X-ray irradiation of the inner ear of the guinea pig. An electron microscopic study of the degenerating outer hair cells of the organ of Corti

Short-term Peripheral Auditory Effects of Cranial Irradiation: A Mouse Model

OBJECTIVES

Assess postcranial irradiation: (1) short-term threshold shift, (2) short-term peripheral auditory histopathology, and (3) the mouse as an experimental model.

METHODS

Adult mice were exposed to single-dose radiation of 10 to 60 Gy. Pre- and post-irradiation (baseline, 2-8 days) audiometric brainstem response data were recorded with analysis of cochlear ultrastructure.

RESULTS

Significant threshold shift occurred at all test frequencies in mice exposed to ≥20 Gy at 4 to 6 days post-irradiation. Ultrastructurally in Rosenthal's canal and the spiral lamina, neuronal density and extracellular matrix decreased dramatically. There was overall preservation of hair cells, stria vascularis, and vasculature. No difference within Gy group was noted in the frequency or severity of pathology along the length of the cochlea.

CONCLUSIONS

The initial impact of radiation in the first week post-exposure focuses on spiral ganglion cell bodies and peripheral projections, resulting in significant threshold shift for irradiation dosages≥20 Gy. This study demonstrates that the mouse is a viable model for study of short-term peripheral auditory effects using single-dose cranial irradiation. Additionally, with access to a precise animal irradiator, the mouse may be used as an experimental model for a fractionated irradiation dosage of 10 Gy, simulating stereotactic therapeutic cranial irradiation.

Osteoradionecrosis of the temporal bone: a case series

OBJECTIVE

To study osteoradionecrosis (ORN) of the temporal bone.

STUDY DESIGN

Retrospective case review.

SETTING

Academic medical center.

PATIENTS

Patients were included who had previously undergone radiation to the head and neck and then developed exposed necrotic bone within the ear canal that persisted at least 3 months.

INTERVENTIONS

Patients were treated with a variety of modalities, including conservative therapy with antibiotic ear drops and in-office debridements, hyperbaric oxygen therapy, and surgery.

MAIN OUTCOME MEASURES

To describe the presentation and management of patients with temporal bone osteoradionecrosis.

RESULTS

Thirty-three patients with temporal bone osteoradionecrosis were included. The most common site of primary tumor was the parotid gland (n = 11), followed by the nasopharynx (n = 7). The time to development of ORN varied between 1 and 22 years, with mean of 7.9 years. The mean radiation dose was 62.6 Gy to the primary tumor, 53.1 Gy to the affected temporal bone, and 65.2 Gy to the affected tympanic bone. The most common symptoms of ORN were otorrhea (n = 15), hearing loss (n = 13), and otalgia (n = 12). Fifteen patients had bacterial superinfection, most commonly Staphylococcus aureus (n = 9). Conservative therapy was successful at managing symptoms but not in eradicating exposed bone in most patients. Surgery was used for recalcitrant pain, infection, cholesteatoma, cranial neuropathies, and intracranial complications.

CONCLUSION

Osteoradionecrosis is a rare complication of radiation to the temporal bone. Management should be aimed at relief of symptoms, eradication of superinfection, and treatment of other commonly present radiation effects like cholesteatoma and hearing loss.

Safety and otoprotection of metformin in radiation-induced sensorineural hearing loss in the guinea pig

OBJECTIVE

There is currently no treatment available to prevent radiation-induced sensorineural hearing loss. Metformin has antineoplastic effects and is able to regulate the mitochondrial production of reactive oxygen species after cellular stress, which is one of the mechanisms involved in apoptosis after radiation damage. The objective of this study was to determine the safety and radioprotective properties of metformin against radiation-induced cochlear damage both in vitro and in vivo.

STUDY DESIGN

In vitro and prospective animal study.

SETTING

Animal Care Facilities of the Montreal Children's Hospital Research Institute.

