Karen Jo Doyle

THE CASE FOR EARLY IDENTIFICATION OF HEARING LOSS IN CHILDREN Auditory System Development, Experimental Auditory Deprivation, and Development of Speech Perception and Hearing

Human infants spend the first year of life learning about their environment through experience. Although it is not visible to observers, infants with hearing are learning to process speech and understand language and are quite linguistically sophisticated by 1 year of age. At this same time, the neurons in the auditory brain stem are maturing, and billions of major neural connections are being formed. During this time, the auditory brain stem and thalamus are just beginning to connect to the auditory cortex. When sensory input to the auditory nervous system is interrupted, especially during early development, the morphology and functional properties of neurons in the central auditory system can break down. In some instances, these deleterious effects of lack of sound input can be ameliorated by reintroduction of stimulation, but critical periods may exist for intervention. Hearing loss in newborn infants can go undetected until as late as 2 years of age without specialized testing. When hearing loss is detected in the newborn period, infants can benefit from amplification (hearing aids) and intervention to facilitate speech and language development. All evidence regarding neural development supports such early intervention for maximum development of communication ability and hearing in infants.

Intratympanic steroid treatment: A review

Objective: To review published literature regarding the use of intratympanic steroids in the treatment of Ménière’s disease and sudden sensorineural hearing loss and to make recommendations regarding their use based on the literature review. Data Sources: Literature review from 1996 to 2003, PubMed, Medline Plus, and Web of Science. Study Selection: Retrospective case series and uncontrolled prospective cohort studies were the only types of studies available for review. Conclusion: On the basis of the available literature, a weak recommendation is made to use intratympanic steroid treatment of sudden hearing loss if oral steroid therapy fails or is con-traindicated. The available studies regarding intratympanic steroid treatment of Ménière’s disease and tinnitus are inadequate to answer the question of the efficacy of this treatment for these conditions. Higher quality studies are needed.

Neonatal middle ear effusion predicts chronic otitis media with effusion

Hypothesis and Aims: The specific aims of the research are to determine whether newborn ears with persistent middle ear effusion at age 30 to 48 hours are more likely to develop chronic otitis media with effusion over the first year of life when compared with ears without persistent middle ear effusion. The hypothesis is that neonates with middle ear effusion persisting to 30 to 48 hours are more likely to develop chronic otitis media with effusion. Study Design: Prospective, case-control design. Loupe-magnified pneumatic otoscopy performed at the time of newborn hearing screening determined presence or absence of effusion. Infants enrolled in the study returned for outpatient examinations. Setting: University medical center well-baby nursery and out-patient audiology clinic. Subjects: From 454 neonates, 14 experimental subjects with neonatal middle ear effusions and 15 control subjects free of neonatal effusion were recruited for the study and followed-up for 1 year. Interventions: Outpatient study tests included transient-evoked otoacoustic emissions, tympanometry, pneumatic otoscopy, and visual reinforcement audiometry (starting at age 6 months), at 3, 6, 9, and 12 months of age. Experimental (neonatal effusion) infants were followed-up starting at age 1 month. Infants found at any follow-up examination to have effusion on otoscopy were followed-up and tested 1 month later. Main Outcome Measures: Chronic otitis media with effusion defined as hypomobile or immobile tympanic membrane on pneumatic otoscopy in one or both ears for three consecutive monthly examinations. Hearing loss defined as greater than 25-dB hearing loss visual reinforcement audiometry thresholds. Results: Eight experimental infants (58%) and three control infants (20%) developed chronic otitis media with effusion (p < 0.04). The average number of effusions was 1.27 for control and 4.14 for experimental infants (average number of effusions for each group at 3-, 6-, 9-, and 12-month visits). Warbled tone and speech visual reinforcement audiometry thresholds averaged 3 dB worse in the experimental group, but these differences were not statistically significant. For the control group, mean visual reinforcement audiometry thresholds never exceeded 25 dB hearing loss. For the experimental group, mean visual reinforcement audiometry thresholds exceeded 25 dB hearing loss at 1,000, 2,000, and 4,000 Hz at 9 months. Conclusions: A majority of infants with persistent neonatal middle ear effusion found by pneumatic otoscopy at 30 to 48 hours will develop chronic otitis media with effusion during the first year of life. However, chronic otitis media with effusion is common in all infants (20% of controls), a time during which infants are examined and tested frequently.

