Diagnostic yield of a routine magnetic resonance imaging in tinnitus and clinical relevance of the anterior inferior cerebellar artery loops.

OBJECTIVE: To assess the diagnostic yield of a routine magnetic resonance imaging (MRI) scan in patients with (unilateral) chronic tinnitus, to define the frequency of incidental findings, and to assess the clinical relevance of potentially found anterior inferior cerebellar artery (AICA) loops. STUDY DESIGN: Retrospective cohort study. SETTINGS: Tertiary Tinnitus Care Group at the University Medical Center Utrecht. PATIENTS: Three hundred twenty-one patients with chronic tinnitus. INTERVENTION: Routine diagnostic magnetic resonance imaging (MRI) and diagnostic auditory brainstem responses (ABR) when an AICA loop was found. MAIN OUTCOME MEASURE: Relationship between abnormalities on MRI and tinnitus. RESULTS: In 138 patients (45%), an abnormality on the MRI scan was described. In only 7 patients (2.2%), the abnormality probably related to the patient's tinnitus. Results were not significantly better in patients with unilateral tinnitus (abnormalities in 3.2%). Incidental findings, not related to the tinnitus, were found in 41% of the patients. In 70 patients (23%), an AICA loop was found in the internal auditory canal. No significant relationships were found between the presence of an AICA loop and the side of the tinnitus, abnormalities on the ABR or complaints specific to nerve compression syndrome. CONCLUSION: A routine MRI is of little or no value in patients with tinnitus with persistent complaints. Anterior inferior cerebellar artery loops are often encountered on an MRI scan but rarely relate to the tinnitus and should thus be considered incidental findings. It is advised to only perform an MRI when on clinical grounds a specific etiology with tinnitus as the symptom seems probable.

The role of electrophonics in electroacoustic stimulation of the guinea pig cochlea.

HYPOTHESIS: Interactions between cochlear responses to combined electrical and acoustic stimulation (EAS) depend on electrically evoked hair cell activity (i.e., electrophonics). BACKGROUND: Although relevant for EAS strategies in cochlear implant users with residual low-frequency hearing, cochlear responses to EAS are not well characterized. Previously, we have shown that acoustically evoked compound action potentials (CAPs) can be suppressed by electrical stimulation. In the present study, we characterized the role of electrophonics in CAP suppression in guinea pigs, under conditions representative of clinically applied EAS. METHODS: Electrophonics depend on the frequency spectrum of the electric pulse train, which is mainly determined by pulse width and, to a lesser extent, by pulse rate. We measured suppression of tone-evoked CAPs by electric pulse trains, while varying the pulse width (80 - 400 µs, n = 5) and the pulse rate (500 - 4000 pps, n = 5). The role of outer hair cells (OHCs) in electrophonics was tested in animals with varying degrees of OHC loss (n = 24). RESULTS: Suppression of acoustically evoked CAPs varied with pulse width, indicating that electrophonics were involved. Short pulse widths resulted in minimal CAP suppression at low acoustic frequencies. Pulse rate did not significantly affect CAP suppression. OHC loss had no significant effect on electrophonic activity. CONCLUSION: Electrophonic activity was present in cochleae with extensive basal hair cell loss, indicating that electrophonics can occur in EAS users. Our results show that short pulse widths are optimal for use in EAS stimulation strategies, on the assumption that minimal suppression is best.

Spatial overlap of combined electroacoustic stimulation determines the electrically evoked response in the guinea pig cochlea.

HYPOTHESIS: Limiting spatial overlap between electrical stimulation (ES) and acoustical stimulation (AS) in the cochlea reduces the effects of AS on electrically evoked auditory nerve activity. BACKGROUND: Some hybrid cochlear implant systems have a regular array, whereas others have short arrays that spatially segregate ES from AS. AS settings in hybrid implants may also affect electroacoustic interaction. METHODS: ES (900 µA) was delivered in the high-frequency part of the cochlea, and the electrically evoked compound action potential (eCAP) was recorded to assess auditory nerve activity. Maximal spatial overlap of ES and AS was tested by using normal-hearing animals (NH, n = 6), whereas minimal overlap was modeled by using animals with high-frequency hearing loss (HFHL, n = 6). AS consisted of broadband (BB) or low-frequency (LF) noise (0-100 dB SPL). Effects of AS on eCAP amplitude were statistically tested using 1-sample t tests (α = 0.05). RESULTS: BB noise at 60 dB SPL significantly suppressed eCAP amplitude in NH animals but not in HFHL animals up to a 30 dB higher level. Suppression with LF noise at 60 dB SPL was not significant in either the NH or the HFHL group, but at 90 dB SPL, suppression was significant in both groups. CONCLUSION: Minimizing spatial overlap between ES and AS reduces eCAP suppression when moderate sound levels are applied. Overlap can be reduced by applying ES in an acoustically insensitive part of the cochlea or by limiting the acoustic spectrum to low frequencies when ES is applied in acoustically sensitive areas.

