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|LETTER TO EDITOR
|Year : 2015 | Volume
| Issue : 4 | Page : 629-631
Chewing induced dystonia: Report of a case and the review of literature
Kalyan B Bhattacharyya1, Indrani Dasgupta2
1 Department of Neuromedicine, G Kar Medical College and Hospital, Kolkata, West Bengal, India
2 Department of Neurology, All Asia Medical Institute, Kolkata, West Bengal, India
|Date of Web Publication||4-Aug-2015|
Kalyan B Bhattacharyya
Department of Neuromedicine, G Kar Medical College and Hospital, Kolkata, West Bengal
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Bhattacharyya KB, Dasgupta I. Chewing induced dystonia: Report of a case and the review of literature. Neurol India 2015;63:629-31
Chewing induced dystonia is a rare form of task-specific dystonia that occurs only on chewing and not with any other activity. Here, we report one such case and review the relevant literature.
A 65-year-old lady reported to the clinic complaining of progressive difficulty in chewing food for the last 1-year. Her husband reported that while chewing, she made grimaces and took an inordinately long time to chew before swallowing the food bolus. Such features were absent while drinking water, speaking or singing. There was no history of abnormal posturing in any other part of the body. Her birth history and subsequent development were uneventful, and she was a housewife by profession. There was no inner urge to carry out the movement that would suggest tics, and no sensory trick abolished it. She was a non-vegetarian and there was no history of any psychiatric illness or intake of any psychotropic drugs, exposure to toxins, facial palsy in the past, no history of jaundice, and the history in general, was non-contributory. She was not using dentures. Her family history was non-contributory.
On examination, we found an elderly lady without any abnormal movements at rest. However, when she was offered a biscuit to eat, there was abnormal contraction of the orbicularis oris, orbicularis oculi, nasalis, corrugator supercilli, risorius and mentalis muscles while chewing. The tongue also did not seem to move with ease and the movements were reproducible. There was no other involuntary movement elsewhere in the body, and there was no finding suggestive of Parkinsonism. However, when she spoke, these abnormal movements were conspicuously absent. The movements were upsetting for the patient since they were socially embarrassing. There were no Kayser-Fleischer ring on the corneas seen on the naked eye examination and rest of the neurological examination was normal. The abnormal movements during chewing and the absence of it while speaking were video-recorded for future documentation [Videos 1 and 2].
The slit-lamp examination did not reveal Kayser-Fleischer rings and the serum ceruloplasmin and 24-h urinary copper level were within normal limits. Magnetic resonance imaging of the brain was within normal limits. The patient did not agree to the idea of needle electromyographic examination of the facial and tongue muscles for documentation of co-contraction of the affected muscles. All other hematological and biochemical examinations like thyroid and liver profiles were within normal limits. She was advised trihexyphenidyl, 2 mg twice daily, and the dose was increased to thrice daily after 1-week. Additionally, tetrabenazine 25 mg, initially half tablet twice daily and then thrice daily after 1-week was also prescribed. The subject of botulinum toxin therapy was discussed with her, but she did not agree to the proposal of facial injections. She did not turn up again and was thus lost to follow-up.
Task-specific dystonias are a group of focal dystonic conditions where the movements occur only during a specific task.  Bernardino Ramazzini provided one of the first descriptions of task-specific dystonia in the form of writer's cramp in 1713 in a book on occupational diseases. He noted that "Scribes and Notaries may develop incessant movement of the hand, always in the same direction…the continuous and almost tonic strain on the muscles…results in failure of power in the right hand."  The term "dystonia" was coined by Hermann Oppenheim in 1911 to indicate an abnormal increase in muscle tone and the contractions that follow.  Initially, the condition was thought to be psychogenic in origin and termed "occupational neuroses." However, the works of Sheehy and Marsden in 1982 incontrovertibly proved the organic nature of the disease. 
The pathophysiology of dystonias is not precisely known. Misfiring of neurons in the sensorimotor cortex is thought to cause the muscular contractions.  The reason for misfiring could be the result of impaired inhibitory mechanisms during contraction of the muscles. Normally, when the motor cortex sends a signal for a muscle to contract, the antagonistic muscles for the intended movement receive inhibitory inputs as well, and they are put to rest. In dystonia, this mechanism seems to be jeopardized and thus, there is simultaneous contraction of the agonists and antagonists.  In addition, it has been proposed that the sensorimotor cortex is organized as discrete templates of the human anatomy and in the physiological state they occupy a distinct area in the cortical motor homunculus. In dystonic conditions, there is a breakdown of this motor memory and the templates lose their identity, and this has been confirmed by imaging findings. , Additionally, it had been a subject of conjecture as to why professionals who excessively use a certain group of muscles for daily activity, are more prone to the development of focal task-specific dystonias. It has been proposed that the excessive motor training in these individuals may contribute to the development of dystonia since their cortical maps enlarge, as a result of cerebral plasticity.  There is one report of a subject who lost both the upper limbs and who thereafter, learnt to write with the pen held between the teeth. Later, he developed task-specific cranio-cervical dystonia while writing, and this has been cited as an example of unusual, unaccustomed overwork in a group of muscles, leading to focal dystonia. 
Task-specific chewing dystonia is a rare entity, and only a handful of cases have been reported so far in the world. , One related case has been reported where dystonic movements were observed in the lower face on closing the jaw.  It has been postulated that the dystonic movements were triggered by pressure on the periodontal mechanoreceptors and a defective central command control of their sensory inputs.  An artificial synapse, like ephaptic transmission between the sensory branches of the trigeminal nerve and the motor root of the facial nerve at their point of exit from the pons has been adduced as another explanation. 
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