Synkinesis
Encyclopedia
Synkinesis is the result from miswiring of nerve
s after trauma
. This result is manifested through involuntary muscular movement
s accompanying voluntary movements. For example, voluntary smiling will induce an involuntary contraction of the eye muscles causing the eye to squint when the subject smiles.
and Marcus Gunn phenomenon
). Trauma to the nerve can be induced in cases such as surgical procedures, nerve inflammation, neuroma
, and physical injury.
The two cases of synkinesis most commonly studied involve the facial muscles
and the extraocular muscles
.
, also called Bell’s Palsy or Facial Palsy. Bell’s Palsy, which occurs due to the compression of the seventh cranial nerve, results in a hemifacial paralysis due to non-functionality of the nerve. As the nerve attempts to recover, nerve miswiring results (see Mechanism of Action below). In patients with severe facial nerve paralysis, facial synkinesis will inevitably develop.
Additionally, a common treatment option for facial palsy is to use electrical stimulation. Unfortunately, this has been shown to be disruptive to normal re-innervation and can promote the development of synkinesis.
The most common symptoms of facial synkinesis include:
) are innervated by three different cranial nerves: Abducens (6th nerve), Trochlear (4th nerve), and Oculomotor (3rd nerve). After nerve trauma around the eye, a combination of any two of these three cranial nerves have been shown to be involved with extra-ocular synkinesis. More over, while the abducens and the trochlear nerve each innervate one specific muscle, the oculomotor nerve has many functions including eyelid retraction and pupil constriction. Thus, during synkinesis, a variable of these functions are involved. Examples include:
Other less common variations of synkinesis involving the cranial nerves include:
Synkinesis is also commonly used in referring to the involuntary convergence of the eyes that accompanies focusing for near. This accommodation-convergence synkinesis can result in esotropia, or eyes that turn in when the ratio between accommodation and convergence is unusually high.
For example: compression to the facial nerve causes a lesion and the set of axons that innervates the orbicularis oris (mouth muscle) degenerate. Once the compression has relieved, regeneration of axons from the lesion site begins. This time though, only 50% of the set of axons that innervate the orbicularis oris successfully reinnervate the original site. The other half aberrantly branched off and innervated the orbicularis oculi(eye muscle). Thus, when the patient purses their lips, the ipsilateral eye will squint.
The hypothesis assumes that disorganized regeneration occurs at the site of the lesion. On the contrary, recent research by Choi and Raisman has provided a more thorough understanding of synkinesis through aberrant axonal regeneration. Their study has shown that regenerating axons become disorganized throughout the length of the nerve and not only at the site of the lesion. 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. The new modification to the hypothesis could allow for better success in developing treatments.
is when two nerves communicate with each other via an artificial synapse between nerves. Healthy peripheral nerves are insulated with a myelin
sheath that helps to both enhance electric transmission and to prevent cross-talk between parallel nerves. After a lesion, it has been observed that regenerating nerves might not be myelinated effectively. Consequently, the two nerve fibers can come into contact and provide a means for an impulse to be directly conducted through the nerve membrane. An example to clarify this is having two uninsulated electrical wires placed adjacent to each other. Thus, the two nerves are able to “cross-talk” and send action potentials in both directions.
) occurs. The post-synaptic cell consequently becomes deprived of input and becomes more sensitive to neurotransmitters (e.g. creating additional receptors). Subsequently, residual undamaged axons in the same premise of the lesion can provide a source of neurotransmitter to the deprived post-synaptic cell. Since the post-synaptic cell is hypersensitive, the neurotransmitters that reach it from an axon of another nerve will successfully provide stimulation. This consequently creates undesired peripheral movement (i.e. synkinesis) .
Although these three mechanisms have been argued for and against in various ways, it has become more accepted that synkinesis develops through a combination of these mechanisms.
