Experimental study of potential adverse effects on the auditory system of rabbits exposed to short-term GSM-1800 radiation.

PURPOSE: The effects of the electromagnetic (EM) radiation emitted by a mobile phone on the central auditory system of rabbits are investigated in this paper. Auditory brainstem response (ABR) measurements were performed before and after short-term exposure to EM radiation. MATERIALS AND METHODS: Excitation was provided by a GSM-1800 emitter placed in contact with the (randomly selected) ear of the anesthetized rabbit/subject. The latency of waves I, II, III, IV, V and the interpeak latencies I-III, I-V, III-V were recorded, for both ears, before (baseline recordings) and after 1, 15, 30, 45 and 60 minutes of exposure to the EM radiation. The repeated measures one-way analysis of variance (ANOVA) followed by the post hoc Tukey test for pairwise comparisons was performed in order to decide about the significance of the results. RESULTS: The statistical tests indicated that, as regards the ear ipsilateral to the radiating module, the mean latencies of waves I, II, III, IV, V, I-III, I-IV after 60 min exposure, the mean latencies of waves I, III, IV, V, I-III, I-IV after 45 min exposure and the mean latencies of waves I, III, IV, V, I-IV after 30 min exposure, were significantly prolonged compared to the corresponding baseline values. Statistically significant differences were also found for certain peak and interpeak latencies for 60 min exposure as compared with the corresponding results for 1 min and 15 min exposure. No statistically significant delay was observed for the latencies before and after the exposure, for the ear contralateral to the radiation source. CONCLUSIONS: Although we found that more than 30 min exposure to GSM-1800 radiation resulted in prolongation of certain ABR components of rabbits, further investigation may be needed into the potential adverse effects on the auditory pathways.

Effects of exposure to 2100 MHz GSM-like radiofrequency electromagnetic field on auditory system of rats / Efeitos da exposição a um campo eletromagnético na radiofrequência de 2100 MHz, similar ao sistema GSM, no sistema auditivo de ratos

Abstract Introduction: The use of mobile phones has become widespread in recent years. Although beneficial from the communication viewpoint, the electromagnetic fields generated by mobile phones may cause unwanted biological changes in the human body. Objective: In this study, we aimed to evaluate the effects of 2100 MHz Global System for Mobile communication (GSM-like) electromagnetic field, generated by an electromagnetic fields generator, on the auditory system of rats by using electrophysiological, histopathologic and immunohistochemical methods. Methods: Fourteen adult Wistar albino rats were included in the study. The rats were divided randomly into two groups of seven rats each. The study group was exposed continuously for 30 days to a 2100 MHz electromagnetic fields with a signal level (power) of 5.4 dBm (3.47 mW) to simulate the talk mode on a mobile phone. The control group was not exposed to the aforementioned electromagnetic fields. After 30 days, the Auditory Brainstem Responses of both groups were recorded and the rats were sacrificed. The cochlear nuclei were evaluated by histopathologic and immunohistochemical methods. Results: The Auditory Brainstem Responses records of the two groups did not differ significantly. The histopathologic analysis showed increased degeneration signs in the study group (p = 0.007). In addition, immunohistochemical analysis revealed increased apoptotic index in the study group compared to that in the control group (p = 0.002). Conclusion: The results support that long-term exposure to a GSM-like 2100 MHz electromagnetic fields causes an increase in neuronal degeneration and apoptosis in the auditory system.

Resumo Introdução: O uso de telefones celulares tornou-se generalizado nos últimos anos. Embora benéfico do ponto de vista da comunicação, os campos eletromagnéticos gerados por celulares pode causar alterações biológicas indesejáveis no corpo humano. Objetivo: Nesse estudo, o objetivo foi avaliar os efeitos do campo eletromagnético na frequência de 2.100 MHz, similar à modulação do Sistema Global para Comunicações Móveis, produzido por um gerador de campo eletromagnético, sobre o sistema auditivo de ratos usando os métodos eletrofisiológico, histopatológico e imunohistoquímico. Método: Foram incluídos no estudo catorze adultos ratos albinos Wistar. Os ratos foram divididos aleatoriamente em dois grupos de sete animais cada. O grupo de estudo foi exposto continuamente por 30 dias a um campo eletromagnético em 2100 MHz com um nível de sinal (potência) de 5,4 dBm (3,47 miliwatts) para simular o modo de conversação em um celular. O grupo controle não foi exposto ao campo eletromagnético acima mencionado. Após 30 dias, o potencial evocado auditivo de tronco encefálico de ambos os grupos foi gravado e os ratos foram sacrificados. Os núcleos cocleares foram avaliados pelos métodos histopatológico e imunohistoquímico. Resultados: Os registros do potencial evocado auditivo de tronco encefálico dos dois grupos não diferiram significativamente. A análise histopatológica mostrou aumento dos sinais de degeneração no grupo de estudo (p = 0,007). Além disso, a análise imuno-histoquímica revelou aumento do índice de apoptose no grupo de estudo em comparação com o grupo controle (p = 0,002). Conclusão: Os resultados confirmam que a exposição a longo prazo a um campo eletromagnético em 2100 MHz similar à modulação do sistema global para comunicações móveis causa um aumento na degeneração neuronal e apoptose no sistema auditivo.