METHODS

Cultured auditory hair cells (HEI-OC1) were exposed to different concentrations of metformin to determine its safety. Cells were incubated with different metformin concentrations and subjected to radiation. Cell viability after experiments was determined with the 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) assay. Sixteen guinea pigs were divided in 2 groups: drinking tap water (n = 8) and drinking water containing metformin (n = 8). The animals were unilaterally irradiated for 20 days (total dose 70 Gy), and the ears were divided in 4 groups: control (n = 8), irradiated (n = 8), metformin (n = 8), and experimental (n = 8). Auditory brainstem responses were assessed before and 1, 6, and 16 weeks after completion of radiotherapy.

RESULTS

Metformin was not cytotoxic or radioprotective in cultured auditory hair cells. Experimental ears had less hearing loss than radiated ones; however, differences were not statistically significant (P > .05).

CONCLUSION

Metformin is not ototoxic or radioprotective in vitro or in vivo. Ears solely subjected to metformin had better hearing thresholds than the rest of the groups.

Ototoxicity after combined platinum and fractionated radiation in a novel guinea pig model

PURPOSE

Cis-platinum and radiation in combination are current organ preservation treatment strategies for head and neck cancer. Their individual ototoxicity has been investigated, with recent demonstration of ototoxicity in clinical studies. Currently, no ototoxicity studies have been performed in animals receiving similar schedules of radiation or cis-platinum to those patients with head and neck cancer.

MATERIALS AND METHODS

In the present study, an animal model was developed to investigate the effects of combined modality therapy on hearing. Albino guinea pigs were given equivalent protocol dosages of cis-platinum (3 parenteral courses), fractionated radiation (25 fractions over 5 weeks), or both. Click and tone burst auditory brainstem response (ABR) measurements were performed before and 6 weeks after the completion of treatment.

RESULTS

Animals receiving radiation or cis-platinum and radiation experienced permanent significant ABR shifts at all frequencies, with 33% of the animals experiencing complete unilateral sensorineural hearing loss at 2 or more frequencies in the ear receiving the full radiation dose (7075 cGy over 25 fractions) (P < .05, paired t test analysis). The animals receiving 3 doses of cis-platinum had no significant ABR threshold shifts at 6 weeks. These data suggest that cis-platinum and radiation cause greater ototoxicity than cis-platinum alone. These findings correlate closely with sensorineural hearing loss in combined modality patients at our institution and in recent studies.

CONCLUSIONS

We conclude that the current animal results parallel those seen clinically and serve as a model for ototoxicity from combined modality therapies in future protocols.

Hearing loss after radiotherapy for pediatric brain tumors: effect of cochlear dose

PURPOSE

To determine the effect of cochlear dose on sensorineural hearing loss in pediatric patients with brain tumor treated by using conformal radiation therapy (CRT).

PATIENTS AND METHODS

We studied 78 pediatric patients (155 ears) with localized brain tumors treated in 1997-2001 who had not received platinum-based chemotherapy and were followed up for at least 48 months. They were evaluated prospectively by means of serial pure-tone audiograms (250 Hz-8 kHz) and/or auditory brainstem response before and every 6 months after CRT.

RESULTS

Hearing loss occurred in 14% (11 of 78) of patients and 11% (17 of 155) of cochleae, with onset most often at 3-5 years after CRT. The incidence of hearing loss was low for a cochlear mean dose of 30 Gy or less and increased at greater than 40-45 Gy. Risk was greater at high frequencies (6-8 kHz). In children who tested abnormal for hearing, average hearing thresholds increased from a less than 25 decibel (dB) hearing level (HL) at baseline to a mean of 46 +/- 13 (SD) dB HL for high frequencies, 41 +/- 7 dB HL for low frequencies, and 38 +/- 6 dB HL for intermediate frequencies.

