Neonatal cochlear hearing loss results in developmental abnormalities of the central auditory pathways

We have used animal models of long term neonatal cochlear hearing loss to study developmental plasticity of the central auditory pathways. Newborn chinchilla pups and feline kittens were treated with the ototoxic drug amikacin, so as to induce basal lesions in the cochlea. At maturity these animals were used in single unit electrophysiological mapping studies, in which the cochleotopic organization of primary auditory cortex (of the cat) and the inferior colliculus of the midbrain (in the chinchilla) were mapped. We have observed, both in the midbrain and auditory cortex, massive reorganization of frequency representation. Most striking were the presence of large monotonic regions (i.e. large areas in which all neurons have similar tuning properties). Cochlear lesions which involve inner hair cells clearly modify the normal development of cochleotopic representation in the midbrain and cortical regions. We suggest that similar abnormal patterns of frequency representation will exist in human subjects with long term neonatal hearing loss.

The MRL-lpr/lpr mouse: a potential model of autoimmune inner ear disease

Most attempts at developing a model of autoimmune inner ear disease have focused on the immunization of healthy animals with cochlear tissue. We have chosen an alternate route of studying this entity utilizing the MRL-lpr/lpr (Lupus) mouse, an animal known to spontaneously develop multisystemic, organ nonspecific autoimmune disease. We report on the auditory pathology found in animals at early stages of this systemic disease. At the onset of clinical signs of illness (cachexia, weight loss, lethargy) animals were sacrificed and their cochleas and kidney prepared for morphologic analysis. Significant pathology was seen in the MRL/lpr animals involving the basal and middle turns of the cochlea which could not be correlated with the presence or degree of glomerulonephritis. Findings included outer and inner haircell degeneration, strial edema and degeneration, and an acellular infiltrate in the tunnel of Corti. Cochlear pathology was not found in control animals. Thus, at early stages of systemic disease, MRL/lpr mice manifest significant cochlear pathology not seen in control animals. The implications of these results with regard to the pathogenesis of these lesions as well as their clinical relevance are discussed.

The effects of long-term cochlear hearing loss on the functional organization of central auditory pathways

We have developed an animal model of neonatal high-frequency cochlear hearing loss to investigate the long-term effects on the central auditory pathways. Specifically, we have induced basal cochlear lesions in newborn kittens using the ototoxic aminoglycoside, amikacin. We have monitored the consequent auditory threshold elevations using auditory brainstem evoked responses (ABR) to tone pip stimuli. In the mature animal we have mapped tonotopic (cochleotopic) representation in primary auditory cortex (AI) using standard micro-electrode recording techniques, and we show that this map becomes massively re-organized. In particular, one frequency area that corresponds to the high frequency cut-off slope of the subject's audiogram appears to become greatly expanded, in some cases to take up 75% of AI surface. In general, the development of normal cochleotopic or tonotopic representation in cortex appears to depend upon the integrity of ascending sensory input from the cochlea particularly during early stages of development. With the clinician in mind, we discuss our findings in relation to practical issues.

Auditory evoked potentials in cats with neonatal high frequency hearing loss. Evidence of abnormal frequency representation in the midbrain

We have recorded auditory evoked potentials, of both neurogenic and myogenic origin, in cats having neonatal high frequency cochlear hearing loss. Using frequency specific stimuli (tone pips) and by measuring responses near to threshold, we have probed tonotopic (or cochleotopic) representation within the brainstem-midbrain auditory pathway. At stimulus frequencies corresponding to the high frequency cut-off of the cats' audiograms we have observed enhanced amplitudes of both auditory brainstem evoked responses (ABR) and postauricular myogenic (PAM) potentials. We interpret our findings as evidence of a larger than normal population of neurons tuned to this frequency region. We suggest that such abnormal frequency representation results from a long-term sensory deficit caused by lesions to the basal, high frequency region of the cochleas.

