Asymmetry of the pupillary light reflex during a cold pressor test.

Pupillary light reflexes were monitored in 20 healthy participants while they immersed one foot in painfully cold water (the cold pressor test) or in warm water for 1 min. Pupillary dilatation was greater during the cold pressor test than during the warm-water immersion. In addition, during the cold pressor test, re-dilation after exposure to bright light proceeded more rapidly for the ipsilateral than contralateral pupil. These findings suggest that sympathetic pupillary drive is greater ipsilateral than contralateral to pain.