Sweep frequency impedance measurements for evaluation of middle ear function in young infants

Abstract

Assessing outer and middle ear status in neonates and young infants is a challenging task for audiologists and medical professionals. Although standard tests such as 226-Hz tympanometry are used successfully to evaluate the function of the outer and middle ear in older children and adults, they are not diagnostically accurate for young infants less than 7 months of age (Kei & Zhao, 2012). Sweep frequency impedance (SFI) is a new emerging technique which can provide useful information about the dynamic behaviour of the outer and middle ear in neonates and young infants. From this dynamic behaviour, the resonance frequency and mobility of the outer ear and middle ear in young infants can be measured. While there are limited case reports and pilot studies exploring the use of SFI with neonates (Murakoshi et al., 2013; Murakoshi, Zhao, & Wada, 2012), further systematic investigation in the clinical applications of SFI in neonates and young infants is essential before SFI can be used as a diagnostic tool for detecting conductive conditions in this population. The present research study aimed to (1) establish normative SFI data for healthy Australian neonates, (2) measure the effect of ear canal static pressure on the dynamic behaviour of the outer and middle ear in healthy newborns, (3) investigate the developmental characteristics of SFI measures in infants from birth to 6 months of age, (4) compare SFI measures obtained from healthy Australian Aboriginal infants with that obtained from Caucasian infants, and (5) evaluate the test performance of SFI against individual and test battery reference standards. Normative SFI data were developed for healthy Australian neonates (Chapter 2). The results revealed two regions of resonance, with the first resonance occurring at 287 Hz, possibly related to outer ear canal wall movement, and the second resonance occurring at 1236 Hz, possibly related to middle ear resonance. The effect of ear canal static pressure on the dynamic behaviour of 122 ears of 86 healthy newborns and 10 ears of 10 newborns with middle ear dysfunction was studied using SFI (Chapter 3). Application of either a positive or negative static pressure to the ear canal of healthy newborns increased the resonance frequency but decreased the mobility of the outer ear and middle ear. In contrast, in ears with middle ear dysfunction, the resonance of the middle ear was absent with no mobility of the middle ear under various static pressures. Application of negative pressure up to minus 200 daPa resulted in collapsed ear canals in more than 90% of ears. Developmental characteristics of SFI data were obtained from 83 healthy infants from birth to 6 months using a cross-sectional study design (Chapter 4). Mean resonance frequency of the outer ear increased from 279 Hz at birth to 545 Hz at 4 months, while the mobility of the outer ear decreased with age. In comparison, the mean resonance frequency and mobility of the middle ear did not change significantly with age from birth to 6 months. Despite Australian Aboriginal children having a higher prevalence of otitis media than Caucasian children, very few studies have compared the acoustic-mechanical properties of the outer and middle ear between Aboriginal and Caucasian neonates. SFI data from 40 ears of 24 Aboriginal neonates were compared with that from 160 ears of 119 Caucasian neonates (Chapter 5). Despite passing the test battery, Aboriginal neonates had significantly lower resonance frequencies of the outer and middle ear than Caucasian neonates. Furthermore, 22.5% of Aboriginal neonates showed no middle ear resonance, indicating the possibility of subtle conductive conditions not detected by the test battery. The predictive accuracy of SFI in identifying conductive conditions in neonates against 4 single reference standards [automated auditory brainstem response (AABR), high frequency tympanometry (HFT), transient evoked otoacoustic emissions (TEOAE), and distortion product otoacoustic emissions (DPOAE)] and 5 test batteries standards (HFT+DPOAE, HFT+TEOAE, DPOAE+TEOAE, DPOAE+AABR and TEOAE+AABR) was evaluated (Chapter 6). The predictive accuracy of SFI was highest when measured against the HFT+DPOAE test battery reference standard, with an area under the receiving operating characteristic curve (AROC) of 0.87. The corresponding sensitivity was 86% and specificity was 88%, with positive likelihood ratio of 7 and negative likelihood ratio of 0.2. Since SFI is an accurate and valid measure of outer and middle ear function in neonates, it may be used for both screening and diagnostic assessments in neonates. In conclusion, this thesis has not only confirmed the feasibility of testing neonates and young infants using the SFI technique, but it has also expanded the clinical application of SFI to detecting conductive conditions in this population. While the SFI technology has shown promising results when assessing young infants, further research is needed to improve the instrumentation and test protocol for screening and diagnostic purposes.