CONCLUSIONS

Sensorineural hearing loss is a late effect of CRT. In the absence of other factors, including ototoxic chemotherapy, increase in cochlear dose correlates positively with hearing loss in pediatric patients with brain tumor. To minimize the risk of hearing loss for children treated with radiation therapy, a cumulative cochlear dose less than 35 Gy is recommended for patients planned to receive 54-59.4 Gy in 30-33 treatment fractions.

Changes in the audiograms of a nasopharyngeal cancer patient during the course of treatment: a temporal bone histopathological study

This report shows the changes that occurred in consecutive audiograms of a patient who underwent chemotherapy and radiotherapy for nasopharyngeal cancer and the histopathological examination of the temporal bones. Both conductive and sensorineural hearing loss developed, but followed different modes of progression. In the left ear, an air-bone gap appeared and deepened, while in the right ear, severe conductive hearing loss was present upon admission and improved after treatment. Sensorineural hearing loss was worse in the right ear, but deteriorated gradually in both ears. Histology revealed tumor invasion in the right temporal bone. Both middle ears showed effusion, but no radiation-induced changes that may be responsible for the conductive loss. The main changes in the cochlea were vascular stria degeneration, spiral ligament atrophy, and spiral ganglion cell depletion, while the hair cells were only occasionally missing. Apart from confirming what has been established previously by other authors, some interesting findings were observed: (1) in addition to the high frequency hearing loss typically caused by cis-platinum and by radiation, there was also low frequency hearing loss, and (2) the cochlear damage was most severe in the vascular stria and spiral ganglions, sparing the hair cells.

Functional and morphological effects of fotemustine on the auditory system of the rat

OBJECTIVE

This study aimed to elucidate the potential inner-ear effects of fotemustine, a chemotherapeutic agent which crosses the blood-brain barrier and is used in the treatment of primary and metastatic brain tumours and metastatic melanoma.

METHODS

This study utilised distortion product otoacoustic emissions and transmission electron microscopy in order to conduct electrophysiological and morphological assessments, using a rat experimental model. Twelve ears of six male rats were examined two months following intraperitoneal slow infusion of fotemustine (100 mg/m2 or 7.4 mg/kg). Pre- and post-treatment measurements were compared. Finally, electron microscopy was performed on three rat temporal bones.

RESULTS

After infusion of fotemustine, distortion product otoacoustic emissions revealed a significant reduction in signal-to-noise ratios only at 3600 Hz (from 11.95 +/- 7.52 to -0.26 +/- 9.45 dB) and at 3961 Hz (from 18.09 +/- 7.49 to 6.74 +/- 12.11 dB) (referenced to 2f1 - f2). Transmission electron microscopy of the temporal bone revealed ultrastructural changes in the outer hair cells, stria vascularis and cochlear ganglion at the cochlear basal turn. The ganglion cell perikarya were unaffected.

CONCLUSIONS

Fotemustine was administered via intraperitoneal slow infusion in a rat experimental model. Twelve ears of six survivors, from 10 rats, were evaluated at the second month. Fotemustine was determined to have a potential for ototoxicity at 3600 and 3961 Hz. Three randomly chosen rats underwent electron microscopy for morphological analysis. Morphological effects in the cochlear basal turn were observed. Oedematous intracytoplasmic spaces and perivascular areas of the stria vascularis, as well as distorted chromatin content, were detected, thereby suggesting potential ototoxic effects for this agent. Further experimental and clinical studies are required in order to determine whether the effect seen in this pilot study is reversible, and to analyse effects in humans.

Ultrastructural and electrophysiologic changes of rat cochlea after irradiation

OBJECTIVE/HYPOTHESIS

To determine the acute and subacute cochlear effects of sublethal total body irradiation.

STUDY DESIGN

The study was designed prospectively as a Sprague-Dawley (SD) rat model with otoacoustic emission measurement.

