation of the action potential results in slowing of conduction through the His-Purkinje system and myocardium. Slowed impulse conduction is responsible for QRS prolongation and atrioventricular block, and contributes to ventricular arrhythmias and hypotension. Therapies that improve conduction, e.g. hypertonic sodium bicarbonate, are useful in treating these toxic effects. Other mechanisms contributing to arrhythmias include abnormal repolarisation, impaired automaticity, cholinergic blockade and inhibition of neuronal catecholamine uptake. Toxicity may be worsened by acidaemia, hypotension or hyperthermia. Sinus tachycardia is due to the anticholinergic effects of the tricyclic antidepressants as well as blockade of neuronal catecholamine reuptake. Sinus tachycardia is generally well-tolerated and requires no therapy. Sinus tachycardia with QRS prolongation may be difficult to distinguish from ventricular tachycardia. Electrocardiograms obtained using oesophageal or atrial electrodes may be useful in determining the relationship of atrial and ventricular activity. Although QRS prolongation alone is not compromising, it is a marker for patients at highest risk of developing seizures, arrhythmias or hypotension. Ventricular tachycardia (monomorphic) is a consequence of impaired myocardial depolarisation and impulse conduction. Hypertonic sodium bicarbonate may partially correct impaired conduction and be of benefit in treating ventricular tachycardia. Since hypertonic sodium bicarbonate appears to act by increasing the extracellular sodium concentration as well as by increasing extracellular pH, hyperventilation may be less effective. Hypertonic sodium bicarbonate is of particular benefit in patients who are acidotic, since acidosis aggravates cardiac toxicity. However, administration of hypertonic sodium bicarbonate is beneficial even when blood pH is normal. Lignocaine (lidocaine) may be useful in treating ventricular tachycardia but should be administered cautiously to avoid precipitating seizures. Ventricular bradyarrhythmias are due to impaired automaticity or depressed atrioventricular conduction and can be treated by placement of a temporary pacemaker, or with a chronotropic agent, e.g. isoprenaline (isoproterenol), with or without concomitant vasoconstrictors.(ABSTRACT TRUNCATED AT 400 WORDS)

The influence of orphenadrine or imipramine on the hypertensive response of physostigmine in the rat

An intravenous infusion of orphenadrine or imipramine to artificially ventilated, urethane anaesthetized rats, completely blocked the physostigmine induced increase in blood pressure and the blood pressure increase induced by electrical stimulation of the posterior hypothalamus; effects mediated via the sympathetic nerve. The noradrenaline induced blood pressure increase was not changed during an infusion with orphenadrine but was markedly depressed during an infusion with imipramine. During an infusion with both orphenadrine or imipramine the pressor response induced by stimulation of the spinal cord were completely blocked in pithed rats. The pattern of the blockade was comparable with the blockade of the pressor response after hypothalamic stimulation. These results show that at least in rats both orphenadrine and imipramine prevents the central stimulatory sympathetic effects on the cardiovascular system by interfering with the sympathetic nervous system. The site of action is discussed.

IN CONCLUSION

the present results show that although physostigmine may be helpful in the treatment of central anticholinergic effects caused by overdoses of orphenadrine and imipramine it is of no use for combating the direct toxic effects of both drugs on the cardiovascular system.