Hearing impairment in children.

This article describes the current standard for infant hearing loss identification and intervention. Since the standard of care was driven by the recommendations made by the Joint Committee on Infant Hearing, a summary of the most recent recommendations is provided, followed by illustrative case studies that highlight how implementation of these guidelines allow access to the critical window for auditory and speech-language development.

Acquired hearing loss in adolescents.

Studies on prevalence of acquired hearing loss across the life span have shown a trend of increase in hearing loss with age. A parallel decline in age of recreational use of loud music and cigarette smoking suggests that these early listening and recreational habits may be major contributing factors to the chronic hearing disability seen in later years. In recognition of these new recreational patterns of adolescents and young adults, Healthy People 2010 has issued a list of objectives for prevention of noise-induced hearing loss and smoking in adolescents and young adults through early education and intervention. In this article, the authors describe the effects of noise- and music-induced and smoking-induced hearing loss and provide guidelines for early identification and hearing conservation. Such an early approach to identification and conservation should ultimately help reduce hearing loss prevalence rates in adults.

Effects of cigarette smoking on distortion product otoacoustic emissions.

This study measured distortion product otoacoustic emissions (DPOAEs) and DPOAE input/output (I/O) curves to assess the effects of smoking on cochlear function. Twenty-four healthy adults, 12 smokers and 12 nonsmokers in the 20-30 years age range were selected based on self-reported histories of five to eight years of smoking or no smoking, respectively. All subjects received tympanometric screening to rule out middle ear pathology. Conventional (0.25-8 kHz) and ultra high frequency (UHF; 10-20 kHz) audiometry showed normal or age-appropriate thresholds across both groups. DPOAE results showed small, but significant, decline in DPOAE levels without concomitant changes in noise floors in smokers as compared to nonsmokers. I/O detection thresholds were also significantly elevated at high frequencies in smokers as compared to their nonsmoking counterparts. These findings indicate that smokers are at greater risk for cochlear damage than nonsmokers, and that DPOAE amplitudes and I/O detection thresholds may identify early changes in cochlear function in smokers.

Connexin 43 expression in glial cells of developing rhombomeres of Xenopus laevis.

Connexin 43 is a gap junctional protein found predominantly in astrocytes. In the mammalian nervous system, it appears to play an organizational role during neural development. In the current study, conducted on the frog, Xenopus laevis, we found that connexin 43 occurs in glial cells during development of rhombomeres and that its expression is spatially and temporally regulated. We used neural (2G9) and cell proliferation (BrdU) markers to identify the overall organization of Xenopus rhombomeres and then tracked expression of connexin 43 and glial fibrillary acidic protein, an intermediate filament protein known to mark glia during rhombomeric development. 2G9 was expressed in rhombomeric centers (ventricular concavities) and outlying neuropil regions, whereas BrdU-labeled cells marked boundary regions (ventricular convexities), as early as stage 35/36. These labeling patterns persisted through premetamorphic stages of hindbrain development. At stage 47, 2G9-labeled profiles were highlighted by the presence of connexin 43, and at stage 49/50, connexin 43-labeled profiles, i.e., rhombomeric centers and neuropil, as well as rhombomeric boundaries, not labeled by connexin 43, became immunoreactive to glial fibrillary acidic protein. Cells of rhombomeric center regions and their processes in the outlying neuropil co-expressed glial fibrillary acidic protein and connexin 43 at a time that is characterized by the emergence of hindbrain auditory neural circuitry. Glial fibrillary acidic protein positive glial cells that appeared at rhombomeric boundaries never expressed connexin 43, but rather appeared to physically bisect ventricular convexities into adjacent rhombomeric regions. Thus, glial cells that express connexin 43 in developing rhombomeric centers may be similar to radial glia, assisting in formation of neural circuitry, while glial cells that do not express connexin 43, situated at rhombomeric boundaries, may be involved in demarcating adjacent rhombomeres.

Prenatal smoke exposure: effects on infant auditory system and placental gene expression.

