A recent study suggests that Hering’s law of equal innervation doesn’t necessarily apply in certain types of eyelid surgery.
The study, conducted by Arie Y. Nemet, M.D., director of oculoplastic service in the Department of Ophthalmology at Meir Medical Center, Kfar Saba, Israel, acknowledges that the Hering’s law effect has significant importance in the planning and outcomes of eyelid surgery. Dr. Nemet designed this study to examine the preoperative and intraoperative effect of Hering’s law in Mullerectomy and levator aponeurosis advancement.
Dr. Nemet conducted a retrospective analysis of 52 patients (median age, 61) with unilateral ptosis who had either levator aponeurosis advancement or Mullerectomy, with or without tarsectomy, from January 2011 through June 2013. Preoperative and postoperative clinical documentation and photographs were evaluated. Preoperative Hering’s dependency and postop changes in positioning of the non-operated eyelid were measured. Decisions to operate on the ptotic eye alone or on both eyelids were based on preoperative Hering’s dependence and intraoperative changes in the contralateral eyelid.
The 14 cases that were not aponeurotic (either congenital, secondary to trauma or due to postoperative ptosis) did not need contralateral repair. In four cases (19%) of Mullerectomy and in nine (52.9%) cases of levator advancement, both eyelids required surgery. Thus it appears that Hering’s law effect was significantly more apparent in the levator advancement surgery than in Mullerectomy.
“Levator surgery resulted in a higher incidence of combined intra-operative and post-operative Hering’s law effect than did Mullerectomy,” Dr. Nemet writes. “Cases with poor levator function or congenital ptosis can be repaired unilaterally with no need for contralateral surgery. The fibrotic levator palpebrae muscle and its special innervations probably explain this phenomenon. This should be considered in surgical planning.”