Aminoglycoside ototoxicity and the medial efferent system: II. Comparison of acute effects of different antibiotics

Wideband Acoustic Reflex Measurement

Acoustic reflex thresholds (ART) obtained using pure-tone probe stimuli as part of a traditional immittance test battery can be used to evaluate site of lesion and provide a cross-check with behavioral results. ARTs obtained as part of a wideband acoustic immittance (WAI) test battery using a click as the probe stimulus can be used in the same way with the added benefit that they may provide lower ARTs than those obtained using a pure-tone probe. Another benefit of the WAI ART test is that it can be completed without requiring a hermetic seal or pressurizing the ear canal. A new adaptive method of obtaining ARTs using WAI techniques may cut test time in half, thus making this an attractive option for future clinical use. More advanced uses of WAI ART tests include the measurement of AR growth functions. These may be used to investigate the possible effects of synaptopathy related to high levels of noise exposure and possible auditory deficits related to ototoxicity.

d-Methionine reduces tobramycin-induced ototoxicity without antimicrobial interference in animal models

BACKGROUND

Tobramycin is a critical cystic fibrosis treatment however it causes ototoxicity. This study tested d-methionine protection from tobramycin-induced ototoxicity and potential antimicrobial interference.

METHODS

Auditory brainstem responses (ABRs) and outer hair cell (OHC) quantifications measured protection in guinea pigs treated with tobramycin and a range of d-methionine doses. In vitro antimicrobial interference studies tested inhibition and post antibiotic effect assays. In vivo antimicrobial interference studies tested normal and neutropenic Escherichia coli murine survival and intraperitoneal lavage bacterial counts.

RESULTS

d-Methionine conferred significant ABR threshold shift reductions. OHC protection was less robust but significant at 20kHz in the 420mg/kg/day group. In vitro studies did not detect d-methionine-induced antimicrobial interference. In vivo studies did not detect d-methionine-induced interference in normal or neutropenic mice.

CONCLUSIONS

d-Methionine protects from tobramycin-induced ototoxicity without antimicrobial interference. The study results suggest d-met as a potential otoprotectant from clinical tobramycin use in cystic fibrosis patients.

Maturation of the auditory system: 2. Transient otoacoustic emission suppression as an index of the medial olivocochlear bundle maturation

Contralateral suppression of transient otoacoustic emissions in 42 premature babies (84 ears; post-conceptional age [PCA] 30-36 weeks) was compared to that of 39 full-term babies (78 ears; PCA: 37-45 weeks). Eighteen healthy adults and ten young children (5-14 years old) were studied as controls. Risk factors for hearing loss were registered in both preterm and full-term groups. An ILO-92 otoacoustic emission recording system was used to deliver linear clicks to the ear examined and broadband noise to the contralateral ear in an alternating on and off mode. Suppression in full-term babies was statistically higher than in preterms, whereas no differences existed between children and adults and children and full-terms. Peripheral auditory lateralization was evident in adults but was observed only as a trend in newborns. Only prematurity at the time of examination and aminoglycoside treatment for more than seven days had a negative impact on suppression. The results support the conclusion that maturation of the efferent system takes place from 30 to 45 weeks PCA. The exact age at which this maturation is accomplished has not yet been clearly determined.

Long-term effects of chemotherapy for acute lymphoblastic leukemia on the medial olivocochlear bundle: effects of different cumulative doses of gentamicin

OBJECTIVE

The treatment of acute lymphoblastic leukemia (ALL) often combines a neurotoxic chemotherapeutic protocol such as Berlin-Frankfurt-Munster-95 (BFM-95) with gentamicin, an antibiotic known to have an early and quickly reversed impact on olivocochlear reflex in animal studies. This study investigates whether this combination has any long-term side effects on the medial olivocochlear bundle (MOCB).

METHODS

In all 47 children of the study suppression of distortion product otoacoustic emissions (DPOAEs) by contralateral application of white noise (WN) was used to assess the function of the MOCB. The population was divided into three groups depending on the time interval between the end of therapy and examination. The group examined shortly after chemotherapy included 12 children who had received low gentamicin doses (less than 13 days). The group evaluated 2 years after therapy involved another 12 children who had required medium gentamicin doses (more than 13, less than 23 days). The group examined 3 years after therapy included a subgroup of 12 children to whom low gentamicin doses were infused and another 11 children with high gentamicin doses (more than 23 days).

RESULTS

Three years after therapy the olivocochlear reflex was efficiently produced in both subgroups of low and high gentamicin doses. Two years after therapy, contralateral WN induced increase of DPOAEs at 4 of the 12 examined frequencies. Shortly after therapy, WN increased, instead of suppressing, DPOAEs at five frequencies.

CONCLUSION

This abnormal result of contralateral noise application perceived as impaired cochlear efferent innervation may indicate that ALL-BFM-95 exerts a toxic effect on the MOCB, which is slowly reversed within the first 3 years after chemotherapy and does not seem to be affected in the long term by different cumulative doses of gentamicin.

