Central auditory pathology

Cochlear nucleus neuron analysis in individuals with presbycusis

OBJECTIVES/HYPOTHESIS

The aim of this study was to analyze the cochlear nucleus neuron population in individuals with normal hearing and presbycusis.

STUDY DESIGN

Retrospective study of archival human temporal bone and brain stem tissues.

METHODS

Using strict inclusion criteria, the temporal bones and cochlear nuclei from six normal hearing individuals and four individuals with presbycusis were selected for analysis. The spiral ganglion cell population, the cochlear nucleus neuron population, and the cell body size of the neurons were quantified in these cases.

RESULTS

A relationship was not observed between age and the spiral ganglion cell population in the normal hearing group. Presbycusis subjects exhibited a reduced spiral ganglion cell population. The mean cochlear nucleus neuron population was observed to be significantly higher in the presbycusis group (mean ± standard deviation: 114,170 ± 10,570) compared to the normal hearing group (91,470 ± 9,510) (P = .019). This difference was predominantly the result of greater multipolar and granule cell neuron populations. Only the fusiform neuron type exhibited a significantly different mean cell body cross-sectional area between the normal hearing group (242 ± 27) and the presbycusis group (300 ± 37) (P = .033).

CONCLUSIONS

This investigation is the first time, to our knowledge, that the populations of the eight neuron types in the cochlear nucleus have been quantified in both normal hearing individuals and individuals with presbycusis. The data support the concept that presbycusis is not an effect of aging alone but instead may be a condition that predisposes one to hearing loss with advancing age and is characterized by a congenitally elevated cochlear nucleus neuron population.

Age-related deficits in auditory confrontation naming

The naming of manipulable objects in older and younger adults was evaluated across auditory, visual, and multisensory conditions. Older adults were less accurate and slower in naming across conditions, and all subjects were more impaired and slower to name action sounds than pictures or audiovisual combinations. Moreover, there was a sensory by age group interaction, revealing lower accuracy and increased latencies in auditory naming for older adults unrelated to hearing insensitivity but modest improvement to multisensory cues. These findings support age-related deficits in object action naming and suggest that auditory confrontation naming may be more sensitive than visual naming.

The jaundiced gunn rat model of auditory neuropathy/dyssynchrony

OBJECTIVE

High levels of bilirubin are neurotoxic and may result in deafness or auditory neuropathy/auditory dyssynchrony (AN/AD). The jaundiced (jj) Gunn rat animal model of kernicterus has electrophysiologic and neuroanatomic abnormalities of brainstem auditory nuclei with normal cochlear microphonic recordings. We examined morphologic changes in the cochlea, spiral ganglion, and auditory nerve and relate these findings to current understanding of AN/AD.

METHODS

At 15 days of age, jj and nonjaundiced (Nj) littermates were injected with sulfadimethoxine (sulfa) and killed 3 days later by transcardial perfusion. Sections were cut through decalcified temporal bones, cochlear nerves, and auditory brainstem and processed for light and electron microscopy and immunohistochemical localization of calbindin-D and parvalbumin.

RESULTS

Spiral ganglion neurons were severely degenerated with a paucity of myelinated axons in jj animals. Electron microscopy of the intramodilar auditory nerve revealed a lack of large caliber axons in jj-sulfa versus Nj-sulfa controls. Large diameter degenerating axons were characterized by an electron-dense atrophied axis cylinder resembling an axonopathy.

CONCLUSIONS

Our findings of abnormal spiral ganglion cells and selective loss of large, myelinated auditory nerve fibers with no abnormalities in cochlear hair cells, support the sulfa-treated jj Gunn rat as a model for bilirubin induced AN/AD. The paucity of large caliber neurons undermines temporal coding of auditory information and neural synchrony and demonstrates that in addition to brainstem auditory nuclei, spiral ganglion neurons are selectively vulnerable to bilirubin toxicity.

