Unilateral ptosis and Hering's law.

In 21 patients with unilateral ptosis, only 1 was found to have lid retraction of the fellow eye--a sign regarded as evidence of Hering's law of equal innervation governing levator function. Hering's law may influence levator function in select cases of ptosis due to disease at or distal to the neuromuscular junction. Alternatively, the relatively autonomous contraction of orbicularis oculi or inhibition of frontalis may mask unilateral lid retraction.