The Müller muscle, also known as the superior tarsal muscle, is a small, involuntary muscle located within the upper eyelid. It sets the final position of the upper eyelid, which is important for vision and appearance. While often overshadowed by the larger, primary eyelid lifter, the Müller muscle maintains a wide-eyed look and reacts to specific neurological signals. Understanding its function provides insight into conditions that cause eyelid drooping, known as ptosis.
Anatomy and Function of the Müller Muscle
The Müller muscle is a thin sheet of smooth muscle tissue nestled deep within the upper eyelid. It originates from the undersurface of the levator palpebrae superioris muscle and inserts onto the superior tarsal plate, a dense band of connective tissue that structures the eyelid. It acts as an accessory retractor of the upper eyelid.
Unlike the large, consciously controlled levator muscle, the Müller muscle is small, measuring 10 to 15 millimeters in length. Its primary function is to provide the final 1 to 2 millimeters of upper eyelid elevation. This action maintains the eyelid’s position against gravity and the opposing orbicularis oculi muscle.
The Müller muscle works synergistically with the levator muscle, but its smooth muscle composition makes it involuntary. It provides a steady, resting tone that keeps the eye fully open. This accessory function ensures the eye remains open at a constant level.
The Sympathetic Control System
The Müller muscle operates entirely outside of conscious control because it is innervated by the sympathetic nervous system. This division of the autonomic nervous system manages involuntary functions related to the “fight or flight” response. The control pathway begins with nerve fibers originating from the superior sympathetic cervical ganglion.
These nerve fibers travel a complex route, wrapping around the internal carotid artery to form the internal carotid plexus. The fibers then enter the orbit, ultimately reaching the Müller muscle. This sympathetic innervation allows the muscle to contract rapidly in response to alertness or surprise, giving the eyes a momentarily wider appearance.
A constant stream of sympathetic nerve activity maintains the muscle’s resting tone, ensuring the eyelid stays elevated. Disruption anywhere along this nerve pathway directly impacts the muscle’s ability to contract.
Recognizing Ptosis Caused by Müller Muscle Dysfunction
Ptosis, or the drooping of the upper eyelid, can result from issues with either the primary levator muscle or the accessory Müller muscle. Ptosis caused by Müller muscle dysfunction is usually milder than that caused by levator muscle failure. Since the Müller muscle contributes a small fraction of the total lift, its failure typically results in a droop of no more than 2 millimeters.
This mild ptosis is a hallmark sign of Horner’s Syndrome, a condition involving damage to the sympathetic nerve pathway. Reduced sympathetic input causes the Müller muscle to relax, leading to the slight eyelid droop. Horner’s Syndrome often includes other signs, such as a constricted pupil (miosis) and reduced sweating on the affected side of the face (anhidrosis).
Distinguishing Müller muscle ptosis involves observing the degree of eyelid drop and performing specific tests. A physician may use phenylephrine, an eye drop that mimics sympathetic stimulation, to see if the eyelid elevates. Significant lifting after the drop suggests the Müller muscle is intact but lacks proper nerve stimulation, confirming sympathetic-related ptosis.
Corrective Procedures
For ptosis attributed to Müller muscle dysfunction, Müller muscle-conjunctival resection (MMCR) is often performed. This procedure is generally reserved for patients with mild to moderate ptosis who show a positive response to the phenylephrine test. The goal of the surgery is to shorten the Müller muscle and the underlying conjunctiva, tightening the posterior layer of the eyelid.
The operation is performed from the underside of the eyelid, avoiding an external skin incision. The surgeon removes a measured portion of the muscle and conjunctiva. The subsequent healing process creates a tightening effect, providing a static, permanent lift to the eyelid margin.
MMCR achieves sufficient eyelid elevation and symmetry between the eyes. While traditionally favored for mild cases, the procedure is increasingly used for a wider range of ptosis severity. A positive response to the phenylephrine test remains a strong predictor of success.