Experimental basis for lidocaine therapy in cochlear disorders

Vestibular effects of lidocaine intratympanic injection in rats

When lidocaine is locally delivered into the inner ear, it rapidly paralyzes the peripheral vestibular afferent neurons and induces unilateral vestibular loss. The goals of this study were to explore the possibility of developing intratympanic injection (IT) of lidocaine as a modality for treating acute vertigo. To evaluate the minimum concentration required, latent time, action duration, and possibility of lidocaine IT readministration to the vestibular system, we compared the development of horizontal nystagmus after IT of 2, 4, 6, 8, and 10% lidocaine solutions in rats. To identify the induction of vestibular compensation, c-Fos-like protein expression was observed in the vestibular nucleus. Results of our investigation showed that lidocaine IT concentrations greater than 4% induced vestibular hyporeflexia in the injected ear. In order to induce hyporeflexia 4 and 6% lidocaine solutions could also be repeatedly injected. Regardless of concentration, effects of the lidocaine IT dissipated gradually over time. Our findings could be used to develop novel methods for symptom control in vestibular disorder patients.

Sustained delivery of lidocaine into the cochlea using poly lactic/glycolic acid microparticles

OBJECTIVES/HYPOTHESIS

Lidocaine is a local anesthetic that is known to suppress tinnitus via systemic or local application; however, this effect has only limited duration. The current study aimed to establish a method for the sustained delivery of lidocaine into the cochlea using poly lactic/glycolic acid (PLGA) microparticles.

STUDY DESIGN

Experimental study.

METHODS

Lidocaine-loaded PLGA microparticles were produced and their in vitro-release profiles were examined. The lidocaine concentrations in the perilymph were measured at different time points following the application of the lidocaine-loaded PLGA microparticles to the round-window membranes of guinea pigs. The possible adverse effects of the local application of lidocaine-loaded PLGA microparticles were also examined.

RESULTS

The in vitro analyses revealed that the microparticles were capable of the sustained delivery of lidocaine. The in vivo experiments demonstrated the sustained delivery of lidocaine into the cochlear fluid, and the maintenance of high lidocaine concentrations in the perilymph for up to 3 days after application. Nystagmus and inflammation in the middle ear mucosa were not detected after the local application of lidocaine-loaded PLGA microparticles, although temporary hearing loss was observed.

CONCLUSIONS

Lidocaine-loaded PLGA microparticles were shown to be capable of the sustained delivery of lidocaine into the cochlea, suggesting that they could be used for the attenuation of peripheral tinnitus.

The effect of intravenously administered mexiletine on tinnitus-a pilot study

The effect of intravenously administered mexiletine on subjective tinnitus and hearing was studied in six patients, who initially responded positively to lidocaine. Distinct mexiletine-induced decreases in tinnitus loudness were demonstrated in three subjects, as reflected by maximum VAS (visual analogue scale) level reduction of 34%, 95%, and 100%, respectively. One subject reported change in tinnitus pitch, another one showed a slight (18% on VAS) tinnitus reduction, and one subject disclosed no effect. Side effects were seen only during one of seven infusions. Mexiletine induced shifts in pure-tone threshold, transient evoked otoacoustic emission, and acoustic reflex threshold, probably reflecting a reversible interference in the function of organ of Corti. The concentration effect relationship remained unclear and no general 'therapeutic' level could be identified. This study confirms the effect of mexiletine on the auditory function and its potential as a possible therapeutic agent or a model for further development in tinnitus pharmacotherapy.

Presbycusis: reversible with anesthesia drugs?

Age-related hearing impairment, or presbycusis, is a degenerative condition not currently treatable by medication. It is therefore significant that the author, as a patient, experienced a reversal of high-frequency hearing loss during a 2-day period following abdominal surgery with general anesthesia. This report documents the surgery and the subsequent restoration of hearing, which was bilateral and is estimated to have exceeded 50dB at 4kHz. A possible role is noted for anesthetic agents such as lidocaine, propofol, or fentanyl. This experience may hold a clue for research toward the development of medical treatments for presbycusis.

Effects of lidocaine on basilar membrane vibration in the guinea pig

The effects of lidocaine on basilar membrane (BM) vibration and compound action potential (CAP) were studied in guinea pigs in order to elucidate the site of lidocaine action in the cochlea. BM vibration was measured with a laser Doppler vibrometer through an opening made in the lateral bony wall of the scala tympani at the basal turn. Ten min after local administration of lidocaine (250 microg) into the scala tympani, the velocity of BM vibration and the CAP amplitude decreased significantly at around the characteristic frequency of the stimulus sound (p < 0.05). The maximum decreases were 4 dB in the velocity of the BM vibration and 40 dB in the CAP amplitude. In contrast, such changes were not observed after i.v. injection of lidocaine (1.5 mg/kg). These results suggest that when lidocaine is administered locally in the cochlea it acts not only on the cochlear nerve but also on the outer hair cells.