Bilirubin toxicity in the developing nervous system

Bilirubin toxicity remains a significant problem despite recent advances in the care of jaundiced (hyperbilirubinemic) neonates. A recent surge in reported cases of classical kernicterus, due in part to earlier hospital discharge and relaxation of treatment criteria for hyperbilirubinemia, and new reports of hyperbilirubinemia-induced auditory dysfunction using evoked potential based infant testing and hearing screening, underscore the need to better understand how hyperbilirubinemia causes brain damage in some infants, especially because the damage is preventable. Recent progress in understanding bilirubin binding and neurotoxicity resulting from unbound or "free" unconjugated bilirubin, how bilirubin affects the central nervous system in vivo and in vitro, and the use of new clinical tools in neonates, for example magnetic resonance imaging revealing bilateral lesions in globus pallidus and subthalamus, and abnormal brainstem auditory evoked potentials with normal inner ear function, may lead to improved detection and prevention of neurologic dysfunction and damage from bilirubin. Finally, the concern is raised that partial or isolated neurologic sequelae, for example auditory neuropathy and other central auditory processing disorders, may result from excessive amount and duration of exposure to free, unconjugated bilirubin at different stages of neurodevelopment.

Changes in calcium-binding protein expression in the auditory brainstem nuclei of the jaundiced Gunn rat

Sensorineural hearing loss and auditory dysfunction are major sequelae of neonatal hyperbilirubinemia. The sites and cellular effects of bilirubin toxicity in the auditory brainstem pathway are not easily detected. Since altered intracellular calcium homeostasis may play a role in neuronal cell death, we hypothesized that the expression of calcium-binding proteins may be altered in the classic animal model of bilirubin neurotoxicity. The expression of the calcium-binding proteins, calbindin-D28k and parvalbumin, in the brainstem auditory pathway of homozygous recessive jaundiced (jj) Gunn rats was examined by light and electron microscopic immunohistochemistry at 18 days postnatally and compared to the findings obtained from age-matched non-jaundiced heterozygous (Nj) littermate control rats. Immunoreactive staining for both calbindin and parvalbumin was reduced in the cochlear nuclei and the superior olivary complex in jj rats. The extent of the reduction in immunoreactivity was related to the severity of the clinical symptoms. By contrast, immunoreactive staining in other brainstem areas (e.g., dorsal and ventral nuclei of the lateral lemniscus, inferior colliculus), thalamic (medial geniculate body) auditory areas, and neighboring non-auditory structures was similar in jaundiced and control rats. Calbindin-immunoreactive staining in the superior paraolivary and medial superior olivary nuclei in Nj rats was associated with myelinated axons, whereas parvalbumin-immunoreactive staining was localized postsynaptically in neuronal somata and dendrites. Immunoreactive staining for the calcium-binding proteins calbindin and parvalbumin in lower brainstem auditory nuclei shows abnormalities in areas susceptible to the effects of hyperbilirubinemia and provides a sensitive new way to assess bilirubin toxicity in the auditory system.

Electrocochleography in auditory neuropathy

Auditory neuropathy (AN) is a disorder characterized by the absence or the severe impairment of the auditory brainstem responses (ABRs) together with the preservation of otoacoustic emissions and/or cochlear microphonic (CM). We recorded transtympanic electrocochleography (ECohG) evoked by 0.1 ms clicks in one young adult and in four children having distortion product otoacoustic emissions and absent ABRs. In all but one patient CM and summating potential (SP) were present with normal threshold, and their amplitudes appeared comparable to or higher than the values obtained from subjects with normal hearing. The compound action potential (CAP) was absent in two patients while in one subject CM and SP were followed by a highly desynchronized neural activity. A broad CAP was found in two children and the threshold appeared clearly elevated in one of them, while it showed only a mild elevation in the other. No correlation was found between CAP and behavioral thresholds. These results suggest that ECohG can be useful in AN diagnoses since it is the only reliable tool in evaluating the auditory peripheral function in the presence of a desynchronized ABR.

Central auditory dysfunction may precede the onset of clinical dementia in people with probable Alzheimer's disease

OBJECTIVES

To document the prognostic significance of a central auditory speech-processing deficit for the subsequent onset of probable Alzheimer's disease.

DESIGN

Prospective cohort study.

SETTING

Framingham Heart Study.

PARTICIPANTS

Seven hundred forty dementia-free volunteers from the Framingham Heart Study cohort with symmetric hearing thresholds at biennial examination 15 (1983-1985).