Correlation of cochlear pathology with auditory brainstem and cortical responses in cats with high frequency hearing loss

Newborn kittens were treated with the aminoglycoside amikacin to produce a bilateral high frequency cochlear hearing loss. The degree and stability of hearing loss were confirmed by recording auditory brainstem evoked potentials (ABR audiograms). After maturation, cochleotopic frequency representation within primary auditory cortex (AI) was mapped using standard microelectrode recording techniques. The cochlear sensory epithelium was assessed with SEM and the pattern of damage compared with the ABR audiograms and cortical frequency maps. Amikacin treatment resulted in various patterns of haircell damage towards the base of the cochlea. A relatively abrupt transition between damaged and undamaged haircell regions resulted in an ABR audiogram with normal threshold to low frequencies and a high frequency elevation with a steep cut-off slope. In the cortical map, low frequency representation was normal, but anterior areas contained only neurons tuned to a common frequency which corresponded to the frequency-place position of the boundary of the haircell lesion and to the cut-off frequency of the audiogram. A large transitional zone of the cochlear lesion correlated with a gradual cut-off slope to the audiogram and again a remapping of the anterior and normally high frequency area to a common lower frequency. Haircell loss or damage (i.e. disarray of stereocilia) in lower frequency regions of the cochlea correlated with a significant reorganization of the lower frequency bands in the cortical map. We conclude from this study that the pattern of cochleotopic organization of the cortex is dependent on the pattern of activity in the ascending sensory pathway during early stages of development.

Developing criteria for ordering common ancillary services

This article describes the process utilized to develop criteria for appropriately ordering five ancillary services frequently performed in hospitals: 1) arterial blood gases, 2) electrocardiograms (ECGs), 3) serum electrolytes, 4) chest x-rays, and 5) complete blood counts (CBCs). The development of each set of criteria involved an initial consultant, a panel of six additional regional expert consultants, and three national reviewers. Each criteria set was developed through a process involving seven steps: 1) an initial working draft, 2) revisions at an initial meeting of the regional experts, 3) revisions at a second meeting of the regional experts, 4) written comments from the regional experts, 5) written comments from national reviewers, 6) additional written comments from regional experts, and 7) application of the criteria to cases in a community hospital. The change in item content was measured between steps and agreement with individual items was measured at steps 2, 3, and 4. The results indicate that appreciable change in content occurred with each step except step 4. Agreement started fairly high and was over 90% by step 4. The discussion considers the utility of each developmental step, factors affecting the utility of the criteria, the extent to which the results can be generalized, and the need for more research to identify optimal processes for the development of criteria to evaluate quality of care.

Three dimensional intracellular structure of the cochlea using the A-O-D-O method

We have observed the three-dimensional intracellular structure of the organ of Corti and the spiral ganglion of the chinchilla using the aldehyde osmium-DMSO-osmium (A-O-D-O) method, and compared our findings with previous studies in the guinea pig. In cells of the spiral ganglion and the organ of Corti, microtubules and other cytoplasmic filaments were digested by OsO4 during processing to provide a clear view of membranous structures such as mitochondria, endoplasmic reticulum and Golgi apparatus. The appropriate digestion time with 0.1% OsO4 solution was found to be 60-80 hrs for the organ of Corti and 70-90 hrs for the spiral ganglion. The A-O-D-O method has been especially useful to show the organization of the endoplasmic reticulum of the outer hair cells. We have made a further characterization of the apical cistern, a reticular network which appears to link the basal body of the (vestigial) kinocilium to both Hensen's body and the subsurface cisternae.

Reorganization of auditory cortex after neonatal high frequency cochlear hearing loss

Cochleotopic representation in cortex (AI) is extensively reorganized in cats having neonatal, bilateral high frequency cochlear hearing loss. Anterior areas of AI, normally devoted to high frequencies, contain neurons which are almost all tuned to one lower frequency. This frequency corresponds, at the level of the cochlea, to the border between normal and damaged haircell regions.

