Sinuses are hollow cavities in the bones of the face. They are lined with a mucous membrane which secretes mucus. This mucus is normally evacuated into the nasal cavity through an opening called the "ostium". If the evacuation is no longer possible, the mucus accumulates, a mucocele is then formed which is literally a "mucus hernia". It deforms the sinus by ballooning it, destroying the bone walls. The most common sinuses involved are the frontal and ethmoidal sinuses.
The treatment is surgical. It consists of marsupializing the mucocele in the nasal cavity, that is to say to widely open it to restore normal drainage and ventilation of the sinus.