Atresia is absence or underdevelopment of the ear canal and middle ear structures. Microtia is almost always accompanied by atresia because the outer ear and the middle ear develop around the same time during fetal development. Some microtia patients have what appears at first look to be a normal canal, but many of them end in a blind pouch (i.e. no connection into middle ear).

Some patients have small ear canals but their middle ear may be normal, and as a result their hearing can be normal or they may have an underdeveloped middle ear resulting in significant conductive hearing loss. Sensorineural function is usually normal as the cochlea and neural elements of hearing are unaffected.