What is dark chocolate sneezing?

Listener Amy asks this question, and Eva Higginbotham has been looking into it...

Eva - So I've had a little read around to see what I can find out about this dark chocolate sneezing, and it's actually a version of something called the photic sneeze reflex, where you walk outside from a dark room into a light room and you immediately sneeze uncontrollably, between one and sometimes up to forty sneezes. Now the interesting thing is, the dark chocolate sneezing is a type of photic sneeze reflex, despite the fact that it's not actually caused by photons, that's where the 'photic' comes from. It actually gets put into kind of a catch-all term, photic sneeze reflex, to mean the light sneezing, or some people have sneezing caused by eating things that are spicy, or things like mint or grapefruit. Basically what's happening is your body is having a reaction to a strong stimulus.

There are lots of different theories about why it happens, but essentially it's a different kind of sneeze, right? Because a normal sneeze happens because you have some sort of irritation in your nose, and this is more just a nerve misfiring in your face producing a sneeze. So one of the reasons this is a really interesting thing to talk about today is that, number one, it's not that well studied, despite the fact that it affects really quite a large proportion of the population - I mean, I've seen numbers anywhere between 11 and 35%, which is quite a lot of people; but also because it's kind of a rare example of something we see out in the world that has kind of a really interesting genetic reason. It's a heritable thing; so therefore, if your parents sneeze when they see light, eat dark chocolate, eat mint, eat grapefruit, whatever, it means that you yourself are more likely to sneeze. And it's a dominant trait, which means that if you have one parent who has it, then you are 50% likely to have it.

Now, in terms of the specifics of the genetics, that's actually less well understood. I've seen online a whole range of different things. Some places say it's something to do with chromosome 2. 23andme actually, the company that will sequence your genome if you give them a sample - for some money - found 54 different markers that they associate with photic sneezing. The fact is, we just don't quite know at the moment. But my favourite fact that I discovered while looking this up is the name of the syndrome that is given to people who have photic sneezing; and it's called Autosomal Compelling Helio-Opthalmic Outburst, which stands for ACHOO syndrome. Which I think is perfect.