Conservation of Hearing by Simultaneous Mutation of Na,K-ATPase and NKCC1

Although drug-induced and age-related hearing losses are frequent otologic problems affecting millions of people, their underlying mechanisms remain uncertain. The inner ear is exclusively endowed with a positive endocochlear potential (EP) that serves as the main driving force for the generation of receptor potential in hair cells to confer hearing. Deterioration of EP leads to hearing loss or deafness. The generation of EP relies on the activity of many ion transporters to establish active potassium (K+) cycling within the inner ear, including K+ channels, the Na–K–2Cl co-transporter (NKCC1), and the α1 and α2 isoforms of Na,K–ATPase. We show that heterozygous deletion of either NKCC1, α1-Na,K–ATPase, or α2–Na,K–ATPase independently results in progressive, age-dependent hearing loss with minimal alteration in cochlear morphology. Double heterozygote deletion of NKCC1 with α1–Na,K–ATPase also shows a progressive, though delayed, age-dependent hearing loss. Remarkably, double heterozygote deletion of NKCC1 with α2–Na,K–ATPase demonstrates a striking preservation of hearing threshold both initially and with age. Measurements of the EP confirm the anticipated drop in potential for genotypes that demonstrate age-dependent hearing loss. The EP generated by the NKCC1 + α2-Na,K–ATPase double heterozygote, however, is maintained at a level comparable to that of the control condition, suggesting a potential advantage in this combination of ion transporter modification. These observations provide insight into the detailed mechanisms of EP generation, and results of combination-knockout experiments may have important implications in the future treatment of drug-induced and age-related hearing losses.

Choristoma of the middle ear: a component of a new syndrome?

Case Report Salivary choristoma of the middle ear is a rare entity. The authors report the 26th known case, which is unique in several respects: the patient had abnormalities of the first and second branchial arches, as well as the otic capsule and facial nerve in ways not yet reported. Our patient presented with bilateral preauricular pits, conchal bands, an ipsilateral facial palsy, and bilateral Mondini-type deformities. A review of the literature revealed salivary choristomas of the middle ear to be frequently associated with branchial arch abnormalities, most commonly the second, as well as abnormalities of the facial nerve. Review of the Literature All 25 cases were reviewed and the results reported with respect to clinical presentation, associated abnormalities, operative findings, and hearing results. It has been proposed that choristoma of the middle ear may represent a component of a syndrome along with unilateral hearing loss, abnormalities of the incus and/or stapes, and anomalies of the facial nerve. Conclusion Eighty-six percent of the reported patients with choristoma have three or four of the four criteria listed to designate middle ear salivary choristoma as part of a syndrome. In the remaining four patients, all of the structures were not assessed.

Functional Consequences of Polyamine Synthesis Inhibition by l-α-Difluoromethylornithine (DFMO) CELLULAR MECHANISMS FOR DFMO-MEDIATED OTOTOXICITY

l-α-Difluoromethylornithine (DFMO) is a chemopreventive agent for colon cancer in clinical trials. Yet, the drug produces an across-frequency elevation of the hearing threshold, suggesting that DFMO may affect a common trait along the cochlear spiral. The mechanism for the ototoxic effects of DFMO remains uncertain. The cochlear duct is exclusively endowed with endocochlear potential (EP). EP is a requisite for normal sound transduction, as it provides the electromotive force that determines the magnitude of the receptor potential of hair cells. EP is generated by the high throughput of K+ across cells of the stria vascularis, conferred partly by the activity of Kir4.1 channels. Here, we show that the ototoxicity of DFMO may be mediated by alteration of the inward rectification of Kir4.1 channels, resulting in a marked reduction in EP. These findings are surprising given that the present model for EP generation asserts that Kir4.1 confers the outflow of K+ in the stria vascularis. We have proposed an alternative model. These findings should also enable the rational design of new pharmaceuticals devoid of the untoward effect of DFMO.

