Detection of tobramycin- and netilmicin-induced ototoxicity in guinea pigs with evoked action potentials

Hearing threshold measurements of five stranded short-finned pilot whales (Globicephala macrorhynchus)

On May 5, 2011, 23 short-finned pilot whales, Globicephala macrorhynchus, were stranded along the coastline near Cudjoe Key, FL. Five animals (two adult females, two juvenile females, and an adult male) were transported to a rehabilitation facility in Key Largo, FL. Auditory evoked potentials (AEPs) were recorded in response to amplitude modulated tone pips modulated at 1000 Hz. AEP thresholds were determined at 10, 20, 40, 80, and 120 kHz for the four females. However, the adult male was euthanized prior to testing. Short-finned pilot whales had peak sensitivity at lower frequencies than other odontocetes such as bottlenose dolphins. Greatest sensitivity was around 40 kHz for all whales, while thresholds for the two adult females were 25-61 dB higher at 80 kHz than the juveniles. Click evoked potentials were similar between the four whales and comparable to other echolocating odontocetes. Click evoked potential data from a fifth short-finned pilot whale that had stranded in Curacao showed no response. These findings add to the limited database of pilot whale (short- and long-finned) hearing studies, of which there are only two others [Schlundt et al. (2011). J. Acoust. Soc. Am. 129, 1111-1116 and Pacini et al. (2010). J. Exp. Biol. 213, 3138-3143].

Audiogram of a formerly stranded long-finned pilot whale (Globicephala melas) measured using auditory evoked potentials

Long-finned pilot whales are highly social odontocetes found in temperate and subpolar regions. This species is particularly known for its interaction with fisheries as well as its mass strandings. Recent tagging work has provided some information about pilot whales in the wild but, even though they have been successfully kept in captivity, little is known about their sensory capabilities. This study investigates the hearing abilities of a rehabilitated 2 year old male long-finned pilot whale. A complete audiogram was collected using auditory evoked potential techniques that included measurements of nine frequencies from 4 to 100 kHz presented as sinusoidally amplitude-modulated tones. The results indicated that the region of best hearing was between 11.2 and 50 kHz and the subject had relatively poor high frequency hearing compared with other odontocete species. This study emphasizes the importance of collecting basic hearing measurements from new species, understanding diagnostic life histories as well as continuously increasing the sample size of audiometry measurements within and between odontocete species as animals become available.

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.

Cochlear permeability of neomycin and gentamicin: an immunohistochemical study

Aminoglycoside antibiotics (AGAs) target specifically the cochleo-vestibular hair cells, but with varied ototoxicity. Differences in their penetration and clearance rates into the membranous labyrinth may play a role. This in turn may be related to a difference in the number of amine groups, the cationic nature, as well as the molecular weight and size of the AGA molecule. Immunohistochemical labeling techniques were used to study the pathways of gentamicin and neomycin from the perilymph into cochlear tissues and target cells. The more cochleotoxic AGA, neomycin, penetrated into cochlear tissues faster than the less cochleotoxic AGA, gentamicin.

Auditory brainstem response in young burn-wound patients treated with ototoxic drugs

Burn-wound patients often require potentially ototoxic doses of aminoglycoside drugs in the treatment of gram-negative sepsis. Cochlear hearing impairment may be an unfortunate consequence of this medical therapy. We evaluated auditory sensitivity with the auditory brainstem response (ABR) in a group of 32 children with acute, severe thermal burns ranging in age from 18 months to 17 years. The mean percent of total body surface area burns was 64%. None of the subjects had a known history of hearing deficits or aminoglycoside therapy, and all yielded a normal baseline ABR upon hospital admission. Eight of the subjects (22%) showed either an abnormal ABR, or no response, at 40 dB prior to hospital discharge. The medical treatment for this group of subjects (gentamicin, amikacin, vancomycin, amphotericin B) was compared to that of a second subgroup of 7 subjects without auditory deficit but with a statistically comparable percentage of burns. The mean dosage of vancomycin was higher for the auditory impairment group than for the unimpaired group. Prediction of ototoxicity in the acute burned patient is extremely difficult as there are numerous factors that may influence the risk of cochlear damage. We conclude, however, that the ABR can be applied in early detection of auditory deficit. Follow-up audiometric assessment is advisable since auditory deficits in this population may be delayed or progressive after discontinuance of drug therapy.

Alterations of auditory evoked potentials during the course of chloroquine treatment

Chloroquine phosphate is now used extensively in the treatment of active rheumatoid arthritis and in some collagen diseases. The ototoxic potential of this drug during long term use was assessed in 74 patients. At a time when pure tone and speech audiometry failed to demonstrate any hearing change, brainstem audiometry became abnormal in 13 patients. Abnormalities disappeared if chloroquine was withdrawn. Pathogenesis will be discussed on the basis of data obtained from animal experiments.

