Statistics from Altmetric.com
Inverse Marcus Gunn phenomenon is a rare congenital synkinetic movement presenting as eyelid drooping on jaw opening. It has only rarely been reported.1 Often this phenomenon follows peripheral facial palsy,2 suggesting abnormal synkinesis of the facial nerve. However, the precise mechanism remains uncertain. Only one electromyographic study has been reported in this condition.3 In that case, trigemino-oculomotor synkinesis was the supposed mechanism, rather than trigemino-facial synkinesis. We report a patient with familial inverse Marcus Gunn phenomenon and speculate on the neuronal mechanism with the support of electromyographic results.
A 31 year old woman visited our clinic complaining of involuntary winking of her left eyelid on jaw opening. The symptom was most prominent when she opened her mouth forcefully, such as when eating or yawning. She stated that the symptom had been noticed from birth, and that her mother and uncle were also affected, but not her two sons. She had no history of previous peripheral facial palsy.
On neurological examination, her extraocular movements were normal and ptosis was not noted. There was no facial paralysis or sensory changes. Masticatory movement or forced mouth opening induced closure of the left eye. Her mother showed the same feature on neurological examination. Brain MRI showed no abnormal findings. Facial nerve conduction studies and the blink reflex were normal. Two channel electromyography showed co-contraction of the left lateral pterygoid muscle and the orbicularis oculi muscle on repetitive masticatory movements (fig 1⇓).
Inverse Marcus Gunn phenomenon, the opposite of the Marcus Gunn phenomenon, is characterised by eyelid closure on jaw opening. Few cases have been reported, most following peripheral facial paralysis. Only one case of congenital inverse Marcus Gunn phenomenon was reported by Lubkin.3 He observed that on electromyography the levator palpebrae muscle was inhibited or inactivated during jaw opening, without any movement of the orbicularis oculi muscle, and concluded that his patient’s symptom was limited to interactions at the level of the third and fifth cranial nerve complexes. Because the Marcus Gunn phenomenon is a pterygoid–levator synkinesis between the pterygoid and the levator palpebrae muscles, Lubkin’s case may be the true inverse Marcus Gunn phenomenon. However, our patient’s electromyographic studies showed that the ipselateral orbicularis oculi muscle co-contracted with the lateral pterygoid muscle when she opened her mouth. Because these findings were consistently reproducible, we think this phenomenon is caused by synkinesis of the trigeminal and facial nerves in our case.
Most cases of inverse Marcus Gunn phenomenon appear to follow peripheral facial nerve paralysis,2,4 with contraction of the orbicularis oculi on jaw opening. However, our patient had no history of peripheral facial paralysis and surprisingly her mother and uncle showed the same phenomenon. We therefore conclude that our patient has a congenital trigemino-facial synkinesis with partial autosomal dominant inheritance. The orbicularis oculi muscle is supplied by the facial nerve connected to maxillary and buccal branches of the trigeminal nerve. As Rana pointed out,2 ascending proprioceptive impulses from muscle stretch during full mouth opening that are carried in the facial nerve could then trigger contraction of the orbicularis oculi muscle owing to faulty regeneration after a traumatic nerve injury. More recently there has been a tendency to explain these synkinetic movements in terms of plastic change in the central nervous system following deafferentiation. As our patient’s symptom is congenital, we suppose that the trigemino-facial synkinesis is the result of an abnormal partial connection between the motor neurones of the trigeminal and facial nerves.
Competing interests: none declared
If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.