The Marcus Gunn phenomenon, also known as Jaw-Winking Synkinesis, is a rare, congenital condition involving an involuntary movement between the eyelid and the jaw. It is characterized by ptosis (a droopy upper eyelid) which suddenly and momentarily elevates when the jaw is moved. This unusual connection is present from birth, although it may not be noticed until the child begins activities like feeding or chewing. The condition is considered a congenital cranial dysinnervation disorder, stemming from an abnormal development of the cranial nerves that control muscle movement in the head and face. The phenomenon is typically unilateral, affecting only one eye, and is often discovered when parents observe the rhythmic, eye-lifting motion during infant feeding.
The Underlying Neurological Mechanism
The root cause of the Marcus Gunn phenomenon is neurological miswiring, known as synkinesis—the simultaneous movement of muscles normally innervated independently. This specific synkinesis involves an abnormal connection between two major cranial nerves. The Trigeminal nerve (Cranial Nerve V) controls the muscles used for chewing, such as the pterygoid muscles, and the Oculomotor nerve (Cranial Nerve III) controls the levator palpebrae superioris muscle, which lifts the upper eyelid.
In this condition, nerve fibers meant for jaw muscles are misdirected and connected to the eyelid muscle. When an impulse is sent to initiate jaw movement (during chewing or sucking), the signal inadvertently “leaks” or shunts to the levator muscle. This errant impulse causes the eyelid to contract and quickly lift, creating the characteristic “winking” effect. The phenomenon is thought to originate from an abnormality in the brain stem during fetal development, leading to this anomalous connection.
Recognizing the Physical Signs
The most distinguishing physical sign is the simultaneous presentation of mild-to-moderate ptosis and involuntary eyelid retraction. Ptosis means the upper eyelid rests in a lower position than normal, making the palpebral fissure (the opening between the eyelids) appear smaller in the affected eye when the jaw is still. This droopy eyelid is often the first visible symptom that prompts a medical consultation.
The characteristic “winking” is triggered by activating the jaw muscles, such as opening the mouth widely, moving the jaw side-to-side, or clenching the teeth. The most pronounced elevation is frequently seen when the jaw is moved toward the side opposite the affected eye. The upward jerking motion of the eyelid is rapid and momentary, immediately returning to its lower position once the jaw movement ceases. The severity of the jaw-winking can vary significantly, ranging from a subtle flicker to a very noticeable upward jerk of several millimeters.
Clinical Evaluation and Management Options
Diagnosis of the Marcus Gunn phenomenon is primarily clinical observation. A healthcare provider confirms the presence of ptosis and observes the characteristic synkinetic eyelid movement upon jaw action. During the physical examination, the physician asks the patient to perform movements like chewing or opening the mouth to elicit the involuntary eyelid elevation. This observation helps differentiate it from other causes of congenital ptosis.
The evaluation process also includes a comprehensive eye exam to check for associated vision issues, such as amblyopia (lazy eye) or strabismus (misaligned eyes). Measuring the degree of both the resting ptosis and the excursion of the eyelid during the jaw-winking helps classify the severity of the condition. Management decisions are based on the degree of visual impairment and the cosmetic impact.
Non-Surgical Management
For mild cases where the ptosis does not obstruct vision and the jaw-winking is not cosmetically bothersome, the primary management approach is observation and close monitoring. Regular follow-up appointments are necessary, particularly in children, to ensure the condition does not lead to the development of amblyopia, which requires treatment like patching or glasses.
Surgical Intervention
If the ptosis is severe enough to block the visual axis or if the jaw-winking is pronounced, surgical intervention may be considered. Surgical treatment aims to address both the ptosis and the synkinesis.
One common surgical technique involves the complete excision of the levator palpebrae superioris muscle in the affected eye, which eliminates the path of the misdirected nerve signal. This is often followed by a frontalis sling procedure. In this procedure, the eyelid is suspended from the eyebrow muscle (frontalis muscle), allowing the patient to lift the eyelid using their forehead. This two-stage procedure typically yields the best functional and aesthetic results by eliminating the unwanted wink and correcting the droopy eyelid.