The primary cause of Marcus Gunn jaw-winking ptosis is believed to be abnormal innervation of the levator palpebrae superioris muscle, which controls eyelid elevation. This abnormal innervation results in a connection between the motor nerve that controls the jaw muscles (specifically the pterygoid muscles) and the levator muscle.
Here's a more detailed breakdown:
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Abnormal Innervation: The core issue is a miswiring in the nerve pathways. Instead of the motor nerve meant for chewing muscles (pterygoid muscles) solely innervating those muscles, it also sends signals to the levator muscle of the eyelid.
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Levator Palpebrae Superioris Muscle: This is the muscle responsible for raising the upper eyelid. In Marcus Gunn, when the jaw moves (e.g., chewing, opening the mouth), the levator muscle is inadvertently stimulated, causing the eyelid to twitch or elevate.
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Not Myopathic: Histopathological studies suggest that the problem is neurological rather than muscular. The muscle itself is usually normal, though some studies show variable fibrosis.
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Mechanism: The exact mechanism of how this abnormal connection occurs during development isn't fully understood. It's considered a congenital condition, meaning it's present at birth.
In essence, the "winking" or twitching of the eyelid during jaw movement isn't because the eyelid muscle is diseased but because it's being triggered by the wrong nerve signals originating from the jaw's motor nerve.