Motion sickness. Helping patients tolerate the ups and downs

The sensory conflict theory provides a useful framework for understanding motion sickness. Few modifications of this model have been proposed since its publication. The weak link in the study of motion sickness has been the failure to delineate the anatomic structures that correspond to the model's components. Effective pharmacotherapy is available for treatment of motion sickness. The choice should be based on the need to provide prophylaxis for or treatment of symptoms and on the side effect profile of the drug.

Morphological changes to the cochlea in an animal model of profound deafness

We have used systemic application of the ototoxic drug amikacin, to induce total cochlear haircell loss in the chinchilla, in order to create an animal model of profound deafness. We have produced models of both neonatal deafness (by treatment of new-born pups), as well as of acquired total hearing loss (by treatment of mature animals). We present a description of our techniques for producing cochlear lesions and for evoked potential monitoring of the resulting threshold elevations. We also give a qualitative description of cochlear haircell damage as viewed by scanning electron microscopy, and a quantitative assessment of the spiral ganglion cell survival after total haircell loss.

Effects of total cochlear haircell loss on integrity of cochlear nucleus. A quantitative study

In cochleas of chincillas treated with amikacin, cochlear sensory cells were totally destroyed in all new-born animals. In animals treated as adults some occasional haircells remained in apical turns. In the neonatally treated animals, the resulting auditory deprivation significantly affected the volume of the ventral cochlear nucleus and large-dark spherical cell area. The density of large-dark spherical cells increased significantly from normal in both neonatally and adult treated groups. Our results suggest that the VCN is more dependent on auditory stimulation for proper development than the DCN. In adult chinchillas treated with amikacin there was a significant change in large-dark spherical cell density without a change in total cell numbers or large-dark spherical cell area volume. Our study indicates that the mature cochlear nucleus is much more resistant to the effects of auditory deprivation than the developing cochlear nucleus and that the maintenance of the mature auditory system is not as dependent on auditory stimulation. Studies such as this examining the morphological effects of profound cochlear deafness on higher levels of the auditory system are essential in cochlear implant research.

Cytoarchitecture of cochlear nucleus in the chinchilla

The morphology of the cochlear nucleus in the normal, adult chinchilla, as demonstrated by Nissl staining, was examined. The cytoarchitecture was determined from sections viewed at the light microscope level. The chinchilla cochlear nucleus was found to possess most of the features reported in other mammalian cochlear nuclei. It could easily be divided into dorsal and ventral components due to an intervening layer of granule cells, and most cell types previously reported in mammals were also found in the chinchilla cochlear nucleus. A distinct distribution pattern of cell types exists within each part.

The effects of long and short term profound deafness on the responses of inferior colliculus to electrical stimulation of the cochlea

It is widely assumed that adequate sensory stimulation is important for the proper post-natal development of the central nervous system, particularly during certain critical or sensitive periods in development. To examine the functional effects of long-term profound deafness on the development of the auditory system, the response properties of neurons in the inferior colliculus (IC) to electrical stimulation of the cochlea were investigated in chinchillas (i) deaf from birth and (ii) deafened as adults. In animals deafened as adults there was a 50% reduction in the number of neurons activated by electrical stimulation of the cochlea, compared with studies in normal controls. In contrast, very few or no responses could be elicited from IC of animals made deaf from birth. The results imply that lack of neural activity in the ascending auditory pathways from an early stage of development results in an inactivation of IC neurons. It is not clear if such inactivity is temporary or permanent.

