A patient with a vestibular schwannoma has progressive unilateral hearing loss with poor speech discrimination disproportionate to the pure-tone thresholds, positive SISI (short increment sensitivity index), negative tone decay test (no significant fatigue), and absent stapedial reflexes. Which of these findings specifically indicates RETROCOCHLEAR (nerve/central) rather than cochlear pathology?

• A Absent or elevated stapedial reflexes combined with abnormal tone decay (rapid auditory adaptation) indicate retrocochlear pathology ✓
• B Positive SISI (>70%) indicates recruitment and confirms cochlear (hair cell) pathology, not retrocochlear
• C Pure-tone average alone differentiates cochlear from retrocochlear disease
• D Poor speech discrimination in isolation is pathognomonic of retrocochlear lesion

Explanation

Retrocochlear pathology (acoustic neuroma/vestibular schwannoma) is characterized by: (1) absent or elevated stapedial reflexes (since the afferent arc travels in CN VIII, which is compressed) — this contrasts with cochlear lesions where reflexes are present or even at reduced thresholds due to recruitment; (2) abnormal tone decay (pathological auditory adaptation/fatigue): the nerve cannot sustain firing at constant tone, and sensitivity drops >30 dB over 60 seconds (Carhart test). SISI is typically negative or low (<20%) in retrocochlear lesions because SISI tests for recruitment (a cochlear phenomenon reflecting loss of outer hair cell compression), while in cochlear disease SISI is positive (>70%). Poor speech discrimination (option D) alone overlaps with cochlear disease and is not pathognomonic. Pure-tone average (option C) does not differentiate the site of lesion.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.