Abstract
Objective Transtympanic electrocochleography (ECochG) was recorded bilaterally in children and adults with auditory neuropathy (AN) to evaluate receptor and neural generators. Methods Test stimuli were clicks from 60 to 120 dB p.e. SPL. Measures obtained from eight AN subjects were compared to 16 normally hearing children. Results Receptor cochlear microphonics (CMs) in AN were of normal or enhanced amplitude. Neural compound action potentials (CAPs) and receptor summating potentials (SPs) were identified in five AN ears. ECochG potentials in those ears without CAPs were of negative polarity and of normal or prolonged duration. We used adaptation to rapid stimulus rates to distinguish whether the generators of the negative potentials were of neural or receptor origin. Adaptation in controls resulted in amplitude reduction of CAP twice that of SP without affecting the duration of ECochG potentials. In seven AN ears without CAP and with prolonged negative potential, adaptation was accompanied by reduction of both amplitude and duration of the negative potential to control values consistent with neural generation. In four ears without CAP and with normal duration potentials, adaptation was without effect consistent with receptor generation. In five AN ears with CAP, there was reduction in amplitude of CAP and SP as controls but with a significant decrease in response duration. Conclusions Three patterns of cochlear potentials were identified in AN: (1) presence of receptor SP without CAP consistent with pre-synaptic disorder of inner hair cells; (2) presence of both SP and CAP consistent with post-synaptic disorder of proximal auditory nerve; (3) presence of prolonged neural potentials without a CAP consistent with post-synaptic disorder of nerve terminals. Significance Cochlear potential measures may identify pre- and post-synaptic disorders of inner hair cells and auditory nerves in AN.
Highlights
Auditory neuropathy (AN) is a disorder of auditory nerve function (Starr et al, 1996) characterized by hearing deficits affecting auditory perceptions dependent on temporal, but not intensity or high frequency spectral cues (Zeng et al, 2005)
The three subjects (#5–7) with an autoimmune etiology had diverse syndromes including scleroderma (#7), an unspecified immune disorder (#6) with abnormal anti-nuclear antibody (ANA) (>1:160, normal < 1:40) identified at onset of deafness while subject #5 with immunological disorder had hearing loss developing at age 5 during a subacute illness marked by pancytopenia and dermatitis
The major conclusions we have drawn from this study of cochlear potentials in eight children and adults with auditory neuropathy (AN) is that the disorder reflects abnormalities of both pre-synaptic (e.g., IHCs) and post-synaptic functions
Summary
Auditory neuropathy (AN) is a disorder of auditory nerve function (Starr et al, 1996) characterized by hearing deficits affecting auditory perceptions dependent on temporal, but not intensity or high frequency spectral cues (Zeng et al, 2005). Clinical criteria for diagnosis include absence or marked abnormality of auditory brainstem potentials (ABRs) beyond that expected for the hearing loss, preserved cochlear receptor outer hair cell activities (otoacoustic emissions [OAEs] and/or cochlear microphonics [CMs]), and absent acoustic middle ear muscle reflexes (Starr et al, 1996; Berlin et al, 2005). Temporal bone analyses from patients with Type I AN (Spoendlin, 1974; Hallpike et al, 1980; Starr et al, 2003) reveal loss of auditory ganglion cells and their processes with remaining axons and dendrites showing myelin and axonal abnormalities. On the basis of these findings the expected physiological effects would be absence or marked abnormalities of ABRs due to loss of auditory nerve fiber activity and altered neural synchrony accompanying abnormal conduction in remaining fibers. Activities of hair cells including otoacoustic emissions, the faint sounds emitted by OHCs in response to acoustic stimulation (Probst et al, 1991) as well as cochlear microphonics (CM) generated by both IHCs and OHCs, would likely be preserved
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