The contralateral upper eyelid in ptosis: some observations pertinent to ptosis corrective surgery.

The validity of Hering's law in corrective surgery for ptosis is demonstrated for the first time. It is also shown how Hering's law has been utilised in corrective surgery for unilateral ptosis to elicit a predominant response from the unoperated contralateral upper eyelid. Other implications of reciprocal innervation of the 2 upper eyelids are discussed and a tentative hypothesis is postulated that in a case of true unilateral levator ptosis the fellow upper eyelid will show compensatory retraction.