Oromandibular dystonia involving the lateral pterygoid muscles: four cases with different complexity.

The report describes oromandibular dystonia (OMD) in four women with involuntary activity of the lateral pterygoid muscles (LP), causing incapacitating protrusive and lateral jaw movements and displacements, and treatment with botulinum toxin type A (BTX). For initial survey and treatment control, OMD was analyzed with several, independent, and standardized methods. OMD severity and functional difficulties were evaluated subjectively and scored from videotapes. Jaw movements were assessed graphically with a magnetic tracking system, and electromyographical activity (EMG) of LP was recorded with needle electrodes using an intraoral approach, whereas activity of masseter muscles was recorded with surface electrodes. EMG-guided BTX injections (25-40 units Botox per muscle) into the muscles were performed with cannula electrodes. Compared with reference values for LP, OMD was associated with a markedly increased level of spontaneous activity, but almost normal level of maximum voluntary activity. The central pattern generator for mastication seemed to override the dystonic activity, as all patients were able to chew despite some distortion. BTX reduced both the spontaneous and the maximum activity for 3-9 months. Concomitantly, a marked reduction of the OMD severity, mandibular movements and functional disturbances were also present with the best effect in localized OMD with late onset.