A 22-year-old male from rural India presents with progressive gait ataxia, areflexia, and extensor plantar responses over 3 years. Neuroimaging shows cerebellar and spinal cord atrophy. Pure-tone audiometry reveals the pattern marked **B** in the diagram. Genetic testing confirms homozygous GAA trinucleotide repeat expansion in the FXN gene on chromosome 9q13.11. Which of the following best explains the audiovestibular phenotype in this patient's condition?
A. Auditory neuropathy spectrum disorder with abnormal ABR despite preserved otoacoustic emissions
B. Conductive hearing loss secondary to ossicular fixation
C. Selective loss of cochlear outer hair cells with preserved neural function
D. Sudden sensorineural hearing loss from viral labyrinthitis
Explanation
Why Auditory neuropathy spectrum disorder with abnormal ABR despite preserved otoacoustic emissions is right
Friedreich ataxia (FRDA) causes selective degeneration of large sensory neurons and brainstem pathways due to frataxin deficiency and mitochondrial iron accumulation. The characteristic audiovestibular abnormality in FRDA is auditory neuropathy spectrum disorder (ANSD), not simple cochlear hair cell loss. ANSD in FRDA is defined by the dissociation of preserved otoacoustic emissions (indicating intact cochlear function) with absent or grossly abnormal auditory brainstem responses (ABR), reflecting dysfunction of the auditory nerve and central auditory pathways. While mild bilateral high-frequency SNHL may be present on pure-tone audiometry (as shown in pattern B), the pathophysiology is neural/brainstem dysfunction, not cochlear pathology. This distinction is critical for diagnosis and counseling.
Why each distractor is wrong
Selective loss of cochlear outer hair cells with preserved neural function: This describes the opposite pattern (cochlear dysfunction with preserved neural function) and is not the characteristic FRDA phenotype. FRDA affects neural and brainstem structures, not primarily outer hair cells.
Conductive hearing loss secondary to ossicular fixation: Pattern B is sensorineural (not conductive), and ossicular pathology is not a feature of FRDA. This would present with a conductive or mixed pattern, not the high-frequency SNHL seen here.
Sudden sensorineural hearing loss from viral labyrinthitis: FRDA is a progressive genetic disorder with insidious onset, not acute viral infection. Labyrinthitis would present acutely with vertigo and tinnitus, not the chronic progressive ataxia and neuropathy of FRDA.
High-YieldNEET PG
Friedreich ataxia = ANSD phenotype (preserved OAE + abnormal ABR) from auditory nerve/brainstem degeneration, NOT cochlear hair cell loss.
Harrison 21e Ch 444; AAN Ataxia Guidelines
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