A 38-year-old woman has unilateral low-frequency conductive hearing loss, disabling autophony and vertigo triggered by loud sounds or straining. Tympanometry is normal and acoustic reflexes are PRESENT. High-resolution temporal bone CT confirms dehiscence of the superior semicircular canal. Why are the acoustic reflexes preserved here despite conductive-type loss?
- A The facial nerve innervates the stapedius abnormally in this condition
- B The stapes is mobile, since the loss arises from a third-window shunting of sound energy away from the cochlea ✓
- C Reflexes are always absent in any conductive hearing loss exceeding 20 dB
- D The dehiscence directly stimulates the cochlear nerve, bypassing the middle ear
Correct answer: B. The stapes is mobile, since the loss arises from a third-window shunting of sound energy away from the cochlea
Explanation
Superior semicircular canal dehiscence creates a third mobile window, diverting acoustic energy into the vestibule and producing a conductive-pattern loss with normal middle ear mechanics. Because the stapes and ossicular chain remain mobile, ipsilateral acoustic reflexes are present. This is the key bedside discriminator from otosclerosis, where equivalent air-bone gap abolishes the reflex.
Reference: Cummings Otolaryngology: Head and Neck Surgery, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.