Müller's Muscle as a New Proprioceptive Organ

Objectives: Upper eyelid retraction was believed to be maintained solely by voluntary contraction of the levator palpebrae superioris (LPS) and frontalis muscles, together with involuntary contraction of the sympathetically innervated Müller's muscle (MM). However, several studies have suggested that the LPS also undergoes involuntary contractions, and that a visual stimulus may not be the only trigger for frontalis muscle contractions. Recent studies hypothesized that the MM contains proprioceptive neuronal structures, which elicit involuntary LPS muscle contraction by the mesencephalic trigeminal nucleus via a continuous stretch reflex. We aim to identify proprioceptive structures in MM by means of histological examinations.

Methodology: Prospective study. Collaboration of oculoplastics, ophthalmologists and a neuroanatomy specialist in Sheba Medical Center. 50 fresh MM specimens from patients undergoing Müllerectomies will undergo histologic examinations, including immunohistochemical staining and light and electron microscopy.

Significance: Identification of proprioceptive structures in MM will be the first evidence-based proof of a proprioceptive mechanism in the eyelid. This might have a significant impact on future surgical management of eyelid procedures. Comprehensive understanding of the mechanisms underlying eyelid function is especially important in the management of children with ptosis who may have coexisting amblyopia.