Genetic, Cellular, and Functional Evidence for Ca2+ Inflow through Cav1.2 and Cav1.3 Channels in Murine Spiral Ganglion Neurons

Spiral ganglion neurons (SGNs) of the eighth nerve serve as the bridge between hair cells and the cochlear nucleus. Hair cells use Cav1.3 as the primary channel for Ca2+ inflow to mediate transmitter release. In contrast, SGNs are equipped with multiple Ca2+ channels to mediate Ca2+-dependent functions. We examined directly the role of Cav1.3 channels in SGNs using Cav1.3-deficient mice (Cav1.3−/−). We revealed a surprising finding that SGNs functionally express the cardiac-specific Cav1.2, as well as neuronal Cav1.3 channels. We show that evoked action potentials recorded from SGNs show a significant decrease in the frequency of firing in Cav1.3−/− mice compared with wild-type (Cav1.3+/+) littermates. Although Cav1.3 is the designated L-type channel in neurons, whole-cell currents recorded in isolated SGNs from Cav1.3−/− mice showed a surprising remnant current with sensitivity toward the dihydropyridine (DHP) agonist and antagonist, and a depolarization shift in the voltage-dependent activation compared with that in the Cav1.3+/+ mice. Indeed, direct measurement of the elementary properties of Ca2+ channels, in Cav1.3+/+ neurons, confirmed the existence of two DHP-sensitive single-channel currents, with distinct open probabilities and conductances. We demonstrate that the DHP-sensitive current in Cav1.3−/− mice is derived from Cav1.2 channel activity, providing for the first time, to our knowledge, functional data for the expression of Cav1.2 currents in neurons. Finally, using shRNA gene knockdown methodology, and histological analyses of SGNs from Cav1.2+/− and Cav1.3+/− mice, we were able to establish the differential roles of Cav1.2 and Cav1.3 in SGNs.

Functional significance of K+ channel β-subunit KCNE3 in auditory neurons.

Background: KCNE3 is tentatively associated with inner ear diseases, but its functions are unknown. Results: Null deletion of Kcne3 produces profound changes in the excitability of auditory neurons. Conclusion: KCNE3 regulates the magnitude of K+ conductances responsible for maintaining the electrical phenotype of auditory neurons. Significance: This study is the first to examine the in vivo functions of KCNE3 in the auditory system. The KCNE3 β-subunit interacts with and regulates the voltage-dependent gating, kinetics, and pharmacology of a variety of Kv channels in neurons. Because a single neuron may express multiple KCNE3 partners, it is impossible to predict the overall functional relevance of the single transmembrane domain peptide on the pore-forming K+ channel subunits with which it associates. In the inner ear, the role of KCNE3 is undefined, despite its association with Meniere disease and tinnitus. To gain insights on the functional significance of KCNE3 in auditory neurons, we examined the properties of spiral ganglion neurons (SGNs) in Kcne3 null mutant neurons relative to their age-matched controls. We demonstrate that null deletion of Kcne3 abolishes characteristic wide variations in the resting membrane potentials of SGNs and yields age-dependent alterations in action potential and firing properties of neurons along the contour of the cochlear axis, in comparison with age-matched wild-type neurons. The properties of basal SGNs were markedly altered in Kcne3−/− mice compared with the wild-type controls; these include reduced action potential latency, amplitude, and increased firing frequency. Analyses of the underlying conductance demonstrate that null mutation of Kcne3 results in enhanced outward K+ currents, which is sufficient to explain the ensuing membrane potential changes. Additionally, we have demonstrated that KCNE3 may regulate the activity of Kv4.2 channels in SGNs. Finally, there were developmentally mediated compensatory changes that occurred such that, by 8 weeks after birth, the electrical properties of the null mutant neurons were virtually indistinguishable from the wild-type neurons, suggesting that ion channel remodeling in auditory neurons progresses beyond hearing onset.

Delayed, Reversible Hearing Loss Caused by Difluoromethylornithine (DFMO)

Objectives/Hypothesis Difluoromethylornithine (DFMO) is an anticancer experimental drug that is ototoxic. The objectives of these three experiments were to: 1) determine a dose and dosing schedule of DFMO that produces significant hearing loss (HL) in newborn gerbils; 2) compare the HL level for control and newborn gerbils receiving daily subcutaneous injections of DFMO; and 3) to determine if DFMO‐related HL is significantly reversible following discontinuation of DFMO treatment.

Identification of a key residue in Kv7.1 potassium channel essential for sensing external potassium ions.

A glutamate at position 290 in the human Kv7.1 S5-pore linker is required for its inhibition by high concentrations of extracellular potassium.