The human eye relies on various muscles for proper function, including the Müller muscle. This small but significant muscle plays a role in the subtle movements of the upper eyelid. Understanding its structure and how it contributes to eyelid function is key to recognizing conditions that can affect it and vision.
What is the Müller Muscle?
The Müller muscle, also known as the superior tarsal muscle, is a thin, accessory smooth muscle located within the upper eyelid. It is approximately 10-12 millimeters in length and about 15 mm wide. This muscle originates from the underside of the levator palpebrae superioris muscle and inserts onto the superior tarsal plate, a dense connective tissue that provides support and shape to the upper eyelid.
As a smooth muscle, the Müller muscle operates involuntarily. Its function is regulated by the sympathetic nervous system, which is responsible for automatic bodily responses.
Its Role in Eyelid Function
The Müller muscle contributes to the elevation of the upper eyelid, working in conjunction with the larger levator palpebrae superioris muscle. While the levator palpebrae superioris is the primary muscle for voluntary eyelid lifting, the Müller muscle provides a smaller additional lift, accounting for about 2 millimeters of eyelid elevation. This subtle lift helps maintain the tone and overall position of the upper eyelid.
The sympathetic nervous system’s control over the Müller muscle means it can adjust eyelid position in response to physiological states like excitement, fear, or surprise, leading to a wider visual field. Its continuous tone also works with the opposing orbicularis oculi muscle to maintain the eye open and define the size of the palpebral fissure, the opening between the eyelids.
Conditions Affecting the Müller Muscle
Dysfunction of the Müller muscle can lead to noticeable changes in eyelid position, most commonly manifesting as ptosis, or drooping of the upper eyelid. This type of ptosis, often referred to as sympathetic ptosis, is specifically linked to impaired sympathetic nerve supply to the muscle. It can be a subtle drooping, often less than or equal to 2 millimeters.
Horner’s syndrome is a neurological condition that frequently involves the Müller muscle. It results from an interruption of the sympathetic nerve pathway to the eye and is characterized by a triad of symptoms: partial ptosis, miosis (a constricted pupil), and anhidrosis (loss of sweating) on the affected side of the face.
Another condition where the Müller muscle can be affected is thyroid eye disease, also known as Graves’ ophthalmopathy. In this autoimmune disorder, inflammation and fibrosis can occur within the superior tarsal muscle, leading to upper lid retraction, which is the opposite of ptosis.
Addressing Müller Muscle Issues
Diagnosing conditions involving the Müller muscle often includes specific tests to assess its function. The phenylephrine test is a common diagnostic tool, particularly for evaluating ptosis. In this test, a drop of phenylephrine, an alpha-adrenergic agonist, is instilled into the eye. This stimulates the sympathetically innervated Müller muscle, and if the eyelid elevates, it suggests the muscle is functional and responsive to sympathetic stimulation.
For treatment, surgical options are often considered, especially for ptosis. Müller’s muscle-conjunctival resection is a surgical procedure to address ptosis related to this muscle. This technique involves removing a portion of the Müller muscle and the conjunctiva to elevate the eyelid. This procedure is favored for mild to moderate ptosis in patients who show a positive response to the phenylephrine test.