, hematoma
, and ecchymosis
. Therefore, surgical procedures are very minimally used by doctors and are stored as last-resort options for patients that do not respond well to non-invasive treatments.
therapy for facial synkinesis aims to increase the patient’s awareness of the facial muscle posture and movement. Facial muscles contain few to none intrinsic muscle sensory receptors (used for proprioceptive feedback) and additionally they do not span movable joints and so lack joint receptors (another source for proprioceptive feedback) . Thus, biofeedback allows the patient to actively sense the motion of their muscles. The two common forms of biofeedback used are electromyographic feedback and mirror feedback. Electromyographic feedback includes visual EMG signals (coming from facial muscle sites displayed to the patient from a computer in the form of waveform traces) or auditory signals that indicate strength of muscle contraction. The subsequent role of the patient is to control the movement of undesired muscle during volitional movement by incorporating the information perceived through the EMG. While mirror feedback is a much more basic way of providing the patient feedback on muscle movement, studies have shown that both are very effective options for synkinesis/paresis reduction. Biofeedback is commonly coupled to facial retraining techniques to achieve maximal effectiveness.
A study by Nakamura et al. has shown that biofeedback works better for prevention of synkinesis as opposed to treatment of synkinesis. 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. The desired course of action is to catch the patient shortly after facial nerve trauma and teach the patient biofeedback techniques. This course of action has been experimentally proven to significantly reduce the development of synkinesis.
Current mime therapy consists of a combination of procedures designed to promote symmetry of the face at rest and during movement and control synkinesis. The components include: massage, stretching exercises, exercises to coordinate both halves of the face, etc.. The overall aim of mime therapy is to develop a conscious connection between the use of facial muscles and emotional expression.
So while facial retraining therapy is much more focused on treating slight synkinetic movements, mime therapy aims to increase the overall vigor of the muscles through active exercises, while in the process of doing so, teaching the face to decrease unwanted synkinetic movements.
A more specific synkinesis, crocodile tears syndrome (hyperlacrimation upon eating), has been shown to respond exceedingly well to botox injection. Botox is injected directly into the lacrimal gland and has shown to reduce hyperlacrimation within 24–48 hours. The procedure was shown to be simple and safe with very little chance of side-effects (although rarely ptosis
can occur due to botulinum toxin diffusion) . Furthermore, reduction in hyper-lacrimation was shown to last longer than the expected 3 months (about 12 months) .
Since botox can mimic facial paralysis, an optimized amount of botox has been determined that reduces involuntary synkinesis of the muscle while not affecting muscle tone.
Nerve
A peripheral nerve, or simply nerve, is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...
s after trauma
Physical trauma
Trauma refers to "a body wound or shock produced by sudden physical injury, as from violence or accident." It can also be described as "a physical wound or injury, such as a fracture or blow." Major trauma can result in secondary complications such as circulatory shock, respiratory failure and death...
. This result is manifested through involuntary muscular movement
Spasm
In medicine a spasm is a sudden, involuntary contraction of a muscle, a group of muscles, or a hollow organ, or a similarly sudden contraction of an orifice. It is sometimes accompanied by a sudden burst of pain, but is usually harmless and ceases after a few minutes...
s accompanying voluntary movements. For example, voluntary smiling will induce an involuntary contraction of the eye muscles causing the eye to squint when the subject smiles.
Causes
Almost all cases of synkinesis develop as a sequel to nerve trauma (the exception is when it is congenitally acquired as in Duane-Retraction SyndromeDuane syndrome
Duane syndrome is a rare, congenital eye movement disorder most commonly characterized by the inability of the eye to abduct or move outwards...
and Marcus Gunn phenomenon
Marcus Gunn phenomenon
Marcus Gunn phenomenon is an autosomal-dominant condition with incomplete penetrance, in which nursing infants will have rhythmic upward jerking of their upper eyelid...
). Trauma to the nerve can be induced in cases such as surgical procedures, nerve inflammation, neuroma
Neuroma
A neuroma is a growth or tumor of nerve tissue. Just as the Latin word for swelling is now restricted to neoplasias, the equivalent Greek suffix -oma has shared in that fate. Thus, the typical modern usage of neuroma is for nerve tumors...
