Alternobaric vertigo refers to the dizziness with a spinning sensation that occurs during a dive when one ear compensates for pressure changes before the other. This condition, which is more common during ascent than descent, happens when the permeability of one of the Eustachian tubes is impaired, so that ear does not compensate properly, creating a relative pressure difference between the two ears.
The symptoms are similar to those of any vertigo: dizziness, a sensation that “things are spinning,” sometimes accompanied by nausea and vomiting, and usually subsides spontaneously once compensation is achieved. This often requires returning to a previous depth and avoiding abrupt compensation maneuvers, as they might worsen the condition.
To prevent it, one should avoid diving with upper respiratory congestion issues and, during dives, compensate both ears simultaneously while avoiding very rapid pressure changes.