Aspects on the ototoxic potential of netilmicin

The organ culture system using the 16th gestational day inner ear explant from fetal mice was used for ototoxicity screening of netilmicin. Organ cultures were incubated in 1, 10, 100 and 1000 micrograms/ml, respectively, of netilmicin base in the nutrient solution. After constant exposure to the drug during 5 days, the inner ears (N = 56) were analyzed morphologically at the light and electron microscopic levels. All specimens were serially sectioned. Exposure to 1 and 10 micrograms/ml of netilmicin base, doses that are reached during in vivo conditions in man, did not cause morphological damage to hair cells as observed in the light microscope at the end of the 5-day period. At the ultrastructural level, minor changes characteristic for aminoglycosides occurred in hair cell mitochondria. Incubation with 100 and 1000 micrograms/ml of netilmicin base caused morphological changes in many types of epithelia and not only in vestibular hair cells. This indicates a general sign of toxicity of netilmicin in these doses. The dose response relationship with regard to inner ear pathology was obvious in the vestibular part of the labyrinth. In the cochlea, however, few cytological changes occurred independent of dose. As compared with gentamicin, exposure in the same concentrations as in the present study and under similar experimental conditions, the less ototoxic effect of netilmicin was obvious. The results of the present study indicate that the less ototoxic potential of netilmicin as compared with other aminoglycosides is probably due to the drug itself and not primarily due to a poor penetration of netilmicin into the inner ear.

Aminoglycoside ototoxicity in the chick (Gallus domesticus) inner ear: I. The effects of kanamycin and netilmicin on the basilar papilla

A single dose (100 mg/kg of egg weight) of kanamycin or netilmicin was injected into the yolk sacs of 7-day-old chick (Gallus domesticus) embryos. Embryos were collected every 24 hours and processed for light and transmission electron microscopy. Morphologic study of the medial basilar papilla disclosed that both kanamycin and netilmicin are toxic to the hair cells in this region. Intoxication was manifested by an increased number of dense osmiophilic bodies, swollen mitochondria, agglomerated chromatin, and occasional disorganization of the kinocilium basal bodies. The cytologic changes observed in the hair cells of embryos injected with netilmicin and kanamycin were similar. However, mitochondrial damage was more severe in the chicks after kanamycin than after netilmicin injection. Some of the cytologic alterations described here are comparable to those already reported for aminoglycoside-intoxicated hair cells in several mammalian species. This study and previous work indicate that the chick embryo provides a satisfactory developmental model for testing ototoxicity of drugs in vivo.

Structure side-effect sorting of drugs. VI. Ototoxicities

From a literature survey, over 130 (about 7.8%) drugs and chemicals have been associated with ototoxicities. The major classes are basic aminoglycoside and other antibiotics, anti-inflammatory drugs, antimalarials, beta-blockers, antineoplastic agents, heavy metals, diuretics, some topical agents and various miscellaneous drugs. Possible mechanisms of action are presented and discussed. These include inhibition of protein synthesis, the glycolytic cycle, the TCA cycle, energy utilization, energy generation and the respiratory system within the mitochondria membrane of the hair cell, and also alteration of the permeability of the endolymphatic membrane or alteration of the excretion system for the basic aminoglycosides in the lateral wall of the membranous cochlea. The relative rank order of ototoxicity and reactivity toward mucopolysaccharides of five aminoglycosides is found to be related to the number of basic groups in each molecule.

Netilmicin (Netromycin, Schering-Plough)

Netilmicin sulfate, the 1-N-ethyl derivative of sisomicin, is a new aminoglycoside recently released for use in Canada and not yet released in the U.S. Its place in therapeutics, compared with gentamicin (G), tobramycin (T), and amikacin (A), is not yet established. Preliminary work in animals has suggested a lower incidence of nephrotoxicity and ototoxicity than with other aminoglycosides, and in vitro work has suggested some activity against G/T-resistant organisms. However, netilmicin appears to be virtually identical to G,T, and A in antimicrobial spectrum (except for its poorer activity against P. aeruginosa), human toxicity, and clinical use. For G/T-resistant organisms, amikacin is still the aminoglycoside of choice. In summary, netilmicin has not been demonstrated to have significant advantages over other aminoglycosides (G,T,A), and it is more expensive; thus, its potential value is limited.

The influence of ototoxic drugs on brainstem auditory evoked potentials in man

To examine the short- and long-term consequences of ototoxic drug administration, the brainstem auditory evoked potentials (BAEP) have been recorded in patients undergoing treatment with aminoglycosides antibiotics. It appears that the rapidity of the i.v. injection influences the short-term consequences of the drug administration. The long-term consequences may be reversible. Their reversibility could depend on the status of the ear before the first injection of ototoxic drug. In conclusion, BAEP may be useful as a means of atraumatic monitoring of the auditory function of patients treated with aminoglycosides antibiotics, the non-invasive character of this technique allowing it to be repeatedly used in both adult and young patients.

Altered objective audiometry in aminoglycosides-treated human neonates

To detect the eventual ototoxicity of aminoglycoside treatments in neonates, we compared brain stem response audiometry (BSRA) recordings in 15 neonates treated i.m. with either Gentamicin or Tobramycin at conventional dosages to those of 14 neonates used as a control group. All babies were housed in incubators and were comparable in gestational age (from 29 to 42 weeks). At day 0, BSRA did not significantly differ in the two groups, 90 dB latencies of the prominent wave V measured at 8.51 +/- 0.99 ms in the treated babies and 7.89 +/- 0.84 ms in the controls (p > 0.10), respectively. After 5 days of aminoglycoside treatment, the latencies of the wave V dwelled on 9.13 +/- 1.90 ms while remaining at 7.75 +/- 1.11 ms in the control group (p < 0.01). At day 10 latencies reached 8.73 +/- 1.47 ms in treated babies as compared t 7.31 +/- 1.06 ms in controls (p < 0.01).