Müller’s muscle

Müller’s muscle is smooth muscle innervated by the sympathetic nervous system. Fibres originate from the undersurface of the levator in the region of the aponeurotic muscle junction, travel inferiorly between the levator aponeurosis and conjunctiva, and insert into the superior margin of the tarsus. With age, fatty infiltration may occur, giving the muscle a yellowish colour.

The peripheral vascular arcade of the upper eyelid lies adherent to the lower border of the anterior surface of Müller’s muscle, just above the upper border of the tarsus, and is apparent during blepharoptosis surgery as a plane of dissection is created between the levator aponeurosis and Müller’s muscle5. The action is to widen the palpebral fissure with increased sympathetic tone. Approximately 2 mm of ptosis is observed in Horner syndrome. Sympathetically innervated smooth muscle fibres are also noted in the lower eyelid and constitute the inferior tarsal muscle.

The lower eyelid retractor is a fascial extension from the terminal muscle fibres and tendon of the inferior rectus muscle, originating as the capsulopalpebral head. As it passes anteriorly from its origin, it splits to envelop the inferior oblique muscle and reunites as the inferior transverse ligament (Lockwood’s ligament). From there, the fascial tissue passes anterosuperiorly as the capsulopalpebral fascia. The bulk of the capsulopalpebral fascia inserts on the inferior border of the inferior tarsus. The orbital septum fuses with the capsulopalpebral fascia approximately 5 mm below the inferior tarsal border.

The inferior tarsal muscle (Müller’s muscle) lies just posterior to the fascia and is intimate with its structure. The sympathetically innervated smooth muscle fibres are first noted near the origin of the capsulopalpebral head. In the Asian lower lid, the line of fusion of the orbital septum to the capsulopalpebral fascia is often higher, or indistinct, with anterior and superior orbital fat projection, and over-riding of the pre-septal orbicularis oculi over the pre-tarsal orbicularis.

Upper eyelid pre-aponeurotic fat is found immediately posterior to the orbital septum and anterior to the levator aponeurosis. A central fat pad and a medial fat pad are described in the upper lid, while the lacrimal gland occupies the lateral compartment. The medial fat pad is  usually pale yellow or white, and lies anterior to the levator aponeurosis extending superomedial to the medial horn of the levator6.

[pull_quote align=”left” ]Three retro-septal fat pads are associated with the lower eyelid. The medial and central fat pads are separated by the inferior oblique muscle. [/pull_quote] The central fat pad is yellow and broad. A portion of the lateral end of this pad surrounds the medial aspect of the lacrimal gland. The lacrimal gland has a firm, pinkish, lobulated structure, in contrast to the soft, yellow intraorbital fat. The anterior border is normally just behind the orbital margin, but involutional changes may lead to prolapse anteroinferiorly, which is prominent on external lid examination.

Three retro-septal fat pads are associated with the lower eyelid. The medial and central fat pads are separated by the inferior oblique muscle. However, an isthmus of fat generally lies anterior to the muscle belly. The inferior oblique muscle takes a bony origin from a shallow depression on the anteromedial orbital floor, directly posterior to the orbital margin and lateral to the nasolacrimal canal.

The inferior oblique muscle courses posterolaterally, passing inferior to the inferior rectus muscle, penetrating Tenon’s capsule, and inserting onto the globe near the macula. Its course makes it susceptible to injury during surgical dissection of the surrounding fat pads.

Nerves and arteries

Sensory innervation of the eyelids is subserved by terminal branches of the ophthalmic and maxillary divisions of the trigeminal nerve (CN V). The supraorbital nerve exits the orbit through the supraorbital notch or supraorbital foramen. It subserves sensation to the upper eyelid and forehead skin, except for a mid-line vertical strip, which is supplied by the supratrochlear nerve. The infratrochlear nerve, a terminal branch of the nasociliary nerve, supplies the skin and conjunctiva of the medial canthus, the most medial aspect of the eyelids, and the nasolacrimal sac. The sensory supply of the remaining lower eyelid is provided by the infraorbital nerve and the zygomaticofacial nerve. The zygomaticofacial nerve supplies skin to the lateral lower eyelid, while the palpebral branch of the infraorbital nerve supplies the central lower eyelid skin and conjunctiva.

Branches of the facial nerve innervate the muscles of facial expression. The frontal and zygomatic branches of CN VII innervate the orbicularis oculi muscle; the frontal branch of CN VII innervates the forehead muscles. The orbicularis oculi is innervated by multiple motor branches from the branches of CN VII.

The levator palpebra superioris is innervated by the superior branch of the oculomotor nerve, entering the muscle from its inferior surface in its posterior third. Müller’s muscle requires sympathetic innervation. Postganglionic fibres arise from the superior cervical ganglion and travel superiorly in the neck as a plexus with the internal carotid artery. The fibres take an intracranial course to the cavernous sinus, where they travel through the superior orbital fissure into the orbit via CN branches.

The internal and external carotid arteries contribute to lid arterial supply. The internal carotid arterial supply is from the terminal branches of the ophthalmic artery medially, and the lacrimal artery laterally.