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Rivastigmine for Antimuscarinic Delirium

In the setting of current drug shortage for the single manufacturer medication, IV physostigmine, we need to consider alternate options for routine treatment/diagnosis of antimuscarinic derlium. Interestingly, there is an oral formulation of physostigmine that has been reported to be available; however a quick query of pharmacies in a large metropolitan area (NYC) shows that it is unavailable as of 3/15/2022. Regardless of whether it existed in the past, oral agents are suboptimal for a delirious patient. The treatment of choice should be a tertiary amine acetylcholinesterase inhibitor (ACHe-i), given parenterally. There is only one other on the market: the rivastigmine patch.

Most likely a rivastigmine patch would be untenable for diagnosis of antimuscarinic delirium due to the pharmacokinetics. As per Micromedex®, the Tmax is anywhere between 8 and 16 hours, which is certainly much longer than IV formulation physostigmine, which seems to have peak effect with 5-10 minutes.

Kernebeek, et al recently reported that oral rivastigmine alleviated antimuscarinic delirium in a patient who had a confirmed procyclidine ingestion of over 300 tablets of 5mg. Apparently, they gave rivastigmine because IV physostigmine was not available (https://doi.org/10.1080/15563650.2020.1818768). It is important to note that this patient was intubated and subsequently extubated prior to administration of the PO rivastigmine. Although interesting, this single case report does little to add to the literature regarding an alternative to physostigmine. If the patient was well enough to be extubated due to resolving toxidrome, the benefit of PO rivastigmine is unclear.

From an ethical point of view, if there is a proven safe medication available, which we know is a theoretical antidote for antimuscarinic toxicity, there should be no qualm in recommending a rivastigmine patch, especially if dosed as per FDA regulation.

Other sources:

IBM micromedex