The tympanic isthmus: its anatomy and clinical significance.

The tympanic isthmus is a narrow passage between the tubotympanic cavity and the atticomastoid air space. Obstruction of the tympanic isthmus is frequent in various types of middle ear disease and causes significant air-diffusion disturbance within the temporal bone pneumatic system. Using large numbers of fresh temporal bone specimens, anatomical boundaries of the tympanic isthmus are defined and pathological variations are investigated. Embryological origin of the isthmus and pathophysiological implications of the tympanic isthmus block are discussed. Among factors that cause obstruction of the tympanic isthmus are mucosal folds variations, inflammatory webs and exudate, retracted tympanic membrane, diseased attic mucosa, and cholesteatoma. Each factor is analyzed with representative specimen photographs. Clinical signs due to tympanic isthmus block often simulate or coexist with symptoms of eustachian tube obstruction. Surgical management of the tympanic isthmus block requires clear recognition of these disease processes. Understanding of the tympanic isthmus as an anatomical entity is essential in the management of middle ear and attic disease.