Inverse activity of masticatory muscles with and without trismus: a brainstem syndrome.

Abstract

Clinical and EMG (electromyography) findings in 10 cases of intrinsic brainstem lesions are reported with paradoxical activity of jaw closing muscles during jaw opening, with and without trismus. In 5 cases with trigeminal anesthesia, the inverse activity of jaw closers is interpreted as a manifestation of disturbance in the central programming of mastication in the motor trigeminal area of the brainstem. Stretch reflex mechanisms and disinhibition of the trigeminal motor neurons play no part in the origin of inverse activity. The distinct brainstem syndrome can only be detected by EMG and the special clinical features.