, and physical injury.
Variations
Since synkinesis is ultimately an interaction of nerves with muscles (although glands can also be involved), almost all studied cases are relevant to the cranial nerves where they interact with many small cranial muscles, most of which are responsible for separate and unique functions. This is in contrast to areas of our body where miswiring of the larger muscles is clinically less evident due the size of the muscles.The two cases of synkinesis most commonly studied involve the facial muscles
Facial muscles
The facial muscles are a group of striated muscles innervated by the facial nerve that, among other things, control facial expression. These muscles are also called mimetic muscles.-Structure:...
and the extraocular muscles
Extraocular muscles
The extraocular muscles are the six muscles that control the movements of the eye . The actions of the extraocular muscles depend on the position of the eye at the time of muscle contraction.-List of muscles:-Importance:...
.
Facial synkinesis
Facial Synkinesis is a common sequelae to Idiopathic Facial Nerve ParalysisBell's palsy
Bell's palsy is a form of facial paralysis resulting from a dysfunction of the cranial nerve VII that results in the inability to control facial muscles on the affected side. Several conditions can cause facial paralysis, e.g., brain tumor, stroke, and Lyme disease. However, if no specific cause...
, also called Bell’s Palsy or Facial Palsy. Bell’s Palsy, which occurs due to the compression of the seventh cranial nerve, results in a hemifacial paralysis due to non-functionality of the nerve. As the nerve attempts to recover, nerve miswiring results (see Mechanism of Action below). In patients with severe facial nerve paralysis, facial synkinesis will inevitably develop.
Additionally, a common treatment option for facial palsy is to use electrical stimulation. Unfortunately, this has been shown to be disruptive to normal re-innervation and can promote the development of synkinesis.
The most common symptoms of facial synkinesis include:
- Eye closure with volitional contraction of mouth muscles
- Midfacial movements with volitional eye closure
- Neck tightness (Platysmal contraction) with volitional smiling
- Hyperlacrimation(also called Crocodile Tears)
- A case where eating provokes excessive lacrimation. This has been attributed to neural interaction between the salivary glands and the lacrimal glands .
Extra-ocular muscle synkinesis
The six muscles around the eye (extraocular musclesExtraocular muscles
The extraocular muscles are the six muscles that control the movements of the eye . The actions of the extraocular muscles depend on the position of the eye at the time of muscle contraction.-List of muscles:-Importance:...
) are innervated by three different cranial nerves: Abducens (6th nerve), Trochlear (4th nerve), and Oculomotor (3rd nerve). After nerve trauma around the eye, a combination of any two of these three cranial nerves have been shown to be involved with extra-ocular synkinesis. More over, while the abducens and the trochlear nerve each innervate one specific muscle, the oculomotor nerve has many functions including eyelid retraction and pupil constriction. Thus, during synkinesis, a variable of these functions are involved. Examples include:
- On attempted abduction of an affected eye, the eye adductsAdductionAdduction is a movement which brings a part of the anatomy closer to the middle sagittal plane of the body. It is opposed to abduction.-Upper limb:* of arm at shoulder ** Subscapularis** Teres major** Pectoralis major** Infraspinatus...
and the eyelid retracts.- This is an interaction between the abducens nerve and a branch of the oculomotor nerve. Voluntary activation of the abducens nerve (eye abduction) causes involuntary activation of the oculomotor nerve (eye adduction and eyelid elevation).
- On attempted abduction, the eye’s unreactive pupil constricts
- Another interaction, yet different, between the abducens nerve (eye abduction) and the oculomotor nerve (pupil constriction) .
- On attempted adduction with eye depression, the eyelid retracts.
- This is a case reported in which voluntary activation of the trochlear nerve (eye depression + eye abduction) is involuntarily activating a branch of the oculomotor nerve responsible for eyelid retraction.
