Synkinesis is a movement disorder 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]

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.

Sources:
[01] Salles, A. G., da Costa, E. F., Ferreira, M. C., do Nascimento Remigio, A. F., Moraes, L. B., & Gemperli, R. Epidemiologic Overview of Synkinesis in 353 Patients with Longstanding Facial Paralysis under Treatment with Botulinum Toxin for 11 Years. Plastic and Reconstructive Surgery, 2015;136(6):1289–1298.doi:10.1097/prs.0000000000001802.
[02] Chuang DC, Chang TN, Lu JC. Postparalysis facial synkinesis: clinical classification and surgical strategies. Plast Reconstr Surg Glob Open. 2015 Apr 7;3(3):e320. doi: 10.1097/GOX.0000000000000283. PMID: 25878931; PMCID: PMC4387142.
[03] Colosimo C, Bologna M, Lamberti S et al. Arch Neurol. 2006 Mar; 63(3):441-4.
[04] Valls-Solé J. Electrodiagnostic studies of the facial nerve in peripheral facial palsy and hemifacial spasm. Muscle Nerve. 2007 Jul; 36(1):14-20.
[05] Celik M, Forta H, Vural C. The development of synkinesis after facial nerve paralysis. European Neurology. 2000;43(3):147–51.
[06] Pepper J-P, Kim JC. Selective chemodenervation with botulinum toxin in facial nerve disorders. Operative Techniques in Otolaryngology-Head and Neck Surgery. 2012;23(4):297–305.
[07] Pourmomeny AA, Asadi S. Management of synkinesis and asymmetry in facial nerve palsy: a review article. Iran J Otorhinolaryngol. 2014 Oct;26(77):251-6. PMID: 25320703; PMCID: PMC4196449.
[08] Salles AG, Toledo PN, Ferreira MC. Botulinum toxin injec- tion in long-standing facial paralysis patients: Improvement of facial symmetry observed up to 6 months. Aesthetic Plast Surg. 2009;33:582–590.
[09] Salles AG, Remigio AFN, Cruz DP, Barreiro GC. A sincinesia no paciente com paralisia facial: Estudo de sete anos. Rev Bras Cir Plast. 2010;25(Suppl 1):4.
[10] de Maio M, Benton RF. Botulinum toxin in facial palsy: an effective treatment for contralateral hyperkinesis. Plast Reconstr Surg 2007;120:917e27.
[11] de Maio M, Benton RF. Botulinum toxin in facial palsy: an effective treatment for contralateral hyperkinesis. Plast Reconstr Surg 2007;120:917e27.
[12] Monini S, de Carlo A, Biagini M, et al. Combined protocol for treatment of secondary effects from facial nerve palsy. Acta Oto-Laryngol 2011;131:882e6.
[13] Azuma T, Nakamura K, Takahashi M, et al. Mirror biofeedback rehabilitation after administration of single-dose botulinum toxin for treatment of facial synkinesis. Otolaryngol Head Neck Surg 2012;146(1):40e5.
[14] van Veen MM, Dusseldorp JR, Hadlock TA. Long-term outcome of selective neurectomy for refractory periocular synkinesis. Laryngoscope. 2018 Oct;128(10):2291-2295. doi: 10.1002/lary.27225. Epub 2018 Apr 18. PMID: 29668050; PMCID: PMC6221084.