The clinical utility of neostigmine administration in the diagnosis of acquired myasthenia gravis.

Objective: To assess the clinical utility of neostigmine methylsulfate administration in the diagnosis of suspected acquired myasthenia gravis (MG) in dogs and cats.

Design: Retrospective study (2017-2019).

Setting: Five university teaching hospitals and 2 private referral hospitals.

Animals: Twenty-two dogs and 3 cats. Criteria for inclusion were clinical signs consistent with acquired MG, performance of a neostigmine challenge and acetylcholine receptor antibody titers.

Interventions: None.

Measurements & Main Results: The route of neostigmine administration was recorded. Response to neostigmine challenge was determined via sequential evaluation of muscle strength and ambulation following administration of neostigmine methylsulfate. Response to neostigmine challenge was compared to acetylcholine receptor antibody titers, which were used as the biochemical gold standard in this study. Sixteen out of 22 dogs were diagnosed with acquired MG. Thirteen of 16 had a strong positive response to neostigmine challenge whereas 3 of 16 had no response. Two out of 3 dogs with polymyositis also had a strong positive response to neostigmine challenge. Weak positive results were seen with intracranial neoplasia (n = 1) and a dog with dilated cardiomyopathy and coxofemoral joint disease (n = 1). One cat was diagnosed with acquired MG and had a positive response to neostigmine challenge. Two cats had no response to neostigmine challenge and were diagnosed with alternate conditions. Two cats were premedicated with glycopyrrolate, one of which had a mild adverse response to neostigmine challenge (sialorrhea and mild transient tremors). Three out of 22 dogs had minimal adverse effects (sialorrhea and 1 dog with muscle tremors).

Conclusions: The neostigmine challenge appears to be safe and viable alternative to the previously utilized edrophonium challenge, particularly when weak positive responses are considered negative for acquired MG. Polymyositis cases may have a false positive response to neostigmine challenge.