The latency of auditory nerve-brainstem responses in sensorineural hearing loss

Regulator of G Protein Signalling 4 (RGS4) as a Novel Target for the Treatment of Sensorineural Hearing Loss

We and others have previously identified signalling pathways associated with the adenosine A₁ receptor (A₁R) as important regulators of cellular responses to injury in the cochlea. We have shown that the “post-exposure” treatment with adenosine A₁R agonists confers partial protection against acoustic trauma and other forms of sensorineural hearing loss (SNHL). The aim of this study was to determine if increasing A₁R responsiveness to endogenous adenosine would have the same otoprotective effect. This was achieved by pharmacological targeting of the Regulator of G protein Signalling 4 (RGS4). RGS proteins inhibit signal transduction pathways initiated by G protein-coupled receptors (GPCR) by enhancing GPCR deactivation and receptor desensitisation. A molecular complex between RGS4 and neurabin, an intracellular scaffolding protein expressed in neural and cochlear tissues, is the key negative regulator of A₁R activity in the brain. In this study, Wistar rats (6–8 weeks) were exposed to traumatic noise (110 dBSPL, 8–16 kHz) for 2 h and a small molecule RGS4 inhibitor CCG-4986 was delivered intratympanically in a Poloxamer-407 gel formulation for sustained drug release 24 or 48 h after noise exposure. Intratympanic administration of CCG-4986 48 h after noise exposure attenuated noise-induced permanent auditory threshold shifts by up to 19 dB, whilst the earlier drug administration (24 h) led to even better preservation of auditory thresholds (up to 32 dB). Significant improvement of auditory thresholds and suprathreshold responses was linked to improved survival of sensorineural tissues and afferent synapses in the cochlea. Our studies thus demonstrate that intratympanic administration of CCG-4986 can rescue cochlear injury and hearing loss induced by acoustic overexposure. This research represents a novel paradigm for the treatment of various forms of SNHL based on regulation of GPCR.

Early Treatment with Growth Hormone (GH) and Rehabilitation Recovers Hearing in a Child with Cerebral Palsy

Neonatal hearing loss is one of the most common anomalies and is frequently associated with delivery problems. The effects of growth hormone (GH) on brain regeneration after an injury are well known. This paper looks at a male child diagnosed with cerebral palsy, psychomotor affectation, left spastic hemiparesis, and bilateral sensorineural hearing loss after fetal distress due to ruptured membranes before the delivery of more than 30 hours of evolution and several episodes of severe hypoglycemia. From 3.5 months of age, we treated him with GH (0.04 mg/kg/day), Melatonin (5 mg/day and 6 months later 10 mg/day) and rehabilitation, for a period of 14 months; at discharge, the child fully recovered all the disabilities produced by his cerebral palsy, including normal hearing; GMFM-88 increased from 7.84% to 48.23%; Battelle scores increased from 2 to 9 after 7 months of treatment, and to 30, 1 year after discharge. Most likely hearing loss was recovered due to the effect of GH on the production of hair cells from stem cells (only present in very young children) in the cochlear sensory epithelium. This is the first case of recovery of hearing loss in humans after GH administration. Moreover, GH administration is useful and safe for early treatment of cerebral palsy.

Follow-up of latency and threshold shifts of auditory brainstem responses after single and interrupted acoustic trauma in guinea pig

Thresholds of auditory brainstem responses (ABRs) are widely used to estimate the level of noise-induced hearing loss or the level of acquired resistance to acoustic trauma after repeated exposures, i.e., the "toughening" effect. Less is known about ABR latencies and their relation to threshold changes. Guinea pigs were exposed to a traumatic pure tone at 5 kHz, 120 dB SPL, as either single (2 h, 4 h) or repeated (1 h every 48 h, four times) sessions. Thresholds and latencies of ABRs were monitored up to 45 days following the acoustic trauma. We show that latencies are prolonged in the case of large temporary threshold shifts observed in the days following trauma. The latency shift decreases after several repeated exposures, then stabilizes, similar to thresholds, suggesting that the "toughening" effect also applies to latencies. Permanent latency shift is usually very small compared to the permanent threshold shift. This effect could produce a recovery in the ability to process auditory information through the precise timing of neuronal events. Our study indicates that when estimated at suprathreshold stimulation level (70 dB SPL), latency provides complementary information to the sole threshold.