Micromechanical effects in the cochlea of tetracaine

Local anesthetics applied in the tympanic cavity have earlier been shown to affect the gross receptor potentials in reducing the cochlear microphonics and increasing the positive summating potential. To study the effects of this drug on the mechanical responses in the cochlea, vibrations were measured using laser heterodyne interferometry in an isolated in vitro temporal bone preparation from the guinea pig. Measurements were made at a set of frequencies in the fourth cochlear turn from the Hensen's cells and the outer hair cells in response to sound applied to the ear. The tuning curves of the fundamental and the second harmonic components of the vibratory responses were plotted. When 2 mM tetracaine was applied, the high frequency slope of the second harmonic curve shifted down in frequency, this caused the frequency of the maximum of second harmonic tuning to shift down. These changes were reversible when tetracaine was washed out. Observations were also made in the temporal bone preparation in vitro with a confocal microscope. Fluorescent probes were used to label various structures in the organ of Corti. Optical sections were obtained by tilting the organ permitting a view from the side like a radial section through the organ. Images were acquired before, during and after application of tetracaine and were later analyzed with a computer program. Simultaneously, cochlear microphonics and the summating potential were obtained to monitor the electrical response of the preparation. Although the cochlear microphonics and summating potential decreased when 2 mM tetracaine was applied, structural changes were not measurable in the organ of Corti. The decrease was reversible when tetracaine was washed out. It is concluded that tetracaine affected the high frequency part of the non-linear second harmonic component, possibly by lowering the stiffness of the stereocilia bundle or the body of the outer hair cells.

Auditory evoked responses under total spinal anesthesia in rats

In order to investigate the function of the auditory pathway from the cochlea to the brain stem under total spinal anesthesia, the auditory brain stem response (ABR), compound action potential of the cochlear nerve (CAP), and cochlear microphonics (CM) were simultaneously recorded in rats. Total spinal anesthesia was induced by infusion of 2% lidocaine hydrochloride at a constant rate of 0.10 mL/min into the cerebrospinal fluid through the rats' skulls. The ABR completely disappeared within 1.5 to 4 minutes. After cessation of the injection, the ABR reappeared, starting from wave I and progressing through waves II and III to wave IV. The latency change of the CAP throughout the recording period was quite similar to that of wave I of the ABR. A reduction in amplitude of the CM was observed, but the CM did not disappear during the recording period. Disappearance of the ABR was due, not to loss of cochlear function, but to anesthetic effects on the acoustic nerve and the brain stem. Monitoring of the ABR provided information on the level of neural activity in the brain stem under total spinal anesthesia.

The effect of intravenous lidocaine injection on hearing thresholds

The effect of intravenous injection of lidocaine on hearing thresholds was studied in normal subjects. Continuous and intermittent tones at 1, 4 and 8 kHz were used as stimuli and the threshold change with lidocaine injection was measured using a self-recording audiometer (Békésy audiometer). Both increases and decreases in the threshold were observed. The former occurred more frequently than the latter. In cases of a threshold increase, lidocaine injection exhibited a frequency specific effect; the higher the frequency, the more often the threshold was increased by lidocaine injection. There was no significant difference in threshold changes between continuous and intermittent tones. The present results suggest that lidocaine may act on the inner ear hair cells.

The mechanism and site of action of lidocaine hydrochloride in guinea pig inner ear

