Synkinesis is a movement disorder (mass movements) after a facial palsy that occurs after an injury to the facial nerve due to abnormal healing of the nerve fibers (see Fig. 1). Conspicuous asymmetries of the face are caused by involuntary movement of muscles that were not originally involved in the intended movement.

Especially during the nerve healing of the facial nerve after Bell's paresis and after an expired infection, a faulty interconnection of the nerve cells in the petrosal canal can occur.[1] This leads to the fact that, in addition to the arbitrarily correctly targeted nerve fibers, incorrect nerve fibers are also excited and parallel movements of unintentional muscle groups are initiated. Often, for example, the eye ring muscle is unintentionally contracted when laughing, so that an involuntary closing of the eyes occurs at the same time. Conversely, in synkinesis, unintentional activity of the upper lip-lifting muscles and the cheek muscles can also occur simultaneously. These movement disorders can also occur at rest or during sleep.[2] They must be distinguished from the facial hemispasm, i.e. a unilateral and involuntary contraction of the facial muscles.[3,4] The facial hemispasm is caused by vascular compression at the point where the facial nerve exits the brain stem or by the joining of different facial muscles during the surgical treatment of facial injuries. Data on the frequency of synkinesis in facial nerve palsy patients (prevalence) vary from 8.9% to over 51%.[5] Damage to the temporal region is more often associated with synkinesis.[6]

In this patient, the upper part of the left eye ring muscle is tensed when the mouth region is intentionally controlled. The consequence is an unintentional partial closure movement of the upper eyelid. Synkinesis are generally difficult to treat. In the treatment of synkinesis, for example, rehabilitation measures are useful. Biofeedback training appears to be particularly suitable.[7] Furthermore, the injection of small amounts of botulinum toxin can provide relief.[8,9] In principle, it should be noted that the desired effect of the botulinum toxin injection can be expected after about one month.[10] Furthermore, it can generally be assumed that the effect lasts four to six months, even if results that have already existed for some more time have been described.[11,12] The therapy can be repeated as often as required or at the patient's request. Biofeedback therapy in combination with botolinum toxin injections seems to be particularly promising.[13] In severe forms, surgical interventions are also possible which, although they do not correct the underlying "misconnection" of nerve fibers in the facial nerve trunk, can achieve a "rearrangement" of the nerve impulses through other principles. For example, transplanted nerves can correctly target specifically selected muscle groups via the intact facial nerves of the opposite side. It is also possible to remove some wrongly connected nerve branches. Neurectomies, i.e. the targeted elimination of unphysiologically acting nerve subbranches, can also significantly reduce or eliminate the symptoms.[14]

In synkinesis, involuntary co-movements occur in other parts of the face as part of voluntary movements of facial expressions. One also speaks of "uncontrolled mass movements". These are caused by defect healing of the facial nerve, which is often seen after Bell's palsy (idiopathic facial palsy), but can also occur in the context of other causes of facial paralysis. Pathophysiologically, nerve fibers of the individual five branch systems have become interconnected in the sense of a misconnection and now unintentionally deliver nerve impulses to each other. Synkinetic patients have difficulties in executing desired mimic movements completely or at all, while muscle groups that are not desired are unintentionally activated. Muscularly, agonists and antagonists are thus often activated simultaneously. This leads to the picture of spastic, overshooting and very often restricted facial expressions. In this patient, clear synkinesis can be seen. When asked to merely raise the eyebrows, this is impossible on the side of the former paralysis (left side of the face): due to a dysfunctional frontal brancht the forehead muscle can no longer produce forehead wrinkles. Involuntarily, however, now zygomatic and buccal nerve branches on the left side of the face are simultaneously also triggered, so that an involuntary co-activation of the shincter muscle around the eye (orbicularis oculi)  and the smile muscles (mm. zygomatici) occurs: the left eye is squinted and an involuntary lifting of the corners of the mouth as well as a strong laughter line (nasolabial fold) on the left side are visible. Strictly speaking, there is no facial paralysis in the narrower sense, but a defective condition through false neural connections after a mishealed facial paralysis.

Figure 1
Misdirection of axons during nerve healing can lead to synkinesis.

Source: Modified by author: Azizzadeh B, Frisenda JL. Surgical Management of Postparalysis Facial Palsy and Synkinesis. Otolaryngologic Clinics of North America 2018. doi:10.1016/j.otc.2018.07.012. FIG1.

In this patient, an intentional activation of the mouth region leads to a parallel tension of the upper part of the left eye ring muscle. The result is an involuntary partial closure movement of the upper eyelid.  Interestingly, the functions of two other cranial nerve systems have consequently become interconnected: while mandibular movement is controlled by branches of the trigeminal nerve (V cranial nerve), upper eyelid elevation is mediated by the oculomotor nerve (III cranial nerve). Thus, synkinesias occur not only in the facial nerve region, but also in other cranial nerves and even among themselves. 

This patient also shows faulty connections of different nerve branch systems after a faulty healing of a left facial paresis. For example, during activity in the upper third of the face, mass movements appear in parallel in the lower third and vice versa: if the brow is raised or the eyes are closed, there is involuntary simultaneous activation (cramping) of muscles in the chin region. The smile is restricted on the left side by an uncoordinated activation of opposing muscles; in this case, an involuntary eyebrow elevation is shown in parallel.

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