Modeling Intracochlear Magnetic Stimulation: A Finite-Element Analysis.

This study models induced electric fields, and their gradient, produced by pulsatile current stimulation of submillimeter inductors for cochlear implantation. Using finite-element analysis, the lower chamber of the cochlea, scala tympani, is modeled as a cylindrical structure filled with perilymph bounded by tissue, bone, and cochlear neural elements. Single inductors as well as an array of inductors are modeled. The coil strength (~100 nH) and excitation parameters (peak current of 1-5 A, voltages of 16-20 V) are based on a formative feasibility study conducted by our group. In that study, intracochlear micromagnetic stimulation achieved auditory activation as measured through the auditory brainstem response in a feline model. With respect to the finite element simulations, axial symmetry of the inductor geometry is exploited to improve computation time. It is verified that the inductor coil orientation greatly affects the strength of the induced electric field and thereby the ability to affect the transmembrane potential of nearby neural elements. Furthermore, upon comparing an array of micro-inductors with a typical multi-site electrode array, magnetically excited arrays retain greater focus in terms of the gradient of induced electric fields. Once combined with further in vivo analysis, this modeling study may enable further exploration of the mechanism of magnetically induced, and focused neural stimulation.

[The study on the targets of the optical evoked auditory brainstem response on the cochlea of guinea pig stimulating by infrared laser].

Objective: To identify the targets of the infrared laser stimulating on the cochlea of guinea pig which evoked auditory brainstem response (oABR), and explore the mechanisms of the infrared neurostimulation. Methods: A polished optical fiber with 200 µm diameter (NA=0.22) was planted into the scala tympani of guinea pigs to stimulate the cochlea of both the normal hearing and acute deafened guinea pigs. The direction of the fiber distal was changed to radiate different regions of the scala tympani, recording the oABR respectively. Differences of energy thresholds and amplitudes of oABR between normal hearing and acute deafened animals was concerned, and different responses were recorded as the optical path of laser fiber being changed to investigate the targets of the infrared laser stimulation. Immunofluorescence was used to detect the changes of inner and outer hair cells, and spiral ganglion neurons 7 days post-deafening, to looking for the probable association with the oABR changes at the same stimulus. SPSS 18.0 software was used to analyze the data. Results: Inner and outer hair cells were damaged in basal and middle turn, butresidual hair cells were observed in apical turn.Only when the optical fiber pointed to Rosenthal's canal stimulated the spiral ganglion region directly could the oABR be evoked. No response was recorded while the fiber pointed to other directions. Conclusion: Infrared laser stimulates cochlea evoked oABR generats from the response of spiral ganglion directly, the spiral ganglion neurons are the target of infrared stimulation.

The effect of radiotherapy on gentamicin ototoxicity: an animal model.

OBJECTIVE: Patients undergoing radiotherapy (RT) often present with serious bacterial infections requiring the use of antibiotic treatment. Gentamicin is a commonly used aminoglycoside antibiotic, whose ototoxicity remains a major problem in clinical use. The objective of this study was to determine whether radiation exposure can influence gentamicin-induced ototoxicity. STUDY DESIGN: Prospective animal study. SETTING: Animal care facilities of the Montreal Children's Hospital Research Institute. METHODS: Sixteen guinea pigs received low-dose RT unilaterally for 4 weeks (total: 48 Gy). Animals then received low or high doses of gentamicin (40 mg/kg/d and 80 mg/kg/d) for 10 days. The ears were divided into 4 groups: gentamicin 40 mg, gentamicin 80 mg, gentamicin 40 mg + RT, and gentamicin 80 + RT. Auditory brainstem responses and distortion products otoacoustic emissions were assessed at baseline and before and after gentamicin treatment. Cochlear morphology using light and scanning electron microscopy were evaluated. RESULTS: High-dose gentamicin caused significant auditory brainstem response threshold shifts (P = .020), with greater hearing loss in the irradiated ear (difference of 23.6 + 7.5 dB). All animals exposed to high-dose gentamicin had head tilts toward the radiated side. Cochlear morphology revealed the greatest hair cell damage in the gentamicin 80 + RT group followed by gentamicin 80. CONCLUSION: Results suggest that radiation can exacerbate the ototoxicity of gentamicin at high doses.