The effects of sensory hearing loss on cochlear filter times estimated from auditory brainstem response latencies

Derived-band auditory brainstem responses (ABRs) were obtained in 43 normal-hearing and 80 cochlear hearing-impaired individuals using clicks and high-pass noise masking. The response times across the cochlea [the latency difference between wave V's of the 5.7- and 1.4-kHz center frequency (CF) derived bands] were calculated for five levels of click stimulation ranging from 53 to 93 dB p.-p.e. SPL (23 to 63 dB nHL) in 10-dB steps. Cochlear response times appeared to shorten significantly with hearing loss, especially when the average pure tone (1 to 8 kHz) hearing loss exceeded 30 dB. Examination of derived-band latencies indicates that this shortening is due to a dramatic decrease of wave V latency in the lower CF derived band. Estimates of cochlear filter times in terms of the number of periods to maximum response (Nmax) were calculated from derived-band latencies corrected for gender-dependent cochlear transport and neural conduction times. Nmax decreased as a function of hearing loss, especially for the low CF derived bands. The functions were similar for both males and females. These results are consistent with broader cochlear tuning due to peripheral hearing loss. Estimating filter response times from ABR latencies enhances objective noninvasive diagnosis and allows delineation of the differential effects of pathology on the underlying cochlear mechanisms involved in cochlear transport and filter build-up times.

Auditory brainstem response to bone-conducted clicks in adults and infants with normal hearing and conductive hearing loss

Knowledge concerning auditory brainstem response (ABR) by bone conduction (BC) is limited, and occasionally controversial. The present study was aimed at further elaboration of this issue. The research population consisted of 107 subjects. Four groups were investigated: group 1, normal-hearing adults aged 20-37 years; group 2, 10 children aged 5.6-8.4 years, with confirmed middle ear effusion (MEE); groups 3 and 4, 22 infants, matched by pairs, aged 5-18 months, 11 with normal otoscopy and 11 with suspected MEE. Comparison between ABR by AC and BC for all four groups is discussed. We observed that the AC-ABR threshold of group 2 was statistically significantly elevated compared to group 1. The same tendency was observed for group 4 compared to group 3. In AC ABR, the mean latency of wave V was significantly prolonged, compared to that of BC ABR in children with confirmed MEE, and infants with suspected MEE. We strongly suggest that by combining AC and BC ABR, more information concerning cochlear reserve status can be obtained in infants and young children who are difficult to test, or wherever a behavioral audiogram cannot be achieved.

ABR amplitude and dispersion variables. Relation to audiogram shape and click polarity

ABR amplitude behaviour to condensation (C) and rarefaction (R) clicks was investigated in normal ears, ears affected by Ménière's disease and ears with high-frequency hearing loss (HF ears). The wave IV-V amplitude-intensity function was steeper in ABRs evoked by R than by C clicks. This may suggest that two different cochlear generator components, one intensity-dependent as well as one polarity-dependent, contribute to click-evoked ABRs. Wave IV-V amplitude was significantly higher in Ménière ears compared to HF ears in spite of comparable 2-4 kHz hearing loss in the two groups. Hence, audiometric steepness (which is lower in Ménière ears than in HF ears) seems to predict the wave IV-V amplitude decline more precisely than the 2-4 kHz perception threshold. The wave IV-V dispersion variable (SR IV-V) was close to normal in Ménière ears, while wave IV-V was more dispersed in HF ears. In general, C click ABRs were less affected than R click ABRs by 'peripheral' factors (i.e. intensity and audiometric steepness).

Brainstem response audiometry. I. Its use in distinguishing between conductive and cochlear hearing loss

The auditory brainstem response thresholds and the latency-level curves, l(L)curves, for peak V were determined in 22 subjects with normal hearing, in 40 patients with conductive hearing loss and in 79 patients with cochlear hearing loss. The goal of this study was to investigate the potentials to distinguish between different types of hearing loss on the basis of these auditory brainstem responses. For this purpose the horizontal shift of the l(L) curve, the horizontal shift of its derivative and the latency of peak V at threshold level were plotted against the response threshold. For response thresholds above 30 dB nHL both the horizontal shift of the l(L) curve and the horizontal shift of its derivative give a good separation between cochlear and conductive hearing loss. The combination of the response threshold with the shift of the derivative of the l(L) curve gave a slightly better separation than that of the response threshold with the shift of the l(L) curve itself.