Neurotoxicity of BFM-95 on the Medial Olivocochlear Bundle Assessed by Means of Contralateral Suppression of 2f1-f2 Distortion Product Otoacoustic Emissions

OBJECTIVE

Berlin-Frankfurt-Munster 95 (BFM-95) is a common chemotherapeutic protocol against acute lymphoblastic leukemia (ALL). This prospective study investigates whether this protocol has an adverse effect on the medial olivocochlear bundle (MOCB) and/or outer hair cells' (OHCs) function. The distortion product otoacoustic emissions (DPOAEs) and their suppression by means of contralateral application of white noise were used for assessing the function of OHCs and the MOCB, respectively.

STUDY DESIGN

Prospective study.

SETTING

Oncology and otorhinolaryngology departments in a pediatric hospital.

PATIENTS

Thirty-six children treated with ALL-BFM-95.

INTERVENTIONS

Before chemotherapy, a baseline audiologic evaluation with tympanogram, standard and extended high frequency, pure-tone audiometry, and DPOAEs in the absence and presence of white noise was performed in all children. This population was divided in three groups. In a first group (n = 12), the evaluation was repeated after four sessions of vincristine administration; in the second group (n = 12), after 8 sessions; and in the third group (n = 12), several months after completion of the protocol.

MAIN OUTCOME MEASURE

DPOAEs suppression by contralateral application of white noise.

RESULTS

In the first and the third groups, we observed no changes in DPOAE amplitudes. Nevertheless, in the second group, the DPOAEs demonstrated significant decrease at 1416, 1685, 2002, and 2380 Hz. At baseline examination, all groups presented significant suppression at all frequencies. After eight vincristine sessions, instead of suppression, an increase of amplitudes was noted at 5 of 12 frequencies. Efferent-mediated DPOAE suppression reappeared in the third group at all frequencies (significant at 5 of 12 frequencies).

CONCLUSION

ALL-BFM-95 seems to exert an early and reversible toxic effect on the MOCB, whereas its effects on OHCs are minimal and reversible. These minimal cochleotoxic and neurotoxic properties of ALL-BFM-95 might prove useful for research studies on the role of efferent innervation in hearing.

Gentamicin blocks ACh-evoked K+ current in guinea-pig outer hair cells by impairing Ca2+ entry at the cholinergic receptor

Aminoglycoside antibiotics such as gentamicin are known to block the medial olivocochlear efferent system. In order to determine whether this inhibition takes place at the postsynaptic cholinergic receptors in outer hair cells (OHCs), we studied the effects of these polycationic molecules on cholinergic currents evoked in isolated guinea-pig OHCs. The cholinergic response of OHCs involves nicotinic-like receptors (nAChRs) permeable to Ca2+ ions that activate nearby Ca2+-sensitive K+ channels (KCa(ACh) channels). The extracellular application of gentamicin and neomycin reversibly blocked ACh-evoked K+ current (IK(ACh)) with IC50 values of 5.5 and 3.2 microM, respectively. The results showed that the blocking mechanism of IK(ACh) was due to inhibition of Ca2+ influx via nAChRs. Our study also provides interesting insights into the functional coupling between nAChRs and KCa(ACh) channels in OHCs. By directly recording the cation current flowing through nAChRs (In(ACh)) using an intracellular solution containing 10 mM BAPTA, we measured an EC50 near 110 microM for ACh-evoked In(ACh). This EC50 for ACh is one order of magnitude higher than that measured indirectly on IK(ACh). This reveals a rather low affinity of ACh for its receptor but a very efficient coupling between nAChRs and KCa(ACh) channels. We also show that a high external Ca2+ concentration reverts the gentamicin inhibition of IK(ACh) and that gentamicin directly alters the cation current flowing through the nAChRs of OHCs. We propose that gentamicin acts as a non-competitive cholinergic blocker by displacing Ca2+ from specific binding sites at the nAChRs. This block of the nAChRs at the level of the postsynaptic membrane in OHCs could explain the inhibitory effect of gentamicin reported on the crossed medial olivocochlear efferent system in vivo.

Aminoglycoside ototoxicity and the medial efferent system: I. Comparison of acute and chronic gentamicin treatments

Recently our laboratory has demonstrated, in the guinea pig (GP), that an intramuscular (i.m.) injection of a high dose of gentamicin (GM) (150 mg/kg), can reversibly block the contralateral efferent suppression of ipsilateral cochlear activity. The aims of the present study were: (1) to investigate this effect with lower doses of GM; and (2) to find out whether this effect could constitute an anticipatory sign of ototoxicity during a chronic GM treatment (60 mg/kg i.m., 10 days). The function of the medial olivocochlear efferent system (MOES) was tested by recording the VIIIth nerve ensemble background activity (EBA) without and with contralateral low level (55 dB SPL) broadband noise stimulation. The results show a dose-dependent effect of GM on contralateral suppression, as the dose of 120 mg/kg induced a smaller blockade of the MOES, compared to 150 mg/kg, and no blockade was observed with lower doses. During the ten-day treatment no significant changes in the EBA without acoustic stimulation, nor in contralateral efferent suppression were detected. GPs monitored over several weeks after the treatment showed progressive reduction of the EBA without contralateral stimulation parallel to reduced suppression coefficients of the EBA, and CAP threshold elevations, denoting impaired cochlear function. Thus, this study demonstrated that a chronic treatment with 60 mg/kg of GM, although ototoxic, does not affect the contralateral efferent suppression, at least before the development of ototoxicity.