Alteration of glycine receptor immunoreactivity in the auditory brainstem of mice following three months of exposure to radiofrequency radiation at SAR 4.0 W/kg.

The increasing use of mobile communication has triggered an interest in its possible effects on the regulation of neurotransmitter signals. Due to the close proximity of mobile phones to hearing-related brain regions during usage, its use may lead to a decrease in the ability to segregate sounds, leading to serious auditory dysfunction caused by the prolonged exposure to radiofrequency (RF) radiation. The interplay among auditory processing, excitation and inhibitory molecule interactions plays a major role in auditory function. In particular, inhibitory molecules, such a glycine, are predominantly localized in the auditory brainstem. However, the effects of exposure to RF radiation on auditory function have not been reported to date. Thus, the aim of the present study was to investigate the effects of exposure to RF radiation on glycine receptor (GlyR) immunoreactivity (IR) in the auditory brainstem region at 835 MHz with a specific absorption rate of 4.0 W/kg for three months using free-floating immunohistochemistry. Compared with the sham control (SC) group, a significant loss of staining intensity of neuropils and cells in the different subdivisions of the auditory brainstem regions was observed in the mice exposed to RF radiation (E4 group). A decrease in the number of GlyR immunoreactive cells was also noted in the cochlear nuclear complex [anteroventral cochlear nucleus (AVCN), 31.09%; dorsal cochlear nucleus (DCN), 14.08%; posteroventral cochlear nucleus (PVCN), 32.79%] and the superior olivary complex (SOC) [lateral superior olivary nucleus (LSO), 36.85%; superior paraolivary nucleus (SPN), 24.33%, medial superior olivary nucleus (MSO), 23.23%; medial nucleus of the trapezoid body (MNTB), 10.15%] of the mice in the E4 group. Auditory brainstem response (ABR) analysis also revealed a significant threshold elevation of in the exposed (E4) group, which may be associated with auditory dysfunction. The present study suggests that the auditory brainstem region is susceptible to chronic exposure to RF radiation, which may affect the function of the central auditory system.

The relevance of dosimetry in animal models of cochlear irradiation.

OBJECTIVE: To compare dose measurements of three different irradiation setups for an animal model of unilateral cochlear irradiation using radiochromic films positioned at the cochlear plane. A method of dosimetry is proposed. METHODS: Radiation field simulation was performed to locate the cochlear plane for irradiation experiments using CT scan images. Fifteen film pieces were irradiated at the cochlear plane with 3 different irradiation field sizes. A 12-mm diameter field (n = 5), 5.7 mm diameter field (n = 5), and a 6.5 × 7.2 mm field (n = 5). After obtaining an ideal irradiation field size, 15 film pieces were used to compare dosimetry between tissue substitute materials (PVC n = 5 and PVC + Teflon, 5 each) and real tissue (frozen animal, n = 5). Auditory brainstem responses at 3 frequencies (8, 16, 20, and 25 kHz) were performed on 7 guinea pigs after a cycle of fractionated unilateral irradiation. RESULTS: Dosimetry in real tissue demonstrated an asymmetric dose distribution at the cochlear plane and ultimately a lower dose deposition (30%) when compared with tissue substitute materials. Auditory brainstem responses of ears subjected to radiotherapy demonstrated progressive hearing loss in long-term assessment. CONCLUSION: Asymmetric dose deposition at the cochlear plane highlights the need of comprehensive real tissue dosimetry in animal studies of cochlear irradiation. To avoid misleading discrepancies in dose-deposition between different studies using the same animal model, appropriate planning and confirmatory dosimetry systems are highly desirable.

Auditory brainstem response changes during exposure to GSM-900 radiation: an experimental study.

