What happens inside my body when I hold in a sneeze?

Fire in the hole! A sneeze launches your nasal contents at speeds that can exceed 190km/h. What happens if you try to keep a sneeze from blasting off? You risk serious damage to the launch facility: blocking a sneeze can generate pressure more than 38 times that of a normal sneeze. Damage from blocked sneezes include fractures of the eye socket and the cartilage in the larynx, as well as forceful injection of air into the chest cavity and the tissues of your neck. (Vigorous nose blowing, while nowhere near as explosive, can cause infections of the sinuses and middle ear.) How likely is any of this to happen? No one really knows. But since the only reason to block a sneeze is to avoid spreading viruses (which you can do just as effectively by sneezing into the crook of your elbow), the next time you have a sneeze in 
final count, clear the pad and go for launch.