Influence of acoustic deprivation on recovery of hair cells after acoustic trauma

The purpose of this study was to investigate how the recovery of the cochlea, after acoustic trauma, might be influenced by acoustic stimulation or deprivation. In anaesthetized adult chinchillas, both ears were simultaneously exposed to a traumatizing acoustic stimulus (2 kHz tone, at 117 dB SPL for 15 min). Probe microphones positioned in both bullae were used to ensure identical exposure to the two ears; this was important because the experiment relies on within-animal controls. Cochlear action potential thresholds across frequency (CAP audiograms) were used to verify the similarity of threshold shifts to the two ears. Immediately following, a unilateral ossiculectomy was performed which resulted in one cochlea being acoustically deprived during the recovery period, whilst the other was not. In groups of animals with recovery periods of 1, 3, 6, and 12 weeks, both the acoustically deprived and the normally stimulated cochleas were examined with scanning electron microscopy. To quantify hair cell damage, we used a damage scale based on stereociliar integrity; for each cochlea, a standard region 5.5-8.5 mm from the apex was studied in detail. We found that after acoustic trauma, hair cell damage to the cochlea which is deprived of sound during the recovery period, is significantly greater compared with that in the normally stimulated, contra-lateral cochlea. Our results suggest that mechanical activation of the inner ear acts to inhibit long-term degenerative processes, or influence repair of partially damaged hair cells.

The association between community physician's attendance at a medical center's CME courses and their patient referrals to the medical center

This study attempts to quantify an overall association between CME course attendance and referrals. Attendance at formal CME courses given by the University of Michigan Medical School and referrals to the University Hospitals were examined over a two-year period. Attendance and referrals were linked to physicians in Michigan identified through the Michigan Department of Licensing and Regulation and through the American Medical Association. For physicians who are office-based and likely to be in active practice (age less than 70), those who attended at least one of the University's CME courses referred more patients than those who did not attend one (means of 1.9 referrals per physician and 1.3 referrals per physician, p less than .001). The causal direction of the relationship is not clear, but probably operates in both directions. It is reasonable for medical center marketers to consider CME as an indirect method for marketing clinical services. It is also reasonable for CME directors to identify referring physicians as high-priority groups for marketing CME. Both marketing efforts may be significantly enhanced by linking data bases for referrals and for CME attendance. CME directors must also ensure that marketing efforts do not compromise the objectivity and integrity of the content of the institution's CME program.

Cochlear nerve action potentials during cerebellopontine angle surgery: relationship of latency, amplitude, and threshold measurements to hearing

The cochlear nerve action potential (AP) was monitored during two types of hearing preservation surgery: (1) removal of small cerebellopontine angle tumors and (2) retrolabyrinthine vestibular nerve section. The purpose of this investigation was to study the relationship between changes in intraoperative AP latency, amplitude, and threshold, and changes in audiometric hearing subsequent to surgery. The presence or absence of a cochlear nerve AP toward the end of surgery accurately predicted the presence or absence of postoperative hearing (93%). Regression analyses were performed to explore the relationship between audiometric pre- and postoperative thresholds and intraoperative AP threshold changes. Statistically significant relationships were found for (1) pre-operative audiometric thresholds and initial AP click thresholds (p = 0.0068); (2) final AP click thresholds and postoperative audiometric thresholds (p = 0.0003); (3) AP click (p = 0.0001) and 1 kHz (p = 0.0006) threshold shifts and pre- to postoperative audiometric threshold shifts. No relationship was found between either AP latency or amplitude values and postoperative hearing. Intraoperative threshold measurements can be used as an indicator of hearing change. If amplitude or latency indices are also monitored it may be better to use stimulus levels close to the evoked potential threshold.

Cochlear nerve monitoring during cerebellopontine angle operations

The authors present their experience with intraoperative monitoring of cochlear nerve action potentials (AP) in 30 adult patients. Operative procedures were acoustic neuroma excision with attempted hearing preservation and selective vestibular neurectomy in patients with incapacitating Meniere's disease and serviceable hearing (SRT less than 50 db, discrimination greater than 60%). Loss of AP is detected rapidly and has been demonstrated after manipulation of the cochlear nerve and after coagulation of small arteries on the tumour capsule. Presence of an AP at the end of the procedure usually correlates with postoperative preservation of hearing. AP monitoring appears to be a reliable means of detecting potentially reversible changes in cochlear nerve function intraoperatively.