METHODS AND MATERIALS

Ten rats and 20 ears were included in the study. Distortion product otoacoustic emission (DPOAE) measurements were performed under ketamine anesthesia before and 1 to 8 weeks after sublethal dose total body irradiation. Eight and six rats survived at the first and eighth week measurements, respectively. The frequency bandwidth was set to 1,067 Hz (referenced to f1) - 9,512 Hz (referenced to f2) (2f1 - f2 = 832 - 6,076 Hz) and eight points were sampled per octave, but low-frequency (<2,549 Hz [referenced to 2f1 - f2]) results were not included in the analysis. Averages were obtained at each frequency, and statistical analysis was performed to compare the pre-and postradiation values. Then, three of the rats were killed for transient electron microscopy (TEM) of the temporal bones.

RESULTS

The posttreatment DPOAEs at almost all frequencies showed a trend toward decline at both the first and eighth weeks, but significant reduction was only detected at 3,600 and 3,961 Hz (referenced to 2f1 - f2) in the first week. TEM of the cochlea revealed ultrastructural changes in the outer hair cells (OHCs) and stria vascularis of the basal turn. The cochlear ganglion nuclei and cochlear nerve axons were unaffected.

CONCLUSION

Radiation can affect the cochlea in the acute phase at the mid to high frequencies; further prediction for long-term effects requires longer follow-ups.

Radiation tolerance of normal temporal bone structures: implications for gamma knife stereotactic radiosurgery

Popular current thought states that hearing loss and facial weakness after radiosurgery of vestibular schwannomas is a function of cranial nerve damage. Although this may be true in some cases, the middle and inner ear contain rich networks of other sensitive structures that are at risk after radiotherapy and that may contribute to toxicity afterward. We reviewed the limited reported data regarding radiation tolerance of external, middle, and inner ear structures, and perspectives for therapy with gamma knife stereotactic radiosurgery are addressed.

Radiation exposure of normal temporal bone structures during stereotactically guided gamma knife surgery for vestibular schwannomas

OBJECT

The dosimetry of radiation exposure of healthy inner, middle, and external ear structures that leads to hearing loss, tinnitus, facial weakness, dizziness, vertigo, and imbalance after gamma knife surgery (GKS) for vestibular schwannomas (VSs) is unknown. The authors quantified the dose of radiation received by these structures after GKS for VS to assess the likelihood that these doses contributed to postradiosurgery complications.

METHODS

A retrospective study was performed using a prospectively acquired database of a consecutive series of 54 patients with VS who were treated with GKS during a 3.5-year period at an "open unit" gamma knife center. Point doses were measured for 18 healthy temporal bone structures in each patient, with the anatomical position of each sampling point confirmed by a fellowship-trained neurootologist. These values were compared against single-dose equivalents for the 5-year tolerance dose for a 5% risk of complications and the 5-year tolerance dose for a 50% risk of complications, which were calculated using known 2-Gy/fraction thresholds for chronic otitis, chondromalacia, and osseous necrosis, as well as the tumor margin dose and typical tumor margin prescription doses for patients in whom hearing preservation was attempted. External and middle ear doses were uniformly low. The intratemporal facial nerve is susceptible to unintentionally high radiation exposure at the fundus of the internal auditory canal, with higher than tumor margin doses detected in 26% of cases. In the cochlea, the basal turn near the modiolus and its inferior portion are most susceptible, with doses greater than 12 Gy detected in 10.8 and 14.8% of cases. In the vestibular labyrinth, the ampulated ends of the lateral and posterior semicircular canals are most susceptible, with doses greater than 12 Gy detected in 7.4 and 5.1% of cases.

CONCLUSIONS

Doses delivered to middle and external ear structures are unlikely to contribute to post-GKS complications, but unexpectedly high doses may be delivered to sensitive areas of the intratemporal facial nerve and inner ear. Unintentional delivery of high doses to the stria vascularis, the sensory neuroepithelium of the inner ear organs and/or their ganglia, may play a role in the development of post-GKS tinnitus, hearing loss, dizziness, vertigo, and imbalance. Minimizing treatment complications post-GKS for VS requires precise dose planning conformality with the three-dimensional surface of the tumor.