Other less common variations of synkinesis involving the cranial nerves include:
- Trigeminal-Abducens Synkinesis
- After physical trauma to the skull, the muscle involved in eye abduction can become reinnervated by the branch of the trigeminal nerve involved in innervating the muscles of masticationMuscles of masticationDuring mastication, four muscles of mastication are responsible for adduction and lateral motion of the jaw. Other muscles, usually associated with the hyoid such as the sternohyomastoid, are responsible for opening the jaw.-Muscles:*The masseter...
(chewing muscles). Thus, involuntary abduction of an involved eye will occur upon eating or chewing.
- After physical trauma to the skull, the muscle involved in eye abduction can become reinnervated by the branch of the trigeminal nerve involved in innervating the muscles of mastication
- Trigeminal-Facial Synkinesis
- After surgical trauma, the muscles of mastication can become reinnervated by the facial nerve as opposed to the trigeminal nerve. This causes weakness in voluntary chewing; also, facial movements such as blinking cause the muscles to contract.
Synkinesis is also commonly used in referring to the involuntary convergence of the eyes that accompanies focusing for near. This accommodation-convergence synkinesis can result in esotropia, or eyes that turn in when the ratio between accommodation and convergence is unusually high.
Mechanism of action
There are three proposed mechanisms for synkinesis: aberrant nerve regeneration, interneuronal ephaptic transmission, and nuclear hyperexcitability.Aberrant nerve regeneration
The aberrant nerve regeneration hypothesis is the most widely accepted mechanism for synkinesis. The hypothesis states that, after trauma, axons project from the facial nucleus to incorrect peripheral muscle groups. These aberrant branches can simultaneously innervate different subdivisions of the facial nerve .For example: compression to the facial nerve causes a lesion and the set of axons that innervates the orbicularis oris (mouth muscle) degenerate. Once the compression has relieved, regeneration of axons from the lesion site begins. This time though, only 50% of the set of axons that innervate the orbicularis oris successfully reinnervate the original site. The other half aberrantly branched off and innervated the orbicularis oculi(eye muscle). Thus, when the patient purses their lips, the ipsilateral eye will squint.
The hypothesis assumes that disorganized regeneration occurs at the site of the lesion. On the contrary, recent research by Choi and Raisman has provided a more thorough understanding of synkinesis through aberrant axonal regeneration. Their study has shown that regenerating axons become disorganized throughout the length of the nerve and not only at the site of the lesion. 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. The new modification to the hypothesis could allow for better success in developing treatments.
Ephaptic transmission
Ephaptic transmissionEphaptic coupling
Ephaptic coupling is indirect communication within the nervous system through extracellular electric fields, as opposed to direct communication via electrical synapses or chemical synapses. Ephaptic communication has been observed in myelinated nerve fibers....
is when two nerves communicate with each other via an artificial synapse between nerves. Healthy peripheral nerves are insulated with a myelin
Myelin
Myelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a type of glial cell. The production of the myelin sheath is called myelination...
sheath that helps to both enhance electric transmission and to prevent cross-talk between parallel nerves. After a lesion, it has been observed that regenerating nerves might not be myelinated effectively. Consequently, the two nerve fibers can come into contact and provide a means for an impulse to be directly conducted through the nerve membrane. An example to clarify this is having two uninsulated electrical wires placed adjacent to each other. Thus, the two nerves are able to “cross-talk” and send action potentials in both directions.
Nuclear hyperexcitability
The basis of this hypothesis is as follows: after a lesion, axonal degeneration (via Wallerian degenerationWallerian degeneration
Wallerian degeneration is a process that results when a nerve fiber is cut or crushed, in which the part of the axon separated from the neuron's cell body degenerates distal to the injury. This is also known as anterograde degeneration, or orthograde degeneration...
