When you ascend, the air in your middle ear will expand as the pressure decreases. If your Eustachian tubes become blocked or congested during a dive, they will prevent the expanding air from escaping back out into the airway, which results in a reverse block during ascent.

Early signs of a reverse block include dizziness or vertigo during ascent. This is due to a difference in pressure between your two ears. If the air continues to expand without escaping the middle ear, the pressure can cause extreme pain and possible damage to your ear's internal organs.

The most common cause of a reverse block is a blockage of the Eustachian tubes due to decongestant drugs wearing off during a dive. For this reason, never use decongestant drugs to assist you with equalization.