A 45-year-old man presents with progressive unilateral hearing loss and tinnitus. Audiometry shows sensorineural hearing loss. Auditory Brainstem Response (ABR) testing is performed with click stimuli delivered via earphones, with scalp electrodes recording from vertex (active) and mastoid (reference). The ABR waveform shows a normal Wave I, but the structure marked **C** (Wave V) is significantly delayed compared to the contralateral side, with an interaural latency difference of 0.25 milliseconds. The interpeak latency between Waves I and V is 4.6 milliseconds. Which of the following is the most likely diagnosis?
A. Conductive hearing loss from ossicular fixation
B. Sudden sensorineural hearing loss from cochlear ischemia
C. Auditory neuropathy spectrum disorder with absent cochlear microphonic
D. Vestibular schwannoma with retrocochlear compression
Explanation
Why Vestibular schwannoma with retrocochlear compression is right
Wave V (marked C) is generated by the lateral lemniscus terminating in the inferior colliculus of the midbrain. In retrocochlear pathology such as vestibular schwannoma, the tumor compresses the ascending auditory pathway, classically prolonging Waves III and V and increasing the I-V interpeak latency (>4.4 ms) and the interaural latency difference of Wave V (>0.2 ms). The patient's findings—delayed Wave V with an interaural difference of 0.25 ms and I-V interpeak latency of 4.6 ms—are pathognomonic for retrocochlear lesion. The preserved Wave I confirms cochlear and distal nerve integrity, ruling out cochlear pathology. (Cummings Otolaryngology 7e; Katz Clinical Audiology 7e)
Why each distractor is wrong
Sudden sensorineural hearing loss from cochlear ischemia: Cochlear pathology would affect Wave I (distal cochlear nerve) and produce a delayed or absent Wave I. In this case, Wave I is normal, and the delay is isolated to Wave V, indicating a central (brainstem) lesion, not cochlear.
Conductive hearing loss from ossicular fixation: Conductive hearing loss causes a uniform delay of all ABR waves (I through V) due to reduced stimulus intensity reaching the cochlea. The pattern here—normal Wave I with delayed Wave V—is inconsistent with conductive loss and indicates a central conduction delay.
Auditory neuropathy spectrum disorder with absent cochlear microphonic: ANSD presents with preserved otoacoustic emissions but absent or grossly abnormal ABR (all waves). This patient has a preserved Wave I and only delayed Wave V, which is not consistent with ANSD. Additionally, ANSD is typically seen in neonatal hyperbilirubinemia, Friedreich ataxia, or Charcot-Marie-Tooth disease, not in a 45-year-old with progressive unilateral hearing loss.
High-YieldNEET PG
Wave V (inferior colliculus) delay with I-V interpeak latency >4.4 ms and interaural difference >0.2 ms = retrocochlear pathology (vestibular schwannoma) until proven otherwise.
Cummings Otolaryngology 7e; Katz Clinical Audiology 7e
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