Lidocaine was applied to the round window (RW) in order to localize its site of action in the cochlea. Cochlear microphonic (CM), summating potential (SP), and compound action potential (CAP) input/output functions were measured to a 16 kHz tone burst to assess the functional changes of the cochlea. In separate experiments, the effect of lidocaine on the whole cell current of isolated outer hair cells (OHC) was studied. A dose of 2 microliters of 40 mM lidocaine in saline solution, when applied to the RW, caused a small change in all measured variables, indicating a passage of the drug through the RW membrane to sites of action. However, 160 mM of lidocaine further decreased CM, SP, and CAP by a total of 40% from the control. A partial recovery occurred for CM during the 30 min follow-up period. CAP and SP continued to decline. In isolated OHCs, lidocaine decreased the whole cell current in a dose-dependent fashion. The KD for lidocaine effect on OHCs was 7 mM. Our in vivo results indicate that lidocaine affects OHCs and reduces CM, causing a subsequent reduction in SP and CAP. The increased effect of lidocaine on CAP and SP, while CM is recovering, suggests an additional specific effect of lidocaine on the cochlear nerve and/or on inner hair cells. Considering that lidocaine alters OHC current (in isolated hair cells) and that lidocaine does not affect endocochlear potential [Laurikainen et al. Acta Otolaryngol (Stockh) 1991: 112: 800-9], the observed CM changes are most likely due to an in vivo effect on OHCs. Thus, the early effect of lidocaine on the cochlea appears to be due to a significant change in organ of Corti function, rather than to direct anesthesia of the cochlear nerve. Later, an independent effect of the drug may occur on neural tissues in the inner ear.

The tonic sympathetic input to the cochlear vasculature in guinea pig

Vascular tones is an essential component in maintaining steady regional blood flow and dynamic responsiveness of a vascular bed. Sympathetic innervation can contribute to vascular tone. Although certain studies have reported evoked changes in cochlear blood flow (CBF) with activation of the sympathetic fibers to the cochlear vasculature, other studies have failed to show evidence of sympathetic contribution to CBF regulation when the cervical sympathetic fibers were unilaterally sectioned. We hypothesized that the bilateral 'sympathectomy of the stellate ganglia' would remove sufficient sympathetic input to the cochlea to yield a change in CBF resting level. To test this hypothesis a new technique was used to expose the stellate ganglia (SG) bilaterally and induce a chemical sympathectomy. We observed that unilateral SG blockade with 2 microliters of 4 mM lidocaine hydrochloride on either side produced a 5-10% increase in CBF, which recovered to baseline during the following 2 min. A subsequent blockade of the contralateral SG produced a rapid 25-35% increase, which then recovered partially during the following 3-4 min, remaining 5-15% above the baseline over a 20 min measurement period. Superior cervical ganglion transection did not affect CBF. Our results provide evidence for the existence of a tonic sympathetic component in the control of vascular tone in guinea pig cochlea. This neural effect is derived bilaterally from SG. This result is consistent with previous anatomical studies showing the bilateral innervation of the cochlea by the SG sympathetic fibers and with previous physiological studies on the bilaterality of evoked changes in CBF due to electric stimulation of SG.

Effects of lidocaine on salicylate-induced discharge of neurons in the inferior colliculus of the guinea pig

Using the extracellular recording method, the effects of lidocaine (a local anesthetic known to relieve tinnitus) on discharge of inferior colliculus (IC) neurons of the guinea pig were studied before and after salicylate (200 mg/kg) administration. The salicylate-induced discharge was inhibited by intravenous injection of lidocaine at a concentration (1 mg/kg) clinically used for treating tinnitus. IC neurons could be classified into two groups according to the difference in sensitivity to lidocaine: (1) weakly-sensitive neurons and (2) highly-sensitive neurons. In weakly-sensitive neurons, the duration of the lidocaine effect lasted for less than 5 min, and the inhibitory action on the discharge of neurons was greater when the latency to sound stimulus became longer. In highly-sensitive neurons, on the other hand, the activity of neurons was almost completely inhibited for longer than 30 min, irrespective of the latency to sound stimulus. The clinical relevance of these types of neurons is discussed.

Non-specific effect of beettahistine on cochlear electrophysiology in guinea pig

Beettahistine hydrochloride (BH) is reported to be beneficial in the treatment of certain type of vestibulo-cochlear disorders. Experimentally is shown that BH inhibits vestibular firing rate and improves cochlear blood flow by dilatating the larger feeding vessels. Both these effects are weak, and hardly can explain the clinical benefits reported for the drug. We investigated the effect of BH on the organ of Corti electrophysiology in vivo. Two microliters of 1, 2, 5 and 10% saline solutions of BH was applied (in increasing order) to the round window (RW) membrane in an anesthetized guinea pig (n = 5). Summating potential (SP), action potential (AP), and cochlear microphonics (CM) were recorded at 16 kHz frequency using RW Ag wire electrode. The repeated applications of BH with increasing concentrations produced a dose-dependent threshold shift in CAP. The I/O function for 20, 30, 40, 50, 60, 70, and 80 dB at 16 kHz indicated that there was a saturation before complete blockade of the responsiveness in all measured parameters. SP and CM changes nearly followed CAP changes. The changes recover during 2 hours follow-up period. Our findings indicate that BH affects the organ of Corti electrophysiology. That the drug effect saturates before a complete blockade suggests non-specific effect of BH on the organ of Corti structures and cochlear nerve.