Effects of presbycusis and other types of hearing loss on auditory brainstem responses

Auditory brainstem responses were studied in 373 healthy individuals of various ages and with sensorineural hearing loss of differing degrees. 209 subjects were elderly and had a hearing loss which included a component of presbycusis. The remainder were young or middle-aged with cochlear hearing loss. Altogether 606 ears were tested. The old individuals had generally longer ABR wave latencies than the young subjects. The I-V interpeak latency (IPL) was also prolonged in the older age groups compared with the group of younger individuals, except for subjects with pronounced hearing loss. The results of the study indicate that an age-related dysfunction of the auditory pathway in the brainstem can be present in presbycusis.

Survey of attempts to use auditory evoked potentials to obtain an audiogram. Review article

Click stimuli have been used to elicit ABR in auditory diagnosis but the ABR response is dominated by the activity of high-frequency nerve fibres and it is therefore impossible to deduce hearing at lower frequencies from ABR to clicks. Short tone pips or filtered clicks are also inadequate since the response is still dominated by basal turn, high-frequency nerve fibres. In order to attempt to obtain an audiogram by means of ABR, various noise-masking techniques have been used to restrict the response to more specific regions along the basilar membrane. These included presenting click or filtered click stimuli together with either different high-pass noises in order to obtain a derived ABR or notched noise. Though these techniques have been tested on small groups of normal subjects, they have not been adequately clinically validated on sufficient numbers of hearing-loss patients with various audiogram shapes. When tested on patients with low-frequency hearing loss, inappropriate thresholds have been obtained indicating problems with lower frequencies. Alternatively, other types of auditory evoked responses based on less synchronous neural activity have been studied including the SN10 response and the 40 Hz-MLR. These responses have not been adequately clinically tested. In conclusion the evoked potential techniques have not been completely tested in the clinical situation and several studies have indicated problems with the evaluation of lower frequencies.

Considerations in the differentiation between brainstem lesions and sensori-neural hearing loss using auditory nerve-brainstem evoked responses

When auditory nerve-brainstem evoked responses are used in neurological diagnosis, an important response parameter to be evaluated is brainstem transmission time - the time interval between the auditory nerve response (wave 1) and the response from the rostal brainstem (wave P4). Occasionally the only response wave present is wave P4 and its latency is prolonged. This can be compatible with a conductive or a sensorineural hearing loss (SNHL), or a neurological (brain stem) lesion such as a retrocochlear hearing loss (RCHL). A conductive hearing loss can be excluded by otoscopic, audiometric and tympanometric examination. In order to differentiate between SNHL and RCHL, several techniques have been suggested in order to enhance the visibility of wave 1, but these are not always applicable or successful. In this study, P4 latency was plotted as a function of the audiometric hearing loss at 4 kHz for 26 ears with SNHL only. From this, the correlation coefficient (r = 0.71) was calculated along with the linear regression equation and the standard error of the prediction. This gave an estimation of the range of P4 latency to be expected for a given SNHL for comparison with the actual latency obtained in patients with neurological complaints. An actual latency greater than the expected latency range would be considered corroboratory of a RCHL. Several clinical examples are presented. These considerations also point out the need to bear in mind the otological context of these evoked responses even when used in neurological diagnosis.

Patterns of auditory nerve and brainstem-evoked responses (ABR) in different types of peripheral hearing loss

Auditory brainstem response (ABR) recordings were made in different types of peripheral hearing loss (sensorineural, conductive, and mixed). Scatter diagrams were constructed by plotting different expressions of the hearing loss (audiometric and electrophysiologic) against the obtained wave latencies and for these two sets of variables, correlation coefficients and the parameters of the regression equation were calculated. Consideration is given to the possibility of diagnosing the type of peripheral hearing loss by correlating wave latencies and the electrophysiologic (ABR) threshold.