Effects of tympanic membrane scars on tympanometry: a study in cats.

The efficacy of tympanometry in the diagnosis of ossicular abnormalities and the differentiation of such from eardrum scarring is controversial. This paper presents a study of the tympanometric effects of specific eardrum scars and ossicular abnormalities in cats. For both normal eardrums and healed myringotomies, a normal middle ear could be differentiated from stapes fixation or incudostapedial discontinuity using static immittance parameters and tympanogram morphology. Twenty percent myringectomy scars caused abnormal 660 Hz tympanograms but this effect could be distinguished from changes caused by a concomitant ossicular abnormality. The very low impedance of a 50% myringectomy scar resulted in tympanograms which could not be differentiated from an ossicular discontinuity alone. With this neomembrane, stapes fixation could not even be differentiated from an ossicular discontinuity. This study supports the view that tympanometry should be interpreted as part of a battery of tests which includes otoscopy, audiometry, and acoustic reflex measurements.