Late radiation effects on hearing, vestibular function, and taste in brain tumor patients

PURPOSE

To investigate late radiation effects on hearing, vestibular function, and taste after conventional radiotherapy in brain tumor patients.

METHODS AND MATERIALS

Hearing, vestibular function, and taste were assessed in 33 brain tumor patients irradiated unilaterally to the tumor-bearing hemisphere and the temporal bone. Median observation time after completion of radiotherapy was 13 years; the fraction dose was 1.8 Gy, and mean radiation dose was 53.1 Gy.

RESULTS

Deep ulceration in the external ear canal and osteoradionecrosis on the irradiated side was seen in three patients. Reduced hearing was found for air and bone conduction of the irradiated side compared to the opposite side (0.25-2 kHz: 6.1 dB, 4 kHz: 10.3 dB, 6 kHz: 15.6 dB, and 8 kHz: 16.5 dB). For bone conduction, the corresponding figures were 0.25-2 kHz: 5.5 dB and 4 kHz: 8.2 dB. Three patients had a canal paresis of the irradiated side, and three patients had affection of the chorda tympani.

CONCLUSION

Irradiation of the temporal bone with doses usually given in the treatment of patients with brain tumors may cause osteoradionecrosis, sensorineural hearing loss, dysfunction of the vestibular inner ear, and loss of taste. Head-and-neck examination should be included in the follow-up of long-term survivors.

Histopathology of human temporal bone after cis-platinum, radiation, or both

Preserving organs by use of multiple modalities has become protocol in treating squamous cell carcinomas of the head and neck, but cis-platinum and radiation can impair hearing. To determine the effect of cis-platinum, radiation, or a combination of these treatments on the temporal bone, we studied histopathologic slides of 15 human temporal bones: four after cis-platinum, five after radiation, two after combined treatment, and four from normal controls. Hair cells and cells in spiral ganglia were counted in reconstructed organs of Corti. Lumen-to-diameter indexes in arterioles near facial nerves were quantified for four normal controls and seven irradiated patients. Available audiograms were compared. Decreased spiral ganglion cells, loss of inner and outer hair cells, and atrophy of stria vascularis were demonstrated in groups receiving cis-platinum, radiation, and combinations, compared with age-matched controls. Arterioles around facial nerves demonstrated fibrinous clots within the intima, endothelial proliferation, and hypertrophy and fibrosis of vascular walls in smooth muscle. Fibrosis in connective tissue was clearly progressive after radiation. Cis-platinum and radiation can contribute to otologic sequelae, including sensorineural hearing losses, vascular changes, serous effusion, or fibrosis. Prophylactic treatments and techniques to deliver them should be considered for protection of temporal bones and preservation of hearing after oncologic modalities.

Hearing disability before and after radiotherapy for nasopharyngeal carcinoma

This paper evaluates post-irradiation hearing changes in patients with nasopharyngeal carcinoma (NPC) from a disability orientated approach, which takes into account binaural hearing. Newly diagnosed patients with NPC were studied before radiotherapy, and at four to 12 months (mean 9.2 months) after radiotherapy, provided they remained disease-free. Each patient was examined clinically and with pure tone audiograms. Tympanometry was used to confirm middle ear effusion. Averaged hearing thresholds over 0.5, 1 and 2 kHz were evaluated. If abnormal (> 30 dB), the resultant hearing disability was illustrated by a modified Glasgow Plot. Twenty-three males and 10 females completed the study. Middle ear effusions resulted in 39.3 per cent (binaural in two patients) and 33.3 per cent (binaural in five patients) of patients having hearing disability pre- and post-irradiation respectively. No patient had hearing disability as a result of a sensorineural loss. It is recommended that future reporting of post-irradiation hearing changes in patients with NPC, as in middle ear surgery, be considered from a disability-orientated approach.