An immunohistochemical and electron microscopic study of extra-renal basement membranes in dogs with Samoyed hereditary glomerulopathy

Samoyed hereditary glomerulopathy (SHG) in dogs has been employed as a model for human hereditary nephritis (HN), since affected dogs and patients show splitting of glomerular capillary basement membranes by electron microscopy (EM) and absent staining of glomerular capillaries for Goodpasture antigen (GPA) by immunofluorescence (IF). EM and IF were used to examine basement membranes (BM) in skin, lung, choroid plexus, lens, retina, and inner ear in SHG. By EM, BM in these tissues appeared similar in affected male, carrier female, and unaffected dogs. By IF, GPA could be detected only in lens capsule, internal limiting membrane of retina and basilar membrane of inner ear of unaffected and carrier female dogs, but not in affected male dogs. However, eye abnormalities and hearing loss were not present in any dogs, in contrast to their frequent occurrence in human HN. Our findings on extra-renal BM in SHG suggest that GPA is not required to maintain normal vision or hearing in affected male dogs and permit a greater understanding of the pathogenesis of human HN.

A study of the ototoxicity of deferoxamine in chinchilla

Deferoxamine is a chelating agent used for the treatment of chronic iron overload in patients requiring long-term blood transfusions. Audiological testing of patients with B-thalassemia major and steroid-unresponsive Diamond-Blackfan anemia who were on long-term deferoxamine treatment indicated a possible ototoxic side-effect. In the present study we have investigated this potential toxicity to the cochlea of experimental animals by monitoring electrophysiological responses to sound and also by histological evaluation of the cochlea. In animals having chronic deferoxamine treatment, there were no significant changes in cochlear function or morphology. Data from acutely treated animals indicated an elevation of cochlear response thresholds together with morphological changes at the inner hair-cell level. However, these changes were highly correlated with the respiratory depression caused by an acute general toxicity, rather than a direct ototoxic effect of deferoxamine.

Technical development of an implantable cochlear prosthesis in Canada

A new cochlear implant device is currently being developed by a group of research teams in Canada. This is an update on the progress of this development, including a brief description of the cochlear electrode array, the implantable stimulator, and the software development for the speech processor.

Psychometric properties of component and global measures of structured interview assessed type A behavior in a population sample

Structured interview (SI) assessments of global and component Type A behavior were conducted in a general population sample of 903 respondents. Correlations among all the measures were positive and significant. A factor analysis revealed that the common component variance was explained by a single underlying factor. Interrater reliabilities were approximately 0.5 to 0.7 for the individual components and 0.8 for both the SI global Type A and a second global measure that was assigned independently by assessors who did the component scoring. Prediction of the SI-assessed global measure from the components accounted for 56% of the variance, and prediction of the second global measure from the components accounted for about 83% of the variance. Implications of the psychometric properties of the SI component and global measures of Type A behavior for future research are discussed.

The effects of hypoxia on sensory cells of the cochlea in chinchilla

The effects of hypoxia on the sensory epithelium of the cochlea were investigated in the chinchilla. Systemic hypoxia was produced by increasing the dead space of the respiratory tidal volume. A disarrangement of hair-cell stereocilia, and cytoplasmic protrusions from sensory cells are the main findings in cochleas from hypoxic animals; these changes take place firstly at the inner hair-cells then, with the increase in degree of hypoxia, at the outer hair-cells. These degenerative changes of sensory cells correlate well with both respiratory suppression and with the elevation of auditory threshold to click stimulation as monitored using the compound action potential recording from the cochlear nerve. The latter measure appears to be a useful indicator of cochlear hypoxia. Our morphological findings are similar to other studies including those which have reported on post-mortem cochlear hair-cell degeneration. Our studies indicate the deleterious effects of long term hypoxia on cochlear mechanisms and point to the need for careful monitoring of cardiovascular and respiratory functions in animals under anaesthesia for physiological studies of the auditory system.