The Impact of Hering's Law in Blepharoptosis: Literature Review.

The aim of this literature review is to examine Hering's law, a well-documented phenomenon in blepharoptosis patients, with 10% to 20% noted in cases of unilateral ptosis. Predominantly presenting as contralateral eyelid drop postoperatively, it poses a challenge for eyelid surgeons in the pursuit of symmetry and appropriate eyelid height. Proper preoperative evaluation is of utmost importance, consisting of one of either lifting test, covering test, or phenylephrine test. A deeper understanding of Hering's law further provides adequate information for optimal management of ptosis. In regard to ptosis etiology, congenital ptosis does not appear to have a distinct relation to positive Hering's law, commonly associated with a low incidence, when compared with acquired ptosis. Ptosis in the dominant eye seems to be related to a higher incidence of the phenomenon than ptosis in the nondominant eye, with statistical significance in studies ranging from P < 0.001 to P = 0.09. This can be explained as an innate response for increased innervation to regain the field of vision. Both ptosis severity and levator function appear to be of lesser importance than ptosis etiology, with minimal incidence of Hering's law in congenital ptosis regardless of these factors. It is, however, noted that ptosis severity has direct association with contralateral eyelid position in acquired ptosis, whereas there is a lack of studies for levator function. In the event of preoperative contralateral eyelid drop, surgeons should consider simultaneous surgery instead of delayed surgery for bilateral ptosis (P = 0.002). For unilateral ptosis, although reoperation is done per patient request, it may be more appropriate to first wait for roughly 2 weeks and reassess for self-regulation to a normal eyelid position.