Eustachian tube
When gas pressure within the middle ear is less than atmospheric pressure, the eardrum can become sucked into the middle ear space. This may be caused by disrupted gas exchange in the middle ear mucosa, inadequate opening of the Eustachian tube or a combination of these factors. People with a patulous Eustachian tube may also cause negative middle ear pressure by repeatedly sniffing to try to keep their Eustachian tube closed.