Enhanced Ptosis in Myasthenia Gravis

This is a 20-yo-woman who presented with generalized weakness, ptosis and ophthalmoplegia. She had severe ptosis OU at baseline, but when one eyelid was manually elevated, there was marked enhanced ptosis of the opposite eyelid. This was in accordance with Hering's law of equal innervation of the levator muscles - i.e., manual elevation of a ptotic eyelid decreases the muscle strength necessary to keep the lid elevated, causing contralateral levator relaxation and enhanced ptosis. In MG-related unilateral ptosis, lifting the normal-appearing eyelid commonly leads to ‘enhanced ptosis of the already ptotic eyelid. Alternatively, lifting the ptotic eyelid may cause the normal-appearing eyelid to become ptotic, which is referred to as ‘curtaining'. The baseline equal innervation of the levator muscles due to Hering's law explains both enhanced ptosis and curtaining in MG.