The objective of the present study was to investigate the possible electrophysiological time-related changes in auditory pathway during mobile phone electromagnetic field exposure. Thirty healthy rabbits were enrolled in an experimental study of exposure to GSM-900 radiation for 60 min and auditory brainstem responses (ABRs) were recorded at regular time-intervals during exposure. The study subjects were radiated via an adjustable power and frequency radio transmitter for GSM-900 mobile phone emission simulation, designed and manufactured according to the needs of the experiment. The mean absolute latency of waves III-V showed a statistically significant delay (p < 0.05) after 60, 45 and 15 min of exposure to electromagnetic radiation of 900 MHz, respectively. Interwave latency I-III was found to be prolonged after 60 min of radiation exposure in correspondence to wave III absolute latency delay. Interwave latencies I-V and III-V were found with a statistically significant delay (p < 0.05) after 30 min of radiation. No statistically significant delay was found for the same ABR parameters in recordings from the ear contralateral to the radiation source at 60 min radiation exposure compared with baseline ABR. The ABR measurements returned to baseline recordings 24 h after the exposure to electromagnetic radiation of 900 MHz. The prolongation of interval latencies I-V and III-V indicates that exposure to electromagnetic fields emitted by mobile phone can affect the normal electrophysiological activity of the auditory system, and these findings fit the pattern of general responses to a stressor.

The Effect of CO(2) and KTP laser on the cat saccule and utricle.

OBJECTIVES/HYPOTHESIS: To assess the potential carbon dioxide (CO(2)) and potassium-titanyl-phosphate (KTP) laser-related trauma to the saccule and utricle in a cat model. STUDY DESIGN: Basic science experiment utilizing cat model. METHODS: Twelve adult male cats were divided into two groups-CO(2) and KTP-to assess the potential saccule and/or utricle trauma with direct discharge of laser energy into the vestibule after the stapes was removed. Both groups were subdivided to assess the effects with acute sacrifice and three-month survival. Bone conduction auditory brain-stem response thresholds were used to monitor auditory function. Clinical observation was used to monitor vestibular function. The temporal bones were harvested, processed, and stained with hematoxylin and eosin (H&E) in all animals with the uninvolved side serving as the control. RESULTS: None of the animals demonstrated changes in bone conduction auditory brain-stem responses. None of the animals in the survival group demonstrated clinical vestibular dysfunction. Saccular and utricular wall rupture was observed in all animals sacrificed acutely. None of the saccular and utricular wall ruptures were of a size and location that could be attributed to laser trauma, and none of the saccular and utricular wall ruptures were associated with neuroepithelial trauma. CONCLUSIONS: There is no evidence of a difference between the CO(2) and KTP laser in potential laser-related trauma. Using bone-conducting auditory brain-stem response threshold and clinical monitoring of vestibular function, there was no evidence of clinical auditory or vestibular dysfunction. The histologic evidence of saccular and utricular wall rupture is more consistent with stapes extraction trauma than laser-related trauma.

Glial cell line-derived neurotrophic factor and antioxidants preserve the electrical responsiveness of the spiral ganglion neurons after experimentally induced deafness.

Cochlear implant surgery is currently the therapy of choice for profoundly deaf patients. However, the functionality of cochlear implants depends on the integrity of the auditory spiral ganglion neurons. This study assesses the combined efficacy of two classes of agents found effective in preventing degeneration of the auditory nerve following deafness, neurotrophic factors, and antioxidants. Guinea pigs were deafened and treated for 4 weeks with either local administration of GDNF or a combination of GDNF and systemic injections of the antioxidants ascorbic acid and Trolox. The density of surviving spiral ganglion cells was significantly enhanced and the thresholds for eliciting an electrically evoked brain stem response were significantly reduced in GDNF treated animals compared to deafened-untreated. The addition of antioxidants significantly enhanced the evoked responsiveness over that observed with GDNF alone. The results suggest multiple sites of intervention in the rescue of these cells from deafferentation-induced cell death.

Enriched acoustic environment after noise trauma abolishes neural signs of tinnitus.

Noise-induced hearing loss induces reorganization of the tonotopic map in cat auditory cortex and increases spontaneous firing rate and neural synchrony. We showed previously that keeping cats after noise trauma in an acoustic environment enriched in high frequencies prevents tonotopic map reorganization. Here, we show the effects of low-frequency and high-frequency enriched acoustic environments on spontaneous firing rate and neural synchrony. Exposed cats placed in the quiet environment and in the low-frequency enriched acoustic environment showed increased spontaneous firing rate and synchrony of firing. In contrast, exposed cats placed in the high-frequency enriched acoustic environment did not show significant differences in spontaneous firing rate or synchrony compared with normal hearing controls. This is interpreted as an absence of putative neural signs of tinnitus.