Effects of electrical stimulation on the acoustically evoked auditory-nerve response in guinea pigs with a high-frequency hearing loss.

Criteria for cochlear implantation keep expanding and people with substantial residual low-frequency hearing are considered candidates for implantation nowadays. Therefore, electro-acoustical stimulation in the same ear (EAS) is receiving increasing interest. We have investigated the effects of intracochlear electrical stimulation on acoustically evoked auditory-nerve activity, using a forward masking paradigm. The stimulation electrode was placed in the basal turn of the cochlea. Compound action potential (CAP) recordings were performed in guinea pigs with severe high-frequency hearing loss and in normal-hearing control animals. In normal-hearing animals, electrical stimulation generally suppressed CAPs, especially at high acoustic frequencies (8 and 16 kHz) and low sound levels. At low frequencies (0.5 and 1 kHz), suppression was observed only at high sound levels. In animals with a high-frequency hearing loss, suppression of CAPs at low frequencies was substantially less compared to control animals, even at high current levels and temporal overlap of acoustic and electric stimuli. Hence, effects of electrical stimulation substantially differed between normal-hearing animals and animals with a high-frequency hearing loss. These findings stress the need for a proper animal model when investigating EAS. We conclude that in case of high-frequency loss, the basal part of the cochlea can be stimulated electrically with little effect on responses to low-frequency acoustic stimuli.

Suppression of the acoustically evoked auditory-nerve response by electrical stimulation in the cochlea of the guinea pig.

There is increasing interest in the use of electro-acoustical stimulation in people with a cochlear implant that have residual low-frequency hearing in the implanted ear. This raises the issue of how electrical and acoustical stimulation interact in the cochlea. We have investigated the effect of electrical stimulation on the acoustically evoked compound action potential (CAP) in normal-hearing guinea pigs. CAPs were evoked by tone bursts, and electric stimuli were delivered at the base of the cochlea using extracochlear electrodes. CAPs could be suppressed by electrical stimulation under various conditions. The dependence of CAP suppression on several parameters was investigated, including frequency and level of the acoustic stimulus, current level of the electric stimulus and the interval between electric and acoustic stimulus (EAI). Most pronounced suppression was observed when CAPs were evoked with high-frequency tones of low level. Suppression increased with current level and at high currents low-frequency evoked CAPs could also be suppressed. Suppression was typically absent several milliseconds after the electric stimulus. Suppression mediated by direct neural responses and hair cell mediated (electrophonic) responses is discussed. We conclude that the high-frequency part of the cochlea can be stimulated electrically with little detrimental effects on CAPs evoked by low-frequency tones.

Amplitude and phase of distortion product otoacoustic emissions in the guinea pig in an (f1 ,f2) area study.

Lower sideband distortion product otoacoustic emissions (DPOAEs), measured in the ear canal upon stimulation with two continuous pure tones, are the result of interfering contributions from two different mechanisms, the nonlinear distortion component and the linear reflection component. The two contributors have been shown to have a different amplitude and, in particular, a different phase behavior as a function of the stimulus frequencies. The dominance of either component was investigated in an extensive (f1 ,f2) area study of DPOAE amplitude and phase in the guinea pig, which allows for both qualitative and quantitative analysis of isophase contours. Making a minimum of additional assumptions, simple relations between the direction of constant phase in the (f ,f2) plane and the group delays in f1-sweep, f2-sweep, and fixed f2/f1 paradigms can be derived, both for distortion (wave-fixed) and reflection (place-fixed) components. The experimental data indicate the presence of both components in the lower sideband DPOAEs, with the reflection component as the dominant contributor for low f2/f1 ratios and the distortion component for intermediate ratios. At high ratios the behavior cannot be explained by dominance of either component.