What Is Impedance Testing?
This testing allows the audiologist to measure how well the eardrum is vibrating when the sound strike and how the tiny bones of the ear are working to transmit those vibrations to the organ of hearing.
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It also helps to measure the pressure in the space behind the eardrum that contains those bones. For tympanometry, a soft rubbery tip is inserted in the ear and the pressure in the ear canal is changed, resulting in a feeling like going up in an airplane.
Finding abnormal things on this test may indicate a conductive loss requiring a referral for medical evaluation. An example of a hearing problem with abnormal tympanometry would be a patient with fluid in the ear (otitis media).
Another part of the impedance test battery is an acoustic reflex measurement. Acoustic reflex measurement offers the audiologist to further properly examine the integrity of the middle ear system.
For impedance test loud tones are presented to each ear, to determine whether an acoustic reflex is present or absent, without requiring any response from the patient.
People with the normal hearing, the stapedius muscle found in the middle ear contracts involuntarily in response to loud sounds or tones presented at levels of about 80-100 decibels (dB).
Method of Impedance Testing
Screening for hearing the loss in English children at the entry to school (age 5-6 years) is usually by pure tone audiometry sweep undertaken by school nurses. This study aimed to compare the durability and screening number rates of pure tone audiometry with impedance screening in these children.
The pure tone audiometry and impedance test of screening were compared in 610 school entry children from 19 infant schools in northeast England. Both procedures were completed by school nurses.
The results of screening were validated against the subsequent clinical assessment, including otological examination and actions taken by an independent assessor. Both methods produced broadly similar validation indices after two stages of screening: sensitivity was 74.4% for both methods; specificity was 92.1% and 90.0%; and predicted values of a positive test 42.2% and 35.6% respectively for pure tone audiometry and impedance methods.
The single-stage screening method in both produces higher validity and sensitivity but lower specificity and predictive values of a positive test than two-stage screening. Screening rates were higher with impedance methods in comparison with pure tone audiometry.
In choosing the appropriate method to be used for testing, it must be borne in mind that the impedance method is technically more efficient but takes longer than pure tone audiometry screening. However, the latter method allows an opportunity for other health inquiries in these children.
The study included twenty adult normal hearing subjects of either sex with an age range of 17 to 28 years. Subjects’ selection criteria were normal hearing, with no external and middle ear pathology on clinical examination.
The impedance audiometry was carried out in sound-treated rooms of the Speech and Hearing Unit of ENT OPD, PGIMER, Chandigarh. The equipment used for impedance audiometry was SD 30 tympanometer. The 226 Hz probe-tone was used for tympanometric measurements.
The pressure in the external auditory canal was varied from +200 to -300 data at the rate of 200 data per second. The parameters noted were compliance, ear canal volume (or base volume), and middle ear pressure. The acoustic reflex testing was also conducted. The change in compliance denoted the acoustic reflex in the monitored ear with the probe tip.
In previous studies comparing impedance testing screening with pure tone audiometry found that the sensitivity and productive value of a positive test in two stages impedance testing were better than with pure-tone audiometry. Impedance screen was carried out by a doctor, while the school nurse undertook pure-tone testing.
The apparent superiority of impedance was further examined in a study of children entering infant school who were screened by nurses using both methods.
The aimed therefore to compare the process and outcomes of pure-tone audiometry and impedance investigations undertaken by a school nurse in children entering school. An additional objective was to compare the cost of the two methods.
Observation And Results
A total of twenty subjects with normal hearing and healthy external & middle ear were included in the study. The subjects with age range 17 to 28 years of either sex were assessed for tympanometry and acoustic reflex test in both the ears. Thus the total number of ears assessed was forty (40 ears).
Table 1 shows that highly significant differences were observed between two positions of the probe-tip for ear canal volume. Differences were also observed for middle ear pressure in two positions but could not reach the statistical significance level. Similar results were seen for compliance differences.
In choosing the method to be used, it must be borne in mind that the impedance testing is technically more efficient, but takes longer than pure tone audiometry screening. However, the latter testing allows the opportunity for other health inquiries in these children.
The present study based on data of 40 years, shows that the placement of probe-tip affects the measurements of tympanometry and acoustic reflex testing to some extent. The change of compliance during acoustic reflex is the most affected parameter by a shift of probe-tip position during impedance audiometry.
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