Neural and Receptor Cochlear Potentials Obtained by Transtympanic Electrocochleography in Auditory Neuropathy Q
Total Page：16
File Type：pdf, Size：1020Kb
Clinical Neurophysiology 119 (2008) 1028–1041 www.elsevier.com/locate/clinph Neural and receptor cochlear potentials obtained by transtympanic electrocochleography in auditory neuropathy q Rosamaria Santarelli a,*, Arnold Starr b, Henry J. Michalewski b, Edoardo Arslan a a Department of Medical and Surgical Specialities, Service of Audiology and Phoniatrics, University of Padua, Via Giustiniani 2, I-35128 Padua, Italy b Department of Neurology, University of California, Irvine, CA, USA Accepted 26 January 2008 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 gen- erators 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. Ó 2008 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. Keywords: Pre- and post-synaptic; Auditory nerve synchrony; Adaptation 1. Introduction in understanding speech especially in the presence of noise and the extent of their speech comprehension is impaired Auditory neuropathy (AN) is a disorder of auditory out-of-proportion to the pure tone hearing loss (Sininger nerve function (Starr et al., 1996) characterized by hearing and Oba, 2001, for review). Clinical criteria for diagnosis deficits affecting auditory perceptions dependent on tempo- include absence or marked abnormality of auditory brain- ral, but not intensity or high frequency spectral cues (Zeng stem potentials (ABRs) beyond that expected for the hear- et al., 2005). Adult patients typically complain of difficulty ing loss, preserved cochlear receptor outer hair cell activities (otoacoustic emissions [OAEs] and/or cochlear q microphonics [CMs]), and absent acoustic middle ear mus- The work was done at the University of Padua, Service of Audiology cle reflexes (Starr et al., 1996; Berlin et al., 2005). The dis- and Phoniatrics. * Corresponding author. Tel.: +39 0422 328286; fax: +39 0422 322351. order has a wide range of etiologies (e.g., hereditary, E-mail address: [email protected] (R. Santarelli). infectious, toxic–metabolic, immunological, developmen- 1388-2457/$34.00 Ó 2008 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.clinph.2008.01.018 R. Santarelli et al. / Clinical Neurophysiology 119 (2008) 1028–1041 1029 tal, etc.) and occurs in all age groups (Starr, 2001, for cellular activity, the CM recorded at the promontory is review). deemed to arise from basal portions of the cochlea, while The particulars of the disrupted auditory nerve activity apical regions make almost no contribution to its genera- in this disorder are not known. However, it is known that tion (Withnell, 2001). SP is a slow sustained potential AN accompanies disorders of the auditory nerve (post-syn- whose polarity is highly dependent on the frequency and aptic or Type I AN; Starr et al., 2003), and disorders of intensity of the stimulus (Eggermont, 1976; Ferraro et al., inner hair cells and their synapses with auditory nerve ter- 1994). The SP recorded from the round window in experi- minals (pre-synaptic or Type II AN; Starr et al., 2004; mental animals is believed to be primarily generated by Rodriguez-Ballesteros et al., 2003). Temporal bone analy- IHCs of the basal portion of the cochlea with a minor con- ses from patients with Type I AN (Spoendlin, 1974; Hall- tribution arising from OHCs of both the basal and apical pike et al., 1980; Starr et al., 2003) reveal loss of auditory turns (Zheng et al., 1997; Durrant et al., 1998). ganglion cells and their processes with remaining axons In this study we evaluated auditory nerve (CAP) and and dendrites showing myelin and axonal abnormalities. receptor potentials (CM, SP) recorded by transtympanic The inner and outer hair cells (IHCs, OHCs) appeared nor- ECochG in eight children and young adults with AN. We mal. On the basis of these findings the expected physiolog- hypothesized that both post-synaptic neural and pre-syn- ical effects would be absence or marked abnormalities of aptic receptor abnormalities would be identified reflecting ABRs due to loss of auditory nerve fiber activity and specific cochlear physiological deficits accounting for dis- altered neural synchrony accompanying abnormal conduc- rupted auditory nerve activities in AN. tion in remaining fibers. Activities of hair cells including otoacoustic emissions, the faint sounds emitted by OHCs 2. Methods in response to acoustic stimulation (Probst et al., 1991)as well as cochlear microphonics (CM) generated by both 2.1. Subjects IHCs and OHCs, would likely be preserved. Temporal bone analyses from patients with AN due to We studied eight subjects with AN ranging in age from 5 disordered IHC functions (Type II AN) as might accom- to 48 years (mean = 21.1 years). Details of their clinical pany mutations of otoferlin gene (Varga et al., 2003; and neurological findings, diagnoses, and laboratory mea- Rodriguez-Ballesteros et al., 2003) have not yet been sures are included in Table 1. All patients had absence or reported. The gene mutation is thought to adversely affect severe abnormalities of the ABRs with preserved OAEs. neurotransmitter release and absence of ABRs in this dis- The etiologies of the auditory nerve disorders were heredi- order is consistent with a functional loss of auditory nerve tary (3 subjects), immunological (3 subjects), degenerative input. (1 subject), and congenital (1 subject). ABRs are limited in providing detailed information of The patients’ cochlear functions were evaluated at the cochlear nerve and hair cell activities since the recording University of Padua Service of Audiology and Phoniatrics electrodes are placed at a distance from these generators. by bilateral transtympanic recordings methods as part of Details of cochlear potentials including both receptor their audiological evaluation of hearing disorders. These (SP, CM) and auditory nerve activities (CAP) can be more patients were referred for ECochG to define threshold for effectively evaluated by near-field recordings with an elec- CAP as their ABR results were inconclusive. There have trode placed on the cochlear bony promontory known as been approximately 200 patients tested by this method over transtympanic electrocochleography (ECochG) (Egger- the past 7 years; typically children with prematurity, mont and Odenthal, 1974a). Indeed the amplitude of the hypoxia and hyperbilirubinemia who had been admitted compound action potential (CAP) and summating poten- to NICU, children with delayed speech development tials (SP) reflecting hair cell activation recorded by a needle accompanying autism or mental retardation. In some of electrode on the promontory wall can be as much as 30- the children the transtympanic results did not show objec- fold larger than auditory nerve potentials recorded as tive evidence of a peripheral auditory disorder. Therefore, Wave I of the far-field ABRs. When utilizing click stimula- the ECochG data from 16 subjects with ‘‘normal” thresh- tion the neural fibers in the basal turn are activated at short olds and latency of CAP served as ‘‘controls” for compar- latency and are the major contributors to the recorded ison with the patients with AN. The age of the controls CAP (Eggermont, 1976; Kiang et al., 1976). The SP is typ- ranged from 1 to 7 years with a mean of 3.8 years. There ically not detected in the ABRs. was only one control subject which was younger than 2 Recordings from IHCs and OHCs in experimental ani- years. While the controls were considerably younger than mals have identified intracellular potential changes accom- AN subjects, we do not consider this variable to be a major panying acoustic stimulation that contribute to the SP and limitation. First, the latency of ABR wave I evaluated by CM receptor potentials. CM is believed