When the Eustachian tube is blocked during a cold, allergy, or upper respiratory infection, the presence of bacteria or viruses lead to a build-up of pus and mucus behind the eardrum. This pressure causes earache, swelling, and redness. Because the eardrum cannot vibrate properly, hearing problems can occur.

Although the eardrum may rupture so that the pus can drain out, more commonly the build-up remains due to the swollen and inflamed Eustachian tube. This is called middle ear effusion, or serous otitis media. Often after the acute infection has passed, the effusion remains, and becomes thicker and more difficult to get rid of. This condition makes the child subject to frequent recurrences of the acute infection, and may cause difficulty with hearing and balance.

The following factors may pre-dispose a child to getting otitis media:

• immaturity of the Eustachian tube
• familial allergies
• exposure to secondhand smoke
• attending large day cares