Influence of prenatal irradiation on second generation mice

Pregnant mice were irradiated with 0.5 and 2 Gy on the 13th gestational day. Offspring were raised until sexual maturity at 2 months of age. Mice from prenatally irradiated mothers and unexposed fathers, or from prenatally irradiated fathers and unexposed mothers, or from parents who had both been prenatally irradiated were mated, resulting in the birth of several mice. These second generation mice were tested as adults for hearing thresholds, with the ABR technique, subjectively to balance tests. and investigated morphologically. Hearing levels were found to be normal and the organ of Corti showed only minor changes of its inner and outer hair cells as shown by transmission and scanning electron microscopy. It is concluded that prenatal irradiation does not seem to cause mutations leading to impaired hearing and balance in second generation mice.

Ultrastructural changes in the cochlea of the guinea pig after fast neutron irradiation

Guinea pigs were irradiated with fast neutrons. After a single dose of 2, 6, 10, or 15 Gy was applied, scanning and transmission electron microscopy of the temporal bone was performed to assess the effect of fast neutron irradiation on the cochlea. Outer hair cell damage appeared with neutron irradiation of more than 10 Gy, and inner hair cell damage with neutron irradiation of more than 15 Gy. Outer hair cells were more severely damaged than inner hair cells. No statistically significant differences were found in damage of basal, middle, and apical turns. The second and third rows of outer hair cells were more severely damaged than the first row of outer hair cells. The most significant findings in transmission electron microscopy were clumping of chromatin and extension of the heterochromatin in the nuclei of hair cells. The cytoplasmic changes were sequestration of cytoplasm, various changes of mitochondria, formation of vacuoles, and irregularly arranged stereocilia. The morphologic change in stria vascularis was intercellular and perivascular fluid accumulation. It appeared to be a reversible process.

Structural changes in the middle ear tissues of the rat after fractionated irradiation

Chronic suppurative otitis media often ensues in patients treated with irradiation against a head and neck tumor. In an experimental study, rats were exposed to irradiation to evaluate the sensitivity of the middle ear to an accumulated irradiation dose of 20-45 Gy. Observed otomicroscopically, all animals appeared to have normal tympanic membranes and no fluid developed in the middle ear space. Ten days after the irradiation, minor structural changes had occurred in the pars flaccida. The keratinizing epithelium had thickened and mitoses were seen histologically. The lamina propria was edematous and contained polymorphonuclear cells and macrophages. The middle ear mucosa from all other tissue sites appeared normal. Six months after irradiation only minor changes in the pars flaccida were evident: the lamina propria was thin and inelastic and macrophages were present in the stroma. It is inferred from this study that the middle ear of the rat is relatively resistant to irradiation.

Postirradiation hearing loss

Hearing loss due to irradiation of the head-and-neck region is a rarely reported complication of such a treatment. Although experimental work had been performed in laboratory animals as early as at the turn of the century, substantiated clinical data in large series are lacking. The few reports published are somewhat contradictory as to the incidence, time of onset, type and severity of the hearing loss. Although infrequently encountered, the possibility of radiation-induced hearing loss should be kept in mind. The pertinent literature is reviewed.

Effects of X-ray irradiation on hearing in guinea pigs

Effects of X-ray irradiation on the hearing with CM and ABR to both air- and bone-conduction stimuli were studied continuously from immediately after the irradiation to a few weeks later. At 2, 4, and 6 krad, only slight conductive hearing impairment was found, but at 8 krad or more, CM and ABR disappeared abruptly, after having increased briefly. Advanced sensorineural hearing loss appeared at about 10 hours after irradiation ended at 8 krad, at about 6 hours at 10 krad, and at about 3 hours at 12 krad. More severe hearing impairment was observed at higher frequencies and the endocochlear d.c. potential (EP) decreased at about 7 hours after 10 krad irradiation. Histologically, no discernible changes in the hair cells or sensory hairs were found with a scanning electron microscope at about 6 hours after 10 krad irradiation, while with a transmission electron microscope, the outer hair cells in the basal coil of the cochlea were found to be mostly destroyed. However, no changes could be found in the inner hair cells or other supporting cells.

