From the Merck Veterinary Manual:
Guttural pouch tympany is observed in horses ranging from birth to 1 yr of age and is more common in fillies than in colts. The affected guttural pouch is distended with air and forms a characteristic nonpainful swelling in the parotid region. Breathing may become stertorous in severely affected animals. Tympany may result from inflammation or malformation of the pharyngeal orifice of the eustachian tube, which then acts as a one-way valve by allowing air to enter the pouch but preventing its return into the pharynx. Diagnosis is based on clinical signs and radiographic examination of the skull. Severely affected animals may develop a secondary empyema. Tympany is usually unilateral, but bilateral cases have been reported. Medical management with NSAID and antimicrobial therapy resolves the majority of cases due to upper respiratory tract inflammation. Surgical intervention is warranted in patients with malformation of the guttural pouch opening and involves fenestration of the membrane that separates the affected guttural pouch from the normal one. This provides a route for air in the abnormal guttural pouch to pass to the normal side and be expelled into the pharynx. The postoperative prognosis is good.