) occurs. The post-synaptic cell consequently becomes deprived of input and becomes more sensitive to neurotransmitters (e.g. creating additional receptors). Subsequently, residual undamaged axons in the same premise of the lesion can provide a source of neurotransmitter to the deprived post-synaptic cell. Since the post-synaptic cell is hypersensitive, the neurotransmitters that reach it from an axon of another nerve will successfully provide stimulation. This consequently creates undesired peripheral movement (i.e. synkinesis) .
- Since synkinesis has been reported in patients within 1–2 months, the nuclear hyper-excitability hypothesis is being supported by more researchers . Furthermore, axonal regeneration is a slow process (~1 mm/day growth) and regeneration at this rate of the facial nerve would roughly take 4–8 months. Since synkinesis is observed much earlier, aberrant regeneration and ephaptic communication fail to explain for this observation thus providing evidence that nuclear hyper-excitability is an important factor in the mechanism of synkinesis development..
Although these three mechanisms have been argued for and against in various ways, it has become more accepted that synkinesis develops through a combination of these mechanisms.
Measuring synkinesis
Until May 2007, there was no clinical scale to measure synkinesis. A study led by Mehta et al. has validated the use of a newly designed instrument to evaluate facial synkinesis called the Synkinesis Assessment Questionnaire (SAQ). The instrument, consisting of nine questions, was found to be both reliable and valid. In addition, it is simple, easy to administer, and inexpensive. Its analyses can allow for treatment options to be evaluated.Treatment
Experimental research for treatment has been mostly focused on facial synkinesis due to its abundant prevalence compared to extra-ocular synkinesis. 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). Established treatments for synkinesis in general include surgery; furthermore, facial synkinesis has the benefit of less invasive treatments such as facial retraining, biofeedback, mime therapy, and Botox.Surgery
Practical surgical procedures used for treating synkinesis are neurolysis and selective myectomy. Neurolysis has been shown to be effective in relieving synkinesis but only temporarily and unfortunately symptoms return much worse than originally . Selective myectomy, in which a synkinetic muscle is selectively resected, is a much more effective technique that can provide permanent relief and results in a low recurrence rate; unfortunately, it also has many post-operative complications that can accompany including edemaEdema
Edema or oedema ; both words from the Greek , oídēma "swelling"), formerly known as dropsy or hydropsy, is an abnormal accumulation of fluid beneath the skin or in one or more cavities of the body that produces swelling...
, hematoma
Hematoma
A hematoma, or haematoma, is a localized collection of blood outside the blood vessels, usually in liquid form within the tissue. This distinguishes it from an ecchymosis, which is the spread of blood under the skin in a thin layer, commonly called a bruise...
, and ecchymosis
Ecchymosis
An ecchymosis is the medical term for a subcutaneous purpura larger than 1 centimeter or a hematoma, commonly called a bruise. It can be located in the skin or in a mucous membrane.-Presentation:...
. Therefore, surgical procedures are very minimally used by doctors and are stored as last-resort options for patients that do not respond well to non-invasive treatments.
Facial retraining
Facial retraining therapy builds upon the idea that neurons are constantly in a dynamic state. In other words, there is constant growth and regression of neuronal projections dependent on the stimuli produced. To reduce synkinesis, facial retraining teaches the patient techniques for increasing wanted movements while focusing on restricting unwanted movement. If, for example, the mouth moves whenever the eyes blink voluntarily, facial retraining techniques will teach the patient to slowly close the eyes while actively focusing on keeping the mouth muscles still. Facial retraining has shown to be very successful with almost a 60-70% average decrease in synkinesis reported after 7 months.Biofeedback
BiofeedbackBiofeedback
Biofeedback is the process of becoming aware of various physiological functions using instruments that provide information on the activity of those same systems, with a goal of being able to manipulate them at will...