Changes in cochlear microphonic response after Y-ray irradiation of the inner ear of the guinea-pig

The effects of ionizing irradiation on the cochlear microphonic response of guinea-pigs were studied. The cochlear microphonics (CM) of both ears were recorded in a total of 36 animals. Recording was carried out by the differential electrode technique on the basal turn of the cochlea. One week prior to recording, the left ear of each animal had been exposed to 35-70 Gy radiation in increments of 5 Gy. Doses of 40 Gy and above led to a reduction in CM response. After doses of 60 Gy or more, no CM response could be detected. Damage most probably occurred in the outer and inner hair cells.

Malformations of vestibular organs following low dose gamma irradiation during embryonic development

Pregnant mice were whole-body irradiated on the 12th, 13th and 16th gestational day with single doses of 0.5, 1, 2, 3, 4, 10 and 20 Gy, using a 60Co source. No surviving litters occurred in groups exposed to more than 2 Gy. Below this dose, irradiation can cause malformation of the cristae ampullares and the maculae. Malformations were more severe and more frequent in fetuses exposed on the 12th than on the 13th gestational day, indicating that, prior to or the earlier during morphogenesis irradiation takes place, the more vulnerable is the developing tissue. Inner ears exposed to irradiation on the 16th gestational day did not reveal severe malformations.

Cis-diamminedichloroplatinum (II) ototoxicity in the guinea pig

Cochleas from 12 guinea pigs were evaluated using light, scanning, and transmission electron microscopy after systemic administration of cis-diamminedichloroplatinum (cis-DDP). Administration of cis-DDP resulted in loss of the Preyer reflex and degeneration of outer hair cells (OHC) with increased dose. The OHC degeneration was most pronounced in the basal turns of the cochlea with greatest severity in the inner row. Ultrastructural evidence of OHC degeneration included dilatation of the parietal membranes, softening of the cuticular plate, increased vacuolization and increased numbers of lysosome-like bodies in the apical portion of the cell. Supporting cells appeared more sensitive than OHC. Alteration of supporting cell ultrastructure preceded detectable change in OHC. Injury to the supporting cells was noted with intracellular vesiculation and increased autophagocytosis.

Temporal and spatial sequence of anterograde degeneration in the cochlear nerve fibers of the cat. A light microscopic study

This study deals with anterograde degeneration in the cochlear nerve fibers following cochlear lesions. The observations are based on 2-mum thick sections of material embedded in resin according to procedures used in electron microscopy and stained with toluidine blue. Among the various operative approaches used in this study, sparing of the modiolus afforded the least local reaction and furnished the material best suited for anterograde degneration studies in this nerve only 2 mm long. The anterograde degeneration of the cochlear nerve is characterized by segmental swelling of myelinated nerve fibers followed by shrinkage of the axoplasm and collapse of the distended myelin sheaths. The swelling, which begins at the nodal-paranodal region of the axon, is preceded by accumulation in the cytoplasm of granular organelles, presumably mitochondria and lysosomes. The portions of the cochlear fibers situated in the nerve root, i.e., within the cochlear nuclei and including the axon terminals, follow essentially the same pattern of degeneration as those in the peripheral portion of the nerve. Both peripherally and centrally degenerative changes occur first in the basal, high frequency fibers and centrally degenerative changes occur first in the basal, high frequency fibers and progress to the apical, low frequency fibers. The difference between the two extremes in the onset of degeneration is, approximately, 24 hours. Once initiated, however, the pace of degeneration is the same along the whole fiber spectrum.