Forced mouth opening reaction: a primitive reflex released from cortical inhibition.

We report the case of a 6-year-old girl with congenital adrenal hyperplasia, who showed 'forced mouth opening reaction' during the course of acute encephalopathy due to adrenal crisis. When an object was moved towards her mouth, or when the corner of her mouth was stroked with a tongue depressor, she would immediately open her mouth fully and hold it open. This reaction appeared transiently during the course of her illness in association with other frontal release signs including the rooting, groping and palmomental reflexes. Magnetic resonance imaging showed bilateral widespread lesions involving the gray and white matters in the frontal lobes, and less severe lesions in the temporal and parietal areas. We propose that this unique reaction is a sign of a release phenomenon, and represents the emergence of primitive reflexes in the absence of cortical inhibition in some types of encephalopathies.