MEASUREMENTS

The diagnosis of probable Alzheimer's disease was made prospectively using the National Institute of Neurological and Communicative Disorders and Stroke-Alzheimer's Disease Related Disorder Association criteria. A central auditory speech-processing deficit was defined as a score of 50% of less correct on the Synthetic Sentence Identification with Ipsilateral Competing Message test in at least one ear with normal word recognition ability in both ears. Cox proportional hazards regression assessed the relationship between a central auditory speech-processing deficit and the age at diagnosis of probable Alzheimer's disease.

RESULTS

Forty subjects (5.4%) received a diagnosis of probable Alzheimer's disease during an average of 8.4 years (range 3-12) of follow-up; seven (17.5%) of these had a central auditory speech-processing deficit. The presence of a central auditory speech-processing deficit had an age-adjusted risk ratio for probable Alzheimer's disease of 10.8 (95% CI = 4.6-25.2), and the estimated risk ratio adjusted for age, gender, education level, apolipoprotein allele E4 presence, and hearing level was 23.3 (95% CI =6.6-82.7). A central auditory speech-processing deficit had a positive predictive value for subsequent probable Alzheimer's disease of 47% but the sensitivity was only 17.5%.

CONCLUSION

Central auditory speech-processing deficits may be an early manifestation of probable Alzheimer's disease and may precede the onset of dementia diagnosis by many years.

Reversing click polarity may uncover auditory neuropathy in infants

OBJECTIVE

To identify patients with primary auditory neuropathies whose cochlear potentials to a 100 microsec click persist after click cessation and simulate synchronous auditory brain stem responses (ABRs) at high intensities.

DESIGN

ABRs to condensation and rarefaction clicks, as well as Maximum Length Sequence ABRs and one transtympanic electrocochleogram (ECochG), were collected from five infants with absent middle ear muscle reflexes and normal or near normal otoacoustic emissions. These infants failed ALGO screens, which used alternating polarity clicks, and/or failed full ABRs done elsewhere with alternating polarity clicks.

RESULTS

When ABRs were collected in response to a single polarity pulse, they revealed robust and reproducible wave forms over 4 to 6 msec that initially were mistaken for a normal ABR by the referring agents. However, when condensation and rarefaction click data are compared, the waveforms change polarity when the stimulus is inverted. Furthermore, the waveforms fail to shift in latency as the intensity of the stimulation is reduced. Transtympanic ECochG on one of the children revealed the same polarity reversal and fixed latency functions, confirming that they were cochlear rather than neural responses.

CONCLUSIONS

Comparing responses with positive versus negative polarity clicks may help separate ABRs from cochlear potentials and alert clinicians to the possibility of an auditory neuropathy. Therefore, absent or abnormal ABRs in the presence of normal otoacoustic emissions need not always implicate a purely "central disorder," but might be consistent with dysfunction between outer hair cells and primary afferent fibers.

Auditory neuropathy: a report on three cases with early onsets and major neonatal illnesses

We report 3 children without any brainstem auditory evoked potential (BAEP) neural component who all retained isolated cochlear microphonic potentials as well as click-evoked otoacoustic emissions. Two of them demonstrated only moderately impaired audiometric thresholds. These features correspond to a peculiar pattern of auditory dysfunction recently coined "auditory neuropathy'. In contrast with the published previous cases of auditory neuropathy presenting with an acquired hearing deficit as children or young adults, all 3 children had a history of major neonatal illness and the auditory neuropathy was already demonstrated in the first months of their lives.

Tinnitus and hearing loss in pineal region tumours

The authors report an underestimated symptom and sign arising in pineal region tumours: tinnitus and hearing loss. It has been observed in 13 out of 72 pineal region tumours (18%). Three illustrative cases are reported in this paper. The inferior colliculi, the structure more dense in fibres than any other auditory brain stem site and at which majority of the acoustic pathways relay, is closely adjacent to the pineal body. Displacement of this structure may be responsible for acoustic symptoms together with common visual symptoms.

