Jaw dystonia triggered by biting into hard food.

We report an unusual case of eating dystonia induced by attempts to bite into hard food and resulting in prolonged jaw opening spasms. Mastication was severely impeded. Simultaneous bilateral electromyogram (EMG) of the masticatory muscles during mastication of hard food showed an abnormal prolonged activity in the anterior digastrics with an abnormal cocontraction of these muscles during contraction of the jaw-closers and a prolonged inhibition of the jaw-closers until the hard bolus was softened. This action dystonia seems to be triggered when more than a certain pressure is exerted on the peridontal mechanoreceptors and could result from a defective central command control of their sensory inputs. Local injections of botulinum toxin had little effect.