What causes glue ear?

The build-up of sticky fluid in the middle ear can be caused by a problem with a part of the body called the Eustachian tube.

The Eustachian tube is a thin channel which connects the middle ear with the back of the nose.  Most of the time this tube is closed, but when you swallow or yawn, the tube opens to let the air in and any fluid drain out..

Diagram of the ear:

 

In children with glue ear:

The Eustachian tube can become blocked with mucus, often after a recent cold or ear infection. This stops air entering the middle ear and fluid draining out. 

Young children have narrow and horizontal Eustachian tubes which means they can often get blocked.  

As children get older, the Eustachain tubes become wider and more upright which allows fluid to drain better.  This is why children often grow out of glue ear.