Cochlear microphonics in the jaundiced Gunn rat

INTRODUCTION

Bilirubin toxicity causes brain damage and deafness. Brainstem auditory areas are damaged, but the effects of bilirubin toxicity on the peripheral auditory system are less well defined. As a first step in the study of cochlear physiology, we performed studies of scalp-derived cochlear microphonic (CM) responses in the jaundiced (jj) Gunn rat model of acute bilirubin toxicity.

MATERIALS AND METHODS

CMs were obtained from scalp needle electrodes in response to acoustically delayed 500-, 1,000-, 2,000-, 4,000-, 6,000-, and 8,000-Hz tonebursts delivered by overhead speakers, and brainstem auditory-evoked potentials (BAEPs) were obtained to clicks. At 18 days of age, one nonjaundiced (Nj) and two jj Gunn rats in each of four litters were anesthetized. CMs and BAEPs were performed before and 4 hours after either (1) sulfonamide was injected into jj rats to produce acute bilirubin toxicity, (2) jj controls were injected with saline, or (3) Nj controls were given sulfonamide. In a second experiment, 16-day-old jj-sulfa and Nj-saline littermates were studied with insert speakers at 60 and 75 dB hearing level (HL) at baseline and 6 hours later, and CM amplitude was analyzed with a fast fourier transformation.

RESULTS

No statistically significant differences were found by repeated measures analysis of variance in the CMs in either experiment between groups or after sulfonamide, despite BAEP changes of decreased amplitude of waves II and III and increased latency of I-II and I-III interwave intervals in jjs given sulfa.

CONCLUSION

Alterations of CM after acute bilirubin toxicity did not occur at a time when there was electrophysiologic evidence of brainstem dysfunction. Although more subtle effects might be detected with a larger sample studied at longer intervals after the insult, CM seems insensitive or less sensitive to acute bilirubin toxicity than brainstem auditory function. These results suggest that retrocochlear tests of central auditory function may be more sensitive to the effects of hyperbilirubinemia than tests of peripheral auditory function in humans.

Human cochlear nucleus: comparison of Nissl-stained neurons from deaf and hearing patients

In this study of the effects of deafness on the morphology of the human cochlear nuclei, non-parametric statistical analysis is used to quantify differences in sizes and shapes of neuron somata. Data on 81,007 neuron somata from 11 patients are presented, as well as the total volume and surface area of the cochlear nuclei. Soma size of deaf patients, especially postlinguistically deaf ones, was smaller than that of controls, but not significantly so; the same was true for total cochlear nucleus volume. The data also indicate a greater soma size on the right side (as well as a greater ventral cochlear nucleus volume), a caudal-to-rostral decrease in soma size, and a correlation between soma size and shape. The data base is being continually extended and in future will allow comparisons with measurements from patients suffering from various forms of hearing loss.

Surgical implantation and biocompatibility of central nervous system auditory prostheses

As part of a program to determine the feasibility of a CNS auditory prosthesis, the tissue reaction to electrodes chronically implanted in the cochlear nucleus (CN) of the guinea pig was examined. Varied open operative approaches and microelectrode designs were utilized. Silicon substrate thin film and platinum-iridium wire electrodes, tethered and untethered, were placed successfully in different divisions of the CN. Implantation through a posterior suboccipital approach was most successful. Histologic examinations demonstrated a glial cell proliferation confined to the area of the electrode track that never exceeded 15 microns in width. No neuronal loss or significant effect on cell morphology was seen, and reactive cells were absent. Electrode migration was apparent in a minority of animal preparations. Although potential problems were identified, our findings lend support to the feasibility of implanting a neuroprosthesis in the CN and have helped to establish methods for future studies of chronic intranuclear stimulation.

Brain stem auditory evoked potentials in jaundiced Gunn rats

Bilirubin encephalopathy causes potentially preventable brain damage and hearing loss. The site of auditory dysfunction is controversial, despite pathologic studies showing damage to brain stem auditory nuclei in humans and experimental animals. We studied the effects of bilirubin toxicity on the auditory system of homozygous jaundiced Gunn rats by use of brain stem auditory evoked potentials. Small but statistically significant abnormalities were found for wave latencies, interwave intervals, and amplitudes.