Sneezing in response to bright light exposure: A case study in a photic sneezer

Abstract Background The photic sneeze reflex (PSR) is a widespread, yet understudied phenomenon characterised by sneezing in response to bright-light exposure, reportedly affecting around 30% of the general population. Our goals were to collect real-world data to characterise PSR-inducing naturalistic light conditions, and to develop an indoor protocol to reliably induce the PSR in affected individuals using parametric stimuli. Methods This study was carried out on one male adult affected by photic sneezing (n=1). To characterise naturalistic light conditions eliciting photic sneezing, real-world light exposure was measured over a 30-day period, while logging PSR events. To study photic sneezing in response to artificial stimuli, a setup including a multi-primary LED source and an integrating sphere was used to present 30-second light stimuli to the participant while collecting pupillometric data with an eye-tracker. Results 82 photic sneeze events were recorded, with an average of 2.73 sneezes per day and a range of 1 to 6 sneezes per event. At a sneeze event, illuminance is on average ten times bigger than five minutes before the sneeze event. A significant increase in illuminance is observed around 2 minutes before the sneeze event. Light levels go back down to pre-sneeze levels within 10 minutes after sneezing. Despite exposure to more than 150 stimuli, no sneeze could be artificially induced in the participant. However, a strong tickling sensation was consistently reported, especially for high illuminance settings. Conclusions Real-world light data confirmed that a sudden increase in environmental lighting conditions can induce photic sneezing. Further analysis could be relevant on instances of illuminance increments not eliciting a photic sneeze. The experimental setup only elicited tickling sensations, but with further testing and optimisation, it is reasonable to assume that it would reliably induce photic sneezes, thereby opening further mechanistic study of this intriguing phenomenon.