Study suggests Müller’s muscle is component of peribulbar smooth muscle network

The authors conducted this study to determine if Müller's muscle has horizontal extensions and connections to the peribulbar smooth muscle network. They examined 20 postmortem orbits of 15 Asian adults. They found that Müller's muscle extended medially and laterally to reach the medial and lateral rectus pulleys, respectively. The authors conclude that Müller's muscle is not an independent structure in the upper eyelid, but rather a component of the peribulbar smooth muscle network.

They performed a full-thickness 360-degree incision of the periosteum around the circumference of each orbit, after which they elevated the periosteum and detached it near the orbital apex. Nerves, blood vessels and the nasolacrimal duct arising from the orbital wall were cut. The lateral orbital wall then was removed at approximately 3 centimeters posterior to the orbital rim, and the retrobulbar content was incised with a sharp scalpel in a coronal plane. The removed orbital content was incised at a plane passing from a point located 15 millimeters superior to the upper eyelid margin and the globe equator at the 3- and 9-o'clock areas. The sliced specimens were dehydrated and embedded in paraffin, cut into 7-µm thickness sections and then stained with Masson trichrome. Examinations were performed by light microscopy.

The authors found that Müller's muscle extended medially and laterally in all specimens. The lateral extension reached the lateral rectus muscle pulley by passing through the lacrimal gland fascia of the palpebral lobe, and in 12 specimens extended directly to the lateral rectus muscle pulley in the posterior part. These extensions intermingled with the peribulbar smooth muscle network and the peribulbar fascia. As in all periorbital structures, the extensions of the smooth muscle are thicker medially and superiorly compared with the lateral and the inferior orbit.

This study analyzed the distribution of Müller muscles and smooth muscle fibers only in specimens from Asian individuals, but it may be different in Caucasian and black people. The implications of the findings to clinical situations and surgery were not mentioned by the authors and may be limited.