The photic sneeze reflex (also known as photoptarmosis, Autosomal Dominant Compelling Helio-Ophthalmic Outburst Syndrome (ACHOO) and colloquially sun sneezing) is a condition of uncontrollable sneezing in response to numerous stimuli, such as looking at bright lights or periocular (surrounding the eyeball) injection. The condition affects 18-35% of the population, but its exact mechanism of action is not well understood.