Facial and extraocular muscles are affected most often; in rare cases, a person's hands might perform mirror movements.
Potential causes include improper healing after nerve trauma or neurodegeneration, as occurs in Parkinson's disease.
Treatment depends on the cause, but is largely conservative with facial retraining or mime therapy, if needed, while Botox and surgery are used as last resort.
[7] There are three proposed mechanisms for synkinesis: aberrant nerve regeneration, interneuronal ephaptic transmission, and nuclear hyperexcitability.
[13] The hypothesis states that, after trauma, axons project from the facial nucleus to incorrect peripheral muscle groups.
[3] For example: compression to the facial nerve causes a lesion and the set of axons that innervates the orbicularis oris (mouth muscle) degenerate.
This time though, only 50% of the set of axons that innervate the orbicularis oris successfully reinnervate the original site.
On the contrary, recent research by Choi and Raisman[14] has provided a more thorough understanding of synkinesis through aberrant axonal regeneration.
Previously, many developed treatment strategies (that inevitably failed) were invented based on the original hypothesis by only focusing on the lesion site for improving the organization of regeneration.
Subsequently, nearby residual undamaged axons can provide a source of neurotransmitter to the deprived post-synaptic cell.
Additionally, since the extra-ocular muscles are hidden within the orbits, there is a limit on the type of practical treatments that can be established (e.g. massage).
Treatments for synkinesis in general include facial retraining, biofeedback, mime therapy, and Botox and surgery, as a last resort.
Due to the extreme efforts needed to achieve improvements during synkinesis, Nakamura et al. observed that patients will often fail to reach their desired goal because of the difficulty of maintaining motivation during training.
[21] It was initially designed to treat facial palsy by improving symmetry of the face both at rest and during movement.
It wasn't until 2003 that Beurskens and Heymans were able to experimentally conclude that mime therapy was indeed a good treatment choice for synkinesis.
[22] Current mime therapy consists of a combination of procedures designed to promote symmetry of the face at rest and during movement to control synkinesis.
[2] The overall aim of mime therapy is to develop a conscious connection between the use of facial muscles and emotional expression.
[24] Due to the short span of Botox effects though, patients must come back to the doctor for re-injection approximately every 3 months.
More notable is that in a majority of patients, various synkinetic movements completely disappeared after 2-3 sessions of trimonthly Botox injections.
[25] A more specific synkinesis, crocodile tears syndrome (hyperlacrimation upon eating), has been shown to respond exceedingly well to Botox injection.
The procedure was shown to be simple and safe with very little chance of side-effects (although on rare occasions ptosis can occur due to botulinum toxin diffusion).