Synkinesis is a neurological symptom in which a voluntary muscle movement causes the simultaneous involuntary contraction of other muscles. An example might be smiling inducing an involuntary contraction of the eye muscles, causing a person to squint when smiling. Facial and extraocular muscles are affected most often; in rare cases, a person's hands might perform mirror movements. Synkinesis is usually caused by dysfunction of a particular nerve. Potential causes include improper healing after nerve trauma or neurodegeneration, as occurs in Parkinson's disease. In congenital cases, mutations of genes involved in nerve growth, specifically axonal growth have been found. Rarely, it occurs as part of syndromes with neuroendocrine problems, such as Kallman syndrome. The prognosis is usually good with normal intelligence and lifespan. 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.
Types Most cases involve the cranial nerves, which innervate many small cranial muscles, such as the facial muscles and the extraocular muscles. This is in contrast to areas of body where miswiring of the larger muscles is less evident due to the size of the muscles. Synkinesis can also involve the upper limbs, especially hands which is quite rare, at 1 case in 1 million. In some cases, nerves improperly regenerate into glands, such as lacrimal glands, leading to a condition known as crocodile tears or Bogorad's syndrome.
Facial synkinesis Facial synkinesis is a common sequela to Idiopathic Facial Nerve Paralysis, also called Bell's Palsy or Facial Palsy. Bell's Palsy, which is thought to occur due to a viral reactivation which can lead (through unknown mechanisms) to diffuse axon demyelination and degeneration 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 frequently develops. 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 muscles) 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. Moreover, 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, one of these functions may be involved. Examples include:
On attempted abduction of an affected eye, the eye adducts 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, is between eye abduction (abducens nerve) and pupil constriction (the oculomotor nerve). On attempted adduction with eye depression, the eyelid retracts. In this case voluntary activation of the trochlear nerve (eye depression + eye abduction) is involuntarily activating a branch of the oculomotor nerve responsible for eyelid retraction. Focusing to the near (accommodation) is accompanied by involuntary convergence of the eyes. This accommodation-convergence synkinesis can result in esotropia, or eyes that turn in when the ratio between accommodation and convergence is unusually high. 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 mastication(chewing muscles). Thus, involuntary abduction of an involved eye will occur upon eating or chewing. 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.
Bimanual Synkinesis Bimanual Synkinesis occurs when left and right upper limbs, especially the hands and fingers execute exactly the same movement even though only one hand is intentionally moved. It is also called "mirror hand movements" and persists throughout life. When it occurs by itself without other associated signs and symptoms it is associated with normal intelligence and lifespan. It can also develop in the course of Parkinson's disease. In association with other abnormalities, mirror hand movements are a hallmark of Kallmann syndrome. Genetic mutations associated with (congenital) mirror hand movements are in the DCC (gene) or RAD51 gene, which account for about 35 percent of cases. In DCC mutation, impaired or missing netrin 1 receptor protein impairs control of axon growth during nervous system development.
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 Syndrome and Marcus Gunn phenomenon.) Trauma to the nerve can be induced in cases such as surgical procedures, nerve inflammation, neuroma, and physical injury.
Mechanism There are three proposed mechanisms for synkinesis: aberrant nerve regeneration, interneuronal ephaptic transmission, and nuclear hyperexcitability.
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