A 62-year-old man with a 10-year history of progressive bilateral high-frequency hearing loss presents to the audiology clinic. Pure-tone audiometry shows symmetric sensorineural hearing loss worse at 3–6 kHz. The SISI test is performed at 1000 Hz, 20 dB above threshold, with 1-dB intensity increments presented every 5 seconds. The patient detects 16 out of 20 increments, yielding a SISI score of 80%. The structure marked **A** in the diagram represents this finding. Which of the following best explains the underlying pathophysiology of this SISI result?

Question

Accepted Answer

A. Loudness recruitment due to outer hair cell damage and loss of cochlear amplifier function, narrowing the dynamic range between threshold and uncomfortable loudness. ## Why "Loudness recruitment due to outer hair cell damage..." is right A SISI score >70% (in this case, 80%) is pathognomonic for **cochlear pathology with loudness recruitment**. The clinical anchor is that SISI exploits the phenomenon of loudness recruitment — an abnormally rapid growth in perceived loudness that occurs when outer hair cell (OHC) damage abolishes the cochlear amplifier. When OHCs are damaged (as in presbycusis, noise-induced hearing loss, Meniere's disease, or ototoxicity), the dynamic range between threshold and uncomfortable loudness narrows dramatically. Consequently, patients can detect small (1-dB) intensity increments near threshold better than normal-hearing listeners because their residual hearing is operating in a compressed loudness space. This patient's symmetric, age-related high-frequency loss is consistent with presbycusis — a classic cochlear etiology — and the SISI score of 80% confirms the presence of recruitment. (Katz Clinical Audiology 7e; Jerger 1959) ## Why each distractor is wrong - **Cochlear nerve fatigue and reduced neural synchrony...**: This describes the mechanism of a **negative SISI (<30%)**, which occurs in normal hearing or retrocochlear lesions. Nerve fatigue prevents the abnormal loudness growth seen in cochlear recruitment. A high SISI score rules out retrocochlear pathology. - **Stiffening of the basilar membrane and reduced frequency selectivity...**: This mechanism is associated with conductive or mixed hearing loss and age-related stiffening of middle-ear structures, not cochlear OHC damage. It does not explain loudness recruitment or a high SISI score. - **Retrocochlear demyelination of the vestibulocochlear nerve...**: Retrocochlear lesions (e.g., acoustic neuroma, demyelination) produce a **negative SISI (<30%)** because central fatigue prevents recruitment. A SISI >70% specifically excludes retrocochlear pathology and points to cochlear origin. **High-Yield:** SISI >70% = cochlear recruitment (OHC damage); SISI <30% = normal or retrocochlear (no recruitment); SISI is presented at 20 dB SL with 1-dB increments and exploits the narrowed dynamic range of cochlear hearing loss. [cite: Katz Clinical Audiology 7e; Jerger 1959 classic]

Answer

B. Cochlear nerve fatigue and reduced neural synchrony preventing abnormal loudness growth in response to intensity increments

Answer

C. Stiffening of the basilar membrane and reduced frequency selectivity, causing decreased sensitivity to small intensity changes

Answer

D. Retrocochlear demyelination of the vestibulocochlear nerve, impairing central auditory processing of intensity cues

Explanation

Why "Loudness recruitment due to outer hair cell damage..." is right

Why each distractor is wrong

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