Autosomal-dominant Compelling Helio-Ophthalmic Outburst syndrome is the need to do what when seeing the Sun?

Sneeze. Autosomal-dominant Compelling Helio-Ophthalmic Outburst (ACHOO) syndrome is the photic sneeze reflex—sudden exposure to bright light (like the Sun or a camera flash) makes some people sneeze. “Autosomal-dominant” means a single gene variant can pass this trait on, and it affects roughly 10–35% of people. The leading idea is “cross-talk” between the optic nerve and the trigeminal nerve: light-triggered signals spill over and activate nasal pathways, producing a sneeze. Remember it by its acronym: ACHOO literally sounds like a sneeze. It’s not yawning, hiccuping, or coughing—and it’s worth noting for safety, as sneezes can be triggered when emerging into sunlight while driving.