A 42-year-old man with a 15-year history of chronic ear discharge and progressive hearing loss presents for audiological assessment. Pure-tone audiometry shows bone conduction thresholds elevated to 35 dB across mid-frequencies and an air-bone gap of 25 dB. The structure marked **A** in the diagram represents the pattern seen in this patient. Which of the following best explains the pathophysiology underlying this audiometric finding in chronic suppurative otitis media?

Explanation

## Why Mixed hearing loss from both conductive and sensorineural mechanisms is right The structure marked **A** in the diagram represents mixed hearing loss with an air-bone gap, which is pathognomonic for simultaneous conductive and sensorineural components. In chronic suppurative otitis media (CSOM), the conductive component arises from tympanic membrane perforation and ossicular chain disruption, while the sensorineural component results from chronic middle ear inflammation, bacterial toxins, and inflammatory mediators damaging the cochlea and affecting round window membrane permeability. The elevated bone conduction threshold (35 dB, >25 dB) confirms cochlear involvement, and the air-bone gap (25 dB, >10-15 dB) confirms the conductive overlay. This mixed pattern is the hallmark of long-standing CSOM with secondary inner ear damage (Cummings Otolaryngology 7e, Chapter 199; Dhingra Diseases of ENT 7e). ## Why each distractor is wrong - **Conductive hearing loss alone from ossicular chain disruption**: While ossicular damage and perforation do cause conductive loss, this option ignores the elevated bone conduction threshold (35 dB), which proves sensorineural involvement. Pure conductive loss would show normal bone conduction (<25 dB). - **Sensorineural hearing loss alone from age-related cochlear degeneration**: This ignores the air-bone gap of 25 dB, which is diagnostic of a conductive component. Presbycusis alone would show an air-bone gap <10 dB with parallel air and bone conduction curves. - **Conductive hearing loss from stapes fixation due to otosclerotic bone remodeling**: Otosclerosis does cause an air-bone gap, but the clinical history (15-year discharge, chronic CSOM) and the magnitude of bone conduction elevation (35 dB) are inconsistent with uncomplicated otosclerosis. Late-stage otosclerosis with cochlear involvement (cochlear otosclerosis) can cause mixed loss, but the primary diagnosis here is CSOM. **High-Yield:** Mixed hearing loss (elevated BC + air-bone gap) in CSOM = conductive component (perforation/ossicles) + sensorineural component (chronic toxic cochlear damage); bone conduction >25 dB + air-bone gap >10–15 dB = mixed loss until proven otherwise. [cite: Cummings Otolaryngology - Head and Neck Surgery 7e, Chapter 199 (CSOM); Dhingra Diseases of Ear Nose and Throat 7e]