therapy for facial synkinesis aims to increase the patient’s awareness of the facial muscle posture and movement. Facial muscles contain few to none intrinsic muscle sensory receptors (used for proprioceptive feedback) and additionally they do not span movable joints and so lack joint receptors (another source for proprioceptive feedback) . Thus, biofeedback allows the patient to actively sense the motion of their muscles. The two common forms of biofeedback used are electromyographic feedback and mirror feedback. Electromyographic feedback includes visual EMG signals (coming from facial muscle sites displayed to the patient from a computer in the form of waveform traces) or auditory signals that indicate strength of muscle contraction. The subsequent role of the patient is to control the movement of undesired muscle during volitional movement by incorporating the information perceived through the EMG. While mirror feedback is a much more basic way of providing the patient feedback on muscle movement, studies have shown that both are very effective options for synkinesis/paresis reduction. Biofeedback is commonly coupled to facial retraining techniques to achieve maximal effectiveness.
A study by Nakamura et al. has shown that biofeedback works better for prevention of synkinesis as opposed to treatment of synkinesis. 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. The desired course of action is to catch the patient shortly after facial nerve trauma and teach the patient biofeedback techniques. This course of action has been experimentally proven to significantly reduce the development of synkinesis.
Mime therapy
Mime therapy was introduced in the Netherlands in 1980. It was initially designed to treat facial palsy by improving symmetry of the face both at rest and during movement. It was then later observed that people who had post-facial palsy synkinesis also benefited from this therapy. 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. Furthermore, later studies by Beurskens et al. have shown that benefits obtained from mime therapy are stable 1 year after therapy.Current mime therapy consists of a combination of procedures designed to promote symmetry of the face at rest and during movement and control synkinesis. The components include: massage, stretching exercises, exercises to coordinate both halves of the face, etc.. The overall aim of mime therapy is to develop a conscious connection between the use of facial muscles and emotional expression.
So while facial retraining therapy is much more focused on treating slight synkinetic movements, mime therapy aims to increase the overall vigor of the muscles through active exercises, while in the process of doing so, teaching the face to decrease unwanted synkinetic movements.
Botox
Botox is a new and versatile tool for the treatment for synkinesis. Initially used for reducing hyperkinesis after facial palsy, botox was later attempted on patients with post-facial palsy synkinesis to reduce unwanted movements. The effects of botox have shown to be remarkable with synkinetic symptoms disappearing within 2 or 3 days. 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 though is that in a majority of patients, various synkinetic movements completely disappeared after 2-3 sessions of botox injections (each session administered after ~3 months) .A more specific synkinesis, crocodile tears syndrome (hyperlacrimation upon eating), has been shown to respond exceedingly well to botox injection. Botox is injected directly into the lacrimal gland and has shown to reduce hyperlacrimation within 24–48 hours. The procedure was shown to be simple and safe with very little chance of side-effects (although rarely ptosis
Ptosis
Ptosis refers to droopiness of a body part. Specifically, it can refer to:* Ptosis * Ptosis * Enteroptosis * Gastroptosis...
can occur due to botulinum toxin diffusion) . Furthermore, reduction in hyper-lacrimation was shown to last longer than the expected 3 months (about 12 months) .
Since botox can mimic facial paralysis, an optimized amount of botox has been determined that reduces involuntary synkinesis of the muscle while not affecting muscle tone.
See also
- Bell's palsyBell's palsyBell's palsy is a form of facial paralysis resulting from a dysfunction of the cranial nerve VII that results in the inability to control facial muscles on the affected side. Several conditions can cause facial paralysis, e.g., brain tumor, stroke, and Lyme disease. However, if no specific cause...
- ParesisParesisParesis is a condition typified by partial loss of voluntary movement or by impaired movement. When used without qualifiers, it usually refers to the limbs, but it also can be used to describe the muscles of the eyes , the stomach , and also the vocal cords...
- ParalysisParalysisParalysis is loss of muscle function for one or more muscles. Paralysis can be accompanied by a loss of feeling in the affected area if there is sensory damage as well as motor. A study conducted by the Christopher & Dana Reeve Foundation, suggests that about 1 in 50 people have been diagnosed...