Basic characteristics of click-evoked otoacoustic emissions in infants and children.

Since Kemp [J. Acoust. Soc. Am. 64, 1386-1391 (1978)] first described click-evoked otoacoustic emissions (COAEs), researchers have advocated their use as an excellent tool for diagnosing hearing loss in infants and children. However, there are few detailed reports of COAEs in this population, and those that do exist suggest that there are age-dependent differences. The purpose of the current study was to determine basic characteristics of COAEs in infants, toddlers, children, and young adults and to define any differences among age groups. An additional goal was to ensure that spontaneous otoacoustic emissions (SOAEs) did not confound any possible developmental effects. COAEs and SOAEs were measured from one ear of 223 normal-hearing subjects. COAE input/ output functions indicated that children aged less than one year have higher COAE levels than older children and adults. Children aged 1-5 yr had higher COAE levels than those aged 12-17 yr and adults. These differences were independent of level and SOAE status, but were dependent on frequency. The results of this study suggest that different clinical norms may be necessary for children aged less than 6 years.

A comparison of transient-evoked and distortion product otoacoustic emissions in normal-hearing and hearing-impaired subjects.

The ability of transient-evoked otoacoustic emissions (TEOAEs) and distortion product otoacoustic emissions (DPOAEs) to distinguish normal hearing from hearing impairment was evaluated in 180 subjects. TEOAEs were analyzed into octave or one-third octave bands for frequencies ranging from 500 to 4000 Hz. Decision theory was used to generate receiver operating characteristic (ROC) curves for each of three measurements (OAE amplitude, OAE/noise, reproducibility) for each OAE measure (octave TEOAEs, 1/3 octave TEOAEs, DPOAEs), for octave frequencies from 500 to 4000 Hz, and for seven audiometric criteria ranging from 10 to 40 dB HL. At 500 Hz, TEOAEs and DPOAEs were unable to separate normal from impaired ears. At 1000 Hz, both TEOAE measures were more accurate in identifying hearing status than DPOAEs. At 2000 Hz, all OAE measures performed equally well. At 4000 Hz, DPOAEs were better able to distinguish normal from impaired ears. Almost without exception, measurements of OAE/noise and reproducibility performed comparably and were superior to measurements of OAE amplitude, although the differences were small. TEOAEs analyzed into octave bands showed better performance than TEOAEs analyzed into 1/3 octaves. Under standard test conditions, OAE test performance appears to be limited by background noise, especially for the low frequencies.

Regulation of skeletal muscle dihydropyridine receptor gene expression by biomechanical unloading.

Biomechanical unloading of the rat soleus by hindlimb unweighting is known to induce atrophy and a slow- to fast-twitch transition of skeletal muscle contractile properties, particularly in slow-twitch muscles such as the soleus. The purpose of this study was to determine whether the expression of the dihydropyridine (DHP) receptor gene is upregulated in unloaded slow-twitch soleus muscles. A rat DHP receptor cDNA was isolated by screening a random-primed cDNA lambda gt10 library from denervated rat skeletal muscle with oligonucleotide probes complementary to the coding region of the rabbit DHP receptor cDNA. Muscle mass and DHP receptor mRNA expression were assessed 1, 4, 7, 14, and 28 days after hindlimb unweighting in rats by tail suspension. Isometric twitch contraction times of soleus muscles were measured at 28 days of unweighting. Northern blot analysis showed that tissue distribution of DHP receptor mRNA was specific for skeletal muscle and expression was 200% greater in control fast-twitch extensor digitorum longus (EDL) than in control soleus muscles. A significant stimulation (80%) in receptor message of the soleus was induced as early as 24 h of unloading without changes in muscle mass. Unloading for 28 days induced marked atrophy (control = 133 +/- 3 vs. unweighted = 62.4 +/- 1.8 mg), and expression of the DHP receptor mRNA in the soleus was indistinguishable from levels normally expressed in EDL muscles. These changes in mRNA expression are in the same direction as the 37% reduction in time to peak tension and 28% decrease in half-relaxation time 28 days after unweighting. Our results suggest that muscle loading necessary for weight support modulates the expression of the DHP receptor gene in the soleus muscle.

Auditory brainstem responses elicited by 1000-Hz tone bursts in patients with sensorineural hearing loss.

Auditory brainstem responses were measured in response to 1000-Hz tone bursts from 115 patients with sensorineural hearing loss, presumably of cochlear origin. Mean wave V latencies and variability were comparable to those observed in normal hearing subjects for similar stimuli. The range of interaural differences in wave V latencies for 1000-Hz tone bursts were slightly greater than those observed for clicks, which may not be surprising, given the greater variability in wave V latencies for tonal stimulation, even in normal-hearing subjects. These differences, however, were not affected either by the magnitude or symmetry of hearing loss for frequencies at and above 1000 Hz. These data suggest that tone burst ABRs might be useful in otoneurologic evaluations, especially for patients with asymmetric hearing loss.

Localization ability in infants with simulated unilateral hearing loss.

This study investigated the feasibility of using a localization task to rule out unilateral hearing loss in infants. Fourcorner localization ability was assessed in 29 normal-hearing infants (9-20 mo) using four different test stimuli. In these same infants, a mild unilateral hearing loss was simulated by occlusion of the external auditory canal and the test sequence was repeated. Analysis of front-back, right-left, and combined errors for each of the test stimuli revealed that this type of task may allow detection of unilateral hearing loss as slight as 25 dB HL.