Teaching Video NeuroImages: Useful bedside testing for myasthenia gravis

During examination of a 62-year-old woman with a diagnosis of myasthenia gravis, left eyelid manual elevation caused exacerbated ptosis on the right (enhanced ptosis) (figure). According to Hering law, there is approximately equal innervation of the levator palpebrae superioris muscles in both eyelids.1 In an asymmetric ptosis, the motor impulses increase to compensate ptosis on the more affected side but also flow to the contralateral eyelid, which mitigates ptosis on that side. With manual elevation of the more affected eyelid, the need to sustain eyelid elevation is relieved, and there is an attenuation of these motor impulses causing ptosis enhancement of the other eyelid (video 1).2