Experimental studies on round window structure: function and permeability

Round window membrane permeability during experimental purulent otitis media: altered Cortisporin ototoxicity

Round window membrane (RWM) permeability is the most critical factor influencing cochlear function following otitis media. Because otic drops are frequently used during purulent otitis media (POM), we investigated RWM permeability and ototoxicity of Cortisporin otic suspension after inducing experimental POM. Unilateral POM was induced in eight chinchillas by inoculating type 7F Streptococcus pneumoniae into the right ears. Left ears were inoculated with phosphate-buffered saline (control). When POM resolved, the animals were divided into two groups. The round window niches of group 1 were covered with Cortisporin otic suspension. Compound action potentials were measured before and after drug application. The RWM permeability was measured in group 2 by use of tetraethylammonium (TEA) ions as tracers, and the arrival time of TEA and the slope of the potassium-selective microelectrode response were measured. Animals with otitis media exhibited less susceptibility to ototoxicity of Cortisporin otic suspension and reduced RWM permeability to the medium-sized molecule TEA.

Outer hair cells as potential targets of inflammatory mediators

Inner ear sequelae with temporary or permanent sensorineural hearing loss can result from inflammatory processes in the middle ear. Loss of outer hair cells in the base of the cochlea has been noted in otitis media, but it is not known how this damage occurs. Evidence supports the permeability of the round window membrane to substances mediating inflammation in the middle ear, and the presence of white blood cells has been reported in the perilymph. In the present study, the potential cytotoxic effects of two representative inflammatory mediators, endotoxin and free radicals, have been evaluated by use of short-term culture of isolated outer hair cells from the guinea pig cochlea model. Incubation with endotoxins from two gram-negative pathogens increased the rate of hair cell death fourfold to sixfold. Free radicals (generated by exposure of cells to UV light or by excitation of intracellular fluorescent dyes) produced morphologic damage to hair cells within 60 seconds. These latter effects were delayed by addition of free-radical scavengers. It is concluded that inflammatory mediators are cytotoxic to hair cells and therefore are potentially ototoxic if permeating the round window membrane.

Historical perspectives and concepts in otitis media research at the University of Minnesota

The University of Minnesota Otitis Media Research Center, formed in 1978, comprises a multidisciplinary research team in the clinical fields of otolaryngology, pediatrics, audiology, and infectious disease as well as the basic science fields of histopathology, epidemiology, microbiology, immunology, biostatistics, pharmacology, physiology, and biochemistry. The team has focused on studies designed to elucidate the pathophysiology of the otitis media continuum, and considerable progress has been made in describing this continuum by using the basic science approaches available in the Center. The collaboration of basic and clinical scientists has created a synergy of goals, ideas, and methods leading to new hypotheses and progress, ultimately resulting in improved patient care methods.

Anatomical perspective, approach, and experience with multichannel intracochlear implantation

An overall review of the authors' anatomical perspective, approach, and experience with multichannel intracochlear implantation is presented. This report includes pertinent anatomical observations and experimental surgical procedures on fresh temporal bones. Although the scala tympani in the basal turn of the cochlea has an adequate space laterally for the insertion of electrodes, in the upper turns the adequate space is located medially. This is, in practice, the single-most limiting factor for an electrode, inserted via the round window, to reach the apical turns without damaging the basilar membrane. An experimental surgical procedure is described in which a second electrode is inserted in the middle turn via a mastoidotomy-tympanotomy approach and reaches a point near the apex. Studies of horizontal sections of human temporal bones include a review of potential structures that could be stimulated by the electrical currents generated by electrodes. The wall of the carotid artery, located at 750 microns from the basal turn at the level of the round window niche, is considered a potential structure to be affected by long-term stimulation. Anatomical principles and surgical considerations for the mastoidotomy-tympanotomy approach are described as an alternative to the facial recess approach. In the authors' experience, this procedure is technically easier, eliminates the possibility of postauricular flap complications, provides a better angle for insertion of electrodes, requires shorter hospitalization, and permits faster recovery than the facial recess approach. The authors' clinical approach, results, and observations are reviewed. Of note is a successful home-based rehabilitation program.

The preparation of acetic acid for use in otic drops and its effect on endocochlear potential and pH in inner ear fluid

The ototoxicity of an otic drop preparation containing 2% acetic acid and 3% propylene glycol (VoSol, Denver Chemical Co., Humacao, PR) was investigated according to measurements of endocochlear potential (EP) and inner ear fluid pH. The application of this preparation to the round window membrane for 30 minutes caused a depression in EP from 80.5 +/- 2.5 mV (mean +/- SD; n = 6) to 11.7 +/- 7.7 mV, and lowered inner ear fluid pH from 7.55 +/- 0.09 to 5.06 +/- 0.19 (n = 6) in perilymph and from 7.52 +/- 0.07 to 5.88 +/- 0.63 (n = 6) in endolymph. Two percent acetic acid produced similar changes after 30 minutes: EP was reduced from 83.0 +/- 2.2 mV to 34.0 +/- 2.9 mV and endolymphatic pH from 7.49 +/- 0.04 to 6.83 +/- 0.21 (n = 4). However, the application of artificial perilymph of pH 4 titrated with HCl induced no significant changes in either EP or endolymphatic pH. We suggest that the mechanisms of ototoxicity in the otic drop preparation are Na+ and K+-ATPase inhibition, and that such inhibition is due to the intracellular acidification of strial cells resulting from the penetration of acetic acid across the cell membrane, and to the direct and synergistic actions of propylene glycol.

Inner ear disturbances following inoculation of endotoxin into the middle ear

Inner ear function was assessed by a frequency-specific (+/-100 Hz) auditory brainstem response (ABR) technique after a single instillation of a suspension of purified E. coli lipopolysaccharide in sterile water into the round window (RW) niche in rats. Instillation of endotoxin caused a transient concentration and tonotopically dependent dysfunction of the inner ear. The largest threshold impairment occurred in the high-frequency region anatomically located close to the RW. At 31.5 kHz the threshold impairment persisted throughout the study. Morphologic damage to the inner ear was not detected at the light microscopic level when using serial sections from decalcified specimens. Our study supports the clinical hypothesis that remnants of non-viable bacteria such as endotoxins, when trapped in the middle ear, can promote middle ear effusion and dysfunction of the inner ear.