The guttural pouch becomes abnormally filled with air, causing a nonpainful swelling just behind the mandibula on one or both sides. It occurs in young horses (from birth to 1 year) and it is caused by inflammation or by a congenital defect that allows air to enter the pouch but prevents it from returning to the pharynx.

If tympany is due to a congenital defect, laser surgery is required to make the pharyngeal orifice bigger. The trapped air will then be expelled from the affected guttural pouch.