The Eustachian tube can be blocked, or obstructed, for a variety of reasons. When that occurs, the middle ear pressure cannot be equalized. The air already there is absorbed and a vacuum occurs, sucking the eardrum inward and stretching it. Such an eardrum cannot vibrate naturally, so sounds are muffled or blocked, and the stretching can be painful.
If the tube remains blocked, fluid will seep into the area from the membranes in an attempt to overcome the vacuum. This is called “fluid in the ear,” serous otitis, or aero-otitis.
The most common cause for a blocked Eustachian tube is the common cold. Sinus infections and nasal allergies (hay fever, etc.) are also causes. A stuffy nose leads to stuffy ears because the swollen membranes block the opening of the Eustachian tube. Children are especially vulnerable to blockages because their Eustachian tubes are narrower than adults.