The palmomental reflex involves contraction of the ipsilateral mentalis muscle and protrusion or wrinkling of the ipsilateral lower lip when the thenar eminence is stroked briskly [1]. This cutaneous-to-motor reflex is elicited most sensitively by stimulating the thenar eminence, though other bodily regions can evoke it as well [2][5]. The wrinkling resembles the start of a wince to more painful stimuli [3]. The reflex was first described in 1920 by Marinesco and Radovici [6].

The neural circuitry underlying the palmomental reflex involves connectivity between the trigeminal nerve, which senses the thenar stimulus, and the facial nerve motor nuclei in the brainstem, which activate the mentalis muscle [4][5]. The reflex is exaggerated in diseases affecting cortical inhibition, like frontal lobe lesions, parkinsonism, and dementia [1][2]. However, the side of the reflex does not necessarily correlate with the side of a cerebral lesion [3][5]. Overall, the palmomental reflex serves as a simple bedside test for impaired cortical control of brainstem reflexes.