Effects of intratympanically delivered lidocaine on the auditory system in humans

Since lidocaine is used to relieve the symptoms of Meniere's disease and tinnitus, its effects on the human cochlea is of specific interest. In experimental animals, topically administered lidocaine affects cochlear electrophysiology in a specific, dose-dependent manner. In the current study, lidocaine (40 mg in 1 ml of saline) was delivered intratympanically in six patients with essentially normal hearing in an attempt to alleviate tinnitus. Auditory function was assessed by pure tone audiometry, auditory evoked brain stem responses (ABR), and transiently evoked otoacoustic emissions (TEOAE) to observe possible drug effect in the auditory system. In five patients, saline was injected prior to lidocaine for control purposes. Saline injection did not create significant changes in any of the measures. After a 2 hr follow-up period, intratympanic injection of lidocaine caused a 2 to 10 dB reduction in TEOAE level at 1 to 3 kHz frequencies. This reduction was at its maximum at the 30 min post-injection sampling point, and was followed by a slow recovery. At 2 hr post-injection, TEOAE amplitude was still 2 to 4 dB below baseline level. Pure tone thresholds were slightly affected 30 min after intratympanic lidocaine injection, but were fully recovered 1 hr after the injection. Lidocaine injection did not cause any changes in ABR latencies or amplitudes in any of the patients. These results suggest that the dose of intratympanically administered lidocaine used here has a specific effect on the organ of Corti structures in human subjects, without significantly affecting the auditory nerve or central auditory pathways. The relationship of the drug effect in conjunction with a possible effect on inner ear disorder might help to localize the site of disorder.

Analysis of auditory brain stem response with lidocaine injection into the cerebrospinal fluid in rats

Auditory brain stem response (ABR) was recorded in 10 rats with total spinal anesthesia induced by injection of 2% lidocaine hydrochloride into the subarachnoid space through the skull. The ABR disappeared immediately (within 4 minutes) after the injection of 13.3 to 40.0 mg/kg lidocaine. The disappearance started with the later waves of the ABR. After cessation of the injection, the ABR reappeared and recovered progressively from wave I to wave IV. The effect of lidocaine on the ABR was reversible and extended in the acoustic nerve to the midbrain.

Temporary latency shifts in auditory evoked potentials by injection of lidocaine in the rat

Temporary latency shifts in auditory evoked potentials by intravenous injection of lidocaine were investigated in albino rats. Auditory brainstem responses (ABR) were recorded with 4000Hz short tone bursts. There was no significant difference of latencies in wave Pl of the ABR between drug-administered group (6 mg/kg body weight of lidocaine) and control group (0.6 ml/kg of 0.9% saline). As for waves P2. P3 and P4 of ABR, the latencies of the waves were significantly prolonged after the lidocaine injection (p < 0.05. U-test). The prolongation of latency of wave P4 was the most prominent in the fast waves and dependent on the dose of lidocaine. The latencies of the slow positive wave of the evoked potentials on the inferior colliculus (wave S) with clicks and the slow positive wave (wave P) and the negative wave following ABR (wave NA) with 4000Hz short tone bursts were also studied. The latencies of waves S. P and NA showed significant temporal prolongation (p < 0.05, U-test) after lidocaine injection. The present study revealed that a relatively small dose of lidocaine can cause temporary prolongation of latencies of auditory evoked potentials not only in the fast components but also in the slow components including wave NA which is a large slow negative wave following the ABR. The effects are considered to be caused by the action of lidocaine which reduces the excitabilities on the auditory nervous system.

Acute effects of cocaine on cochlear function

The effects of a single administration of cocaine on the cochlea was evaluated by measuring amplitude-intensity functions of the N1 response of the auditory nerve. Amplitude-intensity functions of the N1 response to tone-pips of 500 Hz, 1, 2, 4 and 8 kHz were obtained before and after intraperitoneal injection of either saline, 3 mg/kg or 25 mg/kg of cocaine. N1 amplitudes were decreased after the administration of cocaine and this reduction was found to be dose dependent. The influence of cocaine on cochlear blood flow (CBF) was examined in order to test the possibility that cocaine induced reductions in CBF underlie these electro-physiological changes. Corresponding decreases in cochlear blood flow after cocaine exposure were observed.