A 28-year-old man presents with sudden-onset conductive hearing loss and aural fullness 2 weeks after a motor vehicle accident with temporal bone trauma. Otoscopy shows an intact tympanic membrane. Audiometry reveals an air-bone gap of 45 dB. Tympanometry shows the pattern marked **A** in the diagram — an excessively high compliance peak at normal middle ear pressure (0 daPa). Acoustic reflexes are absent bilaterally. High-resolution CT temporal bone shows discontinuity at the incudostapedial joint. Which of the following best explains the mechanism underlying the excessively high compliance peak marked **A**?

Explanation

## Why "Loss of mechanical coupling between the tympanic membrane-malleus complex and the stapes, allowing abnormally free vibration of the TM without normal ossicular load" is right Type Ad tympanogram (excessively high compliance peak at normal middle ear pressure) reflects an abnormally mobile tympanic membrane–ossicular system. In ossicular chain discontinuity (here, incudostapedial joint separation from temporal bone fracture), the tympanic membrane and malleus lose their normal mechanical load and coupling to the stapes and oval window. This allows the TM to vibrate excessively freely, producing the characteristic high-compliance peak (>2.5 mL on 226 Hz probe tone). The intact TM at normal middle ear pressure rules out perforation (Type B) or negative pressure (Type C), confirming the diagnosis of post-traumatic ossicular discontinuity. [Dhingra ENT 7e Ch 12; Cummings Otolaryngology 7e Ch 134] ## Why each distractor is wrong - **Increased stiffness of the tympanic membrane due to post-traumatic fibrosis and scarring**: This would produce a Type As tympanogram (shallow, restricted compliance peak), not Type Ad. Type As is seen in otosclerosis and tympanosclerosis, not ossicular discontinuity. - **Reduced compliance secondary to ossicular fixation from post-traumatic ankylosis**: Ossicular fixation (as in otosclerosis) reduces compliance and produces Type As, not the excessively high compliance of Type Ad. This is the opposite mechanism. - **Negative middle ear pressure causing retraction and decreased compliance of the tympanic membrane**: Negative pressure produces a Type C tympanogram (peak shifted to negative daPa), not Type Ad. Type Ad specifically shows a normal pressure peak (0 daPa) with high compliance. **High-Yield:** Type Ad = hypermobile TM at normal pressure = ossicular discontinuity (post-trauma, incus erosion, stapes superstructure loss); Type As = restricted TM = ossicular fixation (otosclerosis); Type B = flat = perforation or middle ear effusion; Type C = negative pressure peak = Eustachian tube dysfunction. [cite:Dhingra ENT 7e Ch 12; Cummings Otolaryngology 7e Ch 134]