Prenatal smoke exposure has been shown to change cochlear echo response amplitudes and auditory brainstem response (ABR) wave latencies in newborns. Since gene expression changes are often synchronized in different tissue types, the goal of the present work was to determine the relationships between prenatal smoke exposure induced changes in hearing responses with changes in placental gene expression. Results showed significant cotinine level elevations in mothers who smoked ≥10cigarettes/day during their pregnancy compared to no detectable cotinine in nonsmoking mothers. Cochlear echo response amplitudes in the 2-8kHz range and ABR wave latencies, specifically wave V and interpeak interval I-V, were also significantly reduced in newborns of smoking mothers. Functional pathway analysis of upregulated placental genes using the Database for Annotation, Visualization and Integrated Discovery (DAVID) online software showed significant enrichment of terms associated with neurodevelopmental processes including glutamatergic and cholinergic systems and a number of wingless type proteins in the top two tiers with corrected enrichment p-values of ≤0.05. Other relevant functional pathways were significant at unadjusted enrichment p-values of 0.001-0.11 and included calcium signaling, neurotransmission/neurological processes and oxidative stress. The neurological process clusters included 7 genes (EML2, OTOR, SLC26A5, TBL1X, TECTA, USH1C and USH1G) known to modulate cochlear outer hair cell motility. We localized proteins encoded by the top two regulated genes, TBL1X and USH1C, using immunohistochemistry to placental stem and anchoring villi associated with active contractile function. These placental genes may mediate active contraction and relaxation in the placental villi, for example, during maternal-fetal perfusion matching, similar to the active lengthening and shortening of the cochlear outer hair cells during sensory transduction. Thus, the functional consequence of their alteration in the cochlea would be reflected as a decline in cochlear echoes as shown in this study. Such parallel changes suggest the potential utility of placental gene expression as a surrogate for evaluating changes in the developing cochlea related to potential aberrant cochlear function in newborns with prenatal smoke exposure.

Aroclor 1254 impairs the hearing ability of Xenopus laevis.

In this study we assessed the effects of chronic, dietary exposure of Aroclor 1254 (A1254) on the hearing of Xenopus frogs. We used the auditory brainstem response (ABR) to assay changes in hearing physiology; ABR thresholds, as well as latency-intensity and amplitude-intensity profiles of the initial positive (P1) and negative (N1) peaks were measured. Two groups of animals that received 50 ppm and 100 ppm of A1254 in their diet from 5 days post-fertilization through metamorphosis were compared to a control group that received untreated chow. The results showed significant threshold elevations in the 3-4 kHz range and significantly delayed peak latencies and reduced amplitudes at these frequencies in A1254 treated animals as compared to control animals. These findings indicate that A1254 selectively damages the high-frequency sensorineural hearing system associated with the basilar papilla of frogs. This preferential damage may be related to inherent differences in the vulnerability of the basilar versus amphibian papilla in the frog. The overall results of this study are also consistent with the reported A1254-induced auditory deficits in mammals indicating that the basilar papilla of the Xenopus frog may serve as an effective model for studying the effects of A1254 on the auditory system.

Auditory brainstem responses to airborne sounds in the aquatic frog Xenopus laevis: correlation with middle ear characteristics.

In this study we recorded auditory brainstem responses to airborne sounds to determine the hearing sensitivity of Xenopus laevis frogs and correlated their hearing profiles with middle ear characteristics. In newly metamorphosed frogs (body mass 0.5-0.76 gm, snout-vent length 17-20 mm) best hearing sensitivities were measured in the 2.4-2.8 kHz range, whereas optimal hearing sensitivity of older adults (body mass 18-90 gm; snout-vent length 57-100 mm) ranged from 1.0 to 1.2 kHz. Middle ear volumes reconstructed from serial sections showed approximate volume of 0.002 cc and 0.04-0.07 cc in newly metamorphosed and older frogs, respectively. This inverse frequency-volume relationship is consistent with the properties of an acoustic resonator indicating that differences in best hearing sensitivity are at least in part correlated to variation in middle ear volumes for airborne sounds. These results are consistent with peak frequency vibrational velocity profiles of Xenopus tympanic disk that have been shown to be dependent on underlying middle ear volumes and corroborate the occurrence of peak amplitudes of otoacoustic emissions in the 1.0-1.2 kHz region in adult Xenopus frogs.