A study of the effects of cellular telephone microwave radiation on the auditory system in healthy men.

We conducted a study of the effects of mobile cellular telephone microwave radiation on the auditory system in 20 healthy men. After the subjects underwent baseline measurements of transient evoked otoacoustic emission (TEOAE) and auditory brainstem response (ABR), they participated in three sessions of exposure to an electromagnetic field of 900 to 1,800 MHz produced by a cellular phone. Sessions ranged from 15 to 30 minutes in length. TEOAE and ABR were again measured after or during each exposure. Throughout the study, no significant changes in either measurement were noted. We conclude that the use of cellular phones does not alter the auditory system in the short-term.

Cell transplantation to the auditory nerve and cochlear duct.

We have developed a technique to deliver cells to the inner ear without injuring the membranes that seal the endolymphatic and perilymphatic chambers. The integrity of these membranes is essential for normal hearing, and the technique should significantly reduce surgical trauma during cell transplantation. Embryonic stem cells transplanted at the internal auditory meatal portion of an atrophic auditory nerve migrated extensively along it. Four-five weeks after transplantation, the cells were found not only throughout the auditory nerve, but also in Rosenthal's canal and the scala media, the most distal portion of the auditory nervous system where the hair cells reside. Migration of the transplanted cells was more extensive following damage to the auditory nerve. In the undamaged nerve, migration was more limited, but the cells showed more signs of neuronal differentiation. This highlights an important balance between tissue damage and the potential for repair.

Neural stem cells suppress the hearing threshold shift caused by cochlear ischemia.

Neural stem cells are multipotent progenitor cells that show self-renewal activity. In this study, we assessed the use of neural stem cells for ameliorating ischemia-reperfusion injury of the gerbil cochlea. Neural stem cells were injected into one inner ear through the round window 1 day after ischemic insult. Immunostaining for nestin showed that the distribution of neural stem cells was concentrated within the organ of Corti. Seven days after ischemia, the injury-induced auditory brainstem response threshold shift and progressive inner hair cell damage were markedly less on the neural stem cell-transplanted side. These results suggest that the transplantation of neural stem cells is therapeutically useful for preventing damage to hair cells that occurs after transient ischemia of the cochlea.

Radiation tolerance of the cochlear nerve at the gamma-knife in rabbits.

Since the first treatment of acoustic neurinoma using the gamma-knife by Leksell, a series of cases have been reported with good control rates. However, the most frequent complication is delayed hearing loss which occurs in more than 50% of patients. The purpose of this study was to define a safe dose by analyzing the radiosurgical dose-response relationship and histological effects on the normal cochlear nerve in rabbit. The rabbits had computed tomography (CT)-guided stereotactic radiosurgery on their cochlear nerves in the internal auditory canal with a 4 mm collimator focusing of a gamma-unit. Maximum doses of 10, 20, 30, 40, 60, 80, 100, 200 and 500 Gy were administered. After the radiosurgery, auditory brain stem responses (ABR) and the behavior of the rabbits were evaluated periodically. At the conclusion, histological investigations were performed. No physiological or histological findings were observed from doses of 30 Gy or below during the 12 month period after the radiosurgery. A dose of 100 Gy caused a severe ABR threshold elevation, vestibular dysfunction and facial palsy. Necrosis and demyelination of nerves were observed pathologically. In this study, we determined that the safe dose to the normal cochlear nerve during radiosurgery was under 40 Gy in rabbits, and complications seemed to vary due to individual differences in radiation tolerance.

Auditory brainstem responses after radiotherapy for nasopharyngeal carcinoma.

The effect of irradiation for nasopharyngeal carcinoma on auditory brainstem responses and hearing was investigated in 19 otologically normal patients undergoing standard fractionated megavoltage radiotherapy. Auditory brainstem responses and pure tone audiometry were performed before radiotherapy, and at 3 and 12 months after completion of radiotherapy. There were no significant changes in the wave I-III and III-V interpeak intervals, or in sensorineural hearing thresholds (bone conduction at 4 kHz and average of bone conduction at 0.5, 1, 2 and 4 kHz), after radiotherapy. In contrast to previous studies, we found no evoked potential evidence of subclinical brainstem damage arising from irradiation for nasopharyngeal carcinoma.

Morphology of brain stem lesion and bera findings after 60Co irradiation.

