In the present study, we evaluated the validity of intravenous neostigmine administration combined with alternate prism cover test (APCT) measurement as a confirmatory diagnostic method for confusing cases of myasthenia gravis with ocular involvement.

Neostigmine was administered intravenously in 26 suspicious myasthenic diplopia patients under electrocardiographic monitoring. Distance deviation at primary position was evaluated with APCT at 5, 10, 15, 20, and 30 minutes after intravenous injection of neostigmine. Margin reflex distance was also evaluated at each time point.

Seven of 26 patients were diagnosed as myasthenic diplopia based on a positive neostigmine test. Among these patients, 6 had strabismus at the primary position and 5 patients had ptosis. In patients who showed positive results, all 6 patients showed improvement of strabismus. However, ptosis was not improved in 1 patient. The improvement of strabismus and ptosis reached a peak at 10 to 15 minutes after neostigmine administration.

Intravenous neostigmine administration combined with APCT is a rapid, objective and safe method in hard-to-diagnose cases of myasthenia gravis with ocular involvement. When performing the neostigmine test for myasthenia gravis with ocular involvement, not only the lid position but also strabismus should be evaluated quantitatively to avoid a false negative results.