Marcus Gunn syndrome is caused by an abnormal nerve connection that forms before birth, linking the nerves that control jaw movement to the nerve that raises the upper eyelid. This miswiring means that when you move your jaw (to chew, suck, or open your mouth), your eyelid involuntarily lifts or “winks” on one side. The condition accounts for about 5% of all cases of congenital ptosis, the medical term for a drooping eyelid present from birth.
The Nerve Miswiring Behind the Wink
To understand what’s happening, it helps to know that two different cranial nerves are involved. The trigeminal nerve (cranial nerve V) controls your jaw muscles, including the ones used for chewing. The oculomotor nerve (cranial nerve III) controls the muscle that lifts your upper eyelid, called the levator muscle. In Marcus Gunn syndrome, an abnormal bridge exists between the motor branch of the trigeminal nerve that controls the outer chewing muscle (the external pterygoid) and the fibers of the oculomotor nerve that raise the eyelid.
This type of miswiring is called synkinesis, where a nerve signal meant for one muscle accidentally activates another. So when your brain sends a signal to move your jaw, that signal also travels along the aberrant pathway and tells your eyelid to open wider. The result is the characteristic “jaw-winking” that gives the condition its name. Most often, only one eye is affected.
Why the Miswiring Happens
In nearly all cases, Marcus Gunn syndrome is congenital, meaning the abnormal nerve connection forms during fetal development. The exact reason the nerve fibers take a wrong turn during development isn’t fully understood, but evidence points to a genetic component. A study of two families with multiple affected members found that the condition followed an incomplete autosomal dominant inheritance pattern with varied expressivity. In practical terms, that means a parent can pass it on to a child, but not every person who inherits the gene will show symptoms to the same degree, and some carriers may not show symptoms at all.
In one of these families, a woman with the condition had three other relatives on her mother’s side who also had it. In the second family, a child with the syndrome had both a mother and a maternal uncle with jaw-winking. These patterns suggest the trait travels through families, though many cases also appear with no family history at all, likely from new developmental variations during pregnancy.
Rare Acquired Cases
In a small number of reported cases, Marcus Gunn syndrome has developed later in life rather than being present from birth. These acquired cases have been linked to complicated eye surgery and ocular trauma. A history of strabismus surgery (surgery to correct misaligned eyes) has also been noted in some patients. Acquired cases are uncommon enough that they exist mainly as individual case reports in medical literature, so the overwhelming majority of people with the condition were born with it.
What Triggers the Eyelid Movement
The winking is triggered by any movement that engages the pterygoid muscles on the affected side. Common triggers include chewing food, opening the mouth wide, moving the jaw to the opposite side, sucking (which is why parents of infants often notice it during breastfeeding or bottle-feeding), and clenching or thrusting the jaw forward. The eyelid movement is involuntary and happens simultaneously with the jaw movement. It can range from a barely noticeable flicker to a dramatic, obvious lift of the drooping eyelid.
Does It Get Better Over Time?
Marcus Gunn syndrome is generally a stable condition. It does not tend to improve or worsen significantly on its own. A clinical study that followed 14 patients for an average of about two years (with the longest follow-up being eight years) found that none of the patients showed any change in the severity of their jaw-winking, and none developed new drooping of the eyelid. Some parents hope children will “grow out of it,” but the evidence suggests the condition stays essentially the same over time. What can change is how noticeable it appears: as children grow, they may naturally learn to minimize certain jaw movements, making the winking less conspicuous in daily life.
How It Differs From Marcus Gunn Pupil
The name “Marcus Gunn” is attached to two completely different eye conditions, which causes frequent confusion. Marcus Gunn syndrome (also called Marcus Gunn jaw-winking) is the congenital nerve miswiring described here. A Marcus Gunn pupil, on the other hand, is an abnormal pupil response detected during an eye exam. It occurs when one eye’s optic nerve is damaged (from conditions like optic neuritis or glaucoma), causing the pupil to dilate rather than constrict when light is swung from the healthy eye to the affected one. The two conditions share only a name. They involve different nerves, different mechanisms, and different causes.
When Surgery Is Considered
Mild cases of Marcus Gunn syndrome often need no treatment. Many people live comfortably with a slight wink that’s only noticeable during eating or exaggerated jaw movements. Surgery is typically considered when the ptosis is moderate to severe, meaning the drooping eyelid blocks part of the visual field, or when the synkinetic winking is cosmetically significant enough to affect quality of life.
The most commonly performed procedure involves two steps. First, the surgeon disconnects the levator muscle (the one that lifts the eyelid and is receiving the misdirected nerve signals) to eliminate the jaw-winking. Then, a frontalis sling is placed, which is a small band of tissue or synthetic material that connects the eyelid to the forehead muscle, so the patient raises the eyelid by raising the brow instead. This can be done on one side or both. A bilateral approach (operating on both eyes) creates the most symmetrical appearance but means neither eyelid will lower fully when looking down. A unilateral approach preserves the healthy eye’s normal movement but may result in some asymmetry.
Other surgical options exist for less severe cases. Partial shortening of the levator muscle can reduce the amplitude of the wink while preserving some natural eyelid function. One newer technique directly reconnects the detached levator muscle to the forehead muscle rather than discarding it, which has been performed successfully in a small group of patients. The choice of procedure depends on the severity of both the ptosis and the winking, and outcomes are generally good across the different approaches.