Synkinesis

Synkinesis
Classification and external resources
MeSH	D046608
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Synkinesis is the result from miswiring of nerves after trauma. This result is manifested through involuntary muscular movements accompanying voluntary movements. For example, voluntary smiling will induce an involuntary contraction of the eye muscles causing the eye to squint when smiling. Most commonly involved are facial muscles and the extraocular muscles, rarely the hands are performing mirror movements.

Causes are diverse and include nerve trauma with improper healing, or nerve degeneration, as in the course of Parkinson´s disease. In congenital cases, mutations of genes involved in nerve growth, specifically axonal growth have been found. Rarely, it is 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.

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.

Synkinesis is an interaction of nerves with muscles (although glands can also be involved). 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 the size of the muscles. Synkinesis can also involve the upper limbs, especially hands which is quite rare, at 1 case in 1 million.

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 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:

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