Ionizing rays, utilized in radiological diagnostics and oncological therapy affect the central nervous system and may injure auditory pathways and cause hearing disturbances which vary in intensity. On the basis of a stereotactic atlas of the brain of a guinea pig, the trapezoid and geniculate bodies were identified in the skull X-ray pictures. The irradiated region was found to have 10 x 6 x 5 mm in dimensions and to be situated at the depth of 11 mm away from the animals occiput. After introductory recording of the potentials obtained from the brain stems (BERA), the stems of 60 guinea pigs were irradiated in the groups with the doses of 5, 10, 20 Gy. The hearing potentials from the brain stems were recorded on the 1st, 4th, 10th, 21st and 84th day after irradiation. The findings of measurements of hearing potentials were compared with the morphologic picture of the brain stems in the examined animals.

Potenciales evocados auditivos de tallo cerebral en rana catesbeiana / Brainstem auditory evoked responses on rana catesbeiana

Se estudió la sensibilidad de la vía auditiva a través del registro de los potenciales auditivos de tallo cerebral en 10 anfibios sanos de la especie Rana catesbiana. Las respuestas auditivas se efectuaron a 70, 50, 40 y 30 dB NA, en dos grupos de diferente peso. El primero de 17 a 27 gr y el otro grupo de 36 a 86.5 gr, los electrodos fueron insertados subcutáneamente y la estimulación fue por clicks en campo libre dentro de una cámara sonoamortiguada. A 70 dB las respuestas fueron de dos ondas en los primeros milisegundos, a 50 dB la onda II se separó en dos subcomponentes (IIa y IIb). El umbral electrofisiológico se estableció en 40 dB

Effect of irradiation on guinea pig ABR thresholds.

A significant number of patients undergo irradiation to the temporal bone for malignancies. Conflicting reports exist regarding the effects of irradiation on hearing thresholds. Although radiation-induced otitis media and osteoradionecrosis of the ossicles with resultant conductive hearing loss are well-documented, there is disagreement regarding the effect of irradiation on sensorineural hearing. Previous animal models, relying only on behavioral tests and reflex thresholds, have failed to reveal consistent threshold shifts after irradiation. However, with the advent of auditory brainstem response (ABR) testing, a reliable objective measurement of hearing in animals is available. Hearing thresholds were determined bilaterally by ABR testing in 21 albino guinea pigs. The left temporal bones of sixteen animals were then irradiated with a total dose ranging from 5750 to 7000 cGy over 7 weeks. The right ears of these animals, plus both ears of five nonirradiated guinea pigs, served as controls. Follow-up threshold ABRs were obtained immediately post-irradiation (RT), and at 6 and 12 months post-RT. Average thresholds in all groups increased over time: 60 dB in the control group; 53 dB in the control ears of the irradiated animals; and 46 dB in the irradiated ears. There were no statistically significant increases in ABR thresholds for irradiated ears vs. control ears. At the 6-month followup, hearing was actually better in the irradiated ears than the control ears and this difference between ears was significantly greater than the difference at baseline (p < 0.026). Overall, there was no evidence that irradiation produces changes in ABR thresholds.

Auditory brain stem responses in patients after radiation therapy for nasopharyngeal carcinoma.

BACKGROUND: The study evaluated the incidence and severity of brain stem myelopathy occurring after radiation exposure in a cohort of patients who received external radiation exposure for nasopharyngeal carcinoma (NPC). METHODS: Brain stem function was investigated by auditory brain stem responses (ABR). RESULTS: Four of 21 patients who could be examined had aberrations in ABR. Three patients showed highly abnormal ABR, with no distinctive patterns or peaks. Two of these patients also showed clinical symptoms of brain stem dysfunction, including multiple palsies in cranial and peripheral nerves, whereas the third patient had no clinical signs of brain stem disorders. The fourth patient had minor conduction delays in ABR. The remaining group of 17 patients who could be examined had ABR latency and transmission times similar to those of the control group. None of these patients had neurologic symptoms. Dose-response analysis showed that patients who received radiation doses of 59 Gy or less to the brain stem had normal ABR, whereas four of six patients who received a dose of 68 Gy had manifest or subclinical brain stem dysfunction. CONCLUSIONS: The results emphasize the importance of protecting the brain stem from high-dose radiation when possible. The results also demonstrate the usefulness of ABR as a supplement to the clinical examination of patients with possible myelopathy occurring after radiation exposure.