The mechanism of facial sweating and cutaneous vascular responses to painful stimulation of the eye.

The vascular response in the forehead and cheeks to irritating the eye with soapy water was measured in 15 normal subjects. Electrodermal activity, which reflects sweating, was also measured from both sides of the forehead. The mechanism of the response was studied in 15 patients with a unilateral lesion of the facial nerve blocking parasympathetic outflow. Pulse amplitude usually increased briefly on both sides of the forehead after the soap was placed in the eye; the response persisted for several minutes on the ipsilateral side after the soap had been washed from the eye. A facial nerve lesion blocked the vascular response on the lesioned side to stimulation of either eye. No consistent change in pulse amplitude was recorded from the cheeks, although a response was observed in a few subjects. Electrodermal responses to ocular irritation were generally larger on the ipsilateral than contralateral side of the forehead; in patients with facial palsy, electrodermal responses were greater on the normally innervated side than on the lesioned side. The findings suggest that irritating the eye induces a trigeminal-parasympathetic vasodilator reflex and local sweating. The restricted distribution of the response indicates that separate parasympathetic vasodilator reflexes might operate for each division of the trigeminal nerve.