Diagnostic Audiological Tests
Air Conduction and Bone Conduction Comparison
Auditory Evoked Potentials 19-21
- Pure-tone threshold audiometry is the gold standard for diagnostic audiological testing
- From pure-tone air conduction (A/C) and bone conduction (B/C) thresholds, the degree or magnitude (mild, moderate, severe, profound) configuration (sloping, rising, flat) and type (conductive, mixed or sensorineural) of hearing loss are determined
- Results from pure-tone threshold testing are also used to determine parameters and help in interpreting results from additional diagnostic tests.
- The presentation levels for some behavioral diagnostic test are set based on pure-tone thresholds and the classification of speech recognition scores into cochlear or retrocochlear for a given patient is based on the pure-tone threshold sensitivity
- Many conditions have stereotypical findings
- meniere's - low frequency rising sensorineural hearing loss. as the disease progresses, loss changes to flat and then sloping in the latter stages. accompanying the hearing loss are symptoms of low-frequency (roaring) tinnitus, vertigo and nausea
- noise induced hearing loss - gradually when ears are exposed to high intensity sounds. in the region of 4-6kHz with improvement at 8kHz
- presbycusis - advancing age, sensorineural, high frequencies only and gradually invades into the lower frequency
- otosclerosis - spongy material develops in the bony capsule of the inner ear and on the footplate of the stapes which results in a conductive hearing loss. with progression it becomes mixed and sloping. bone conduction thresholds improve with surgery.
- assesses the patient's ability to respond to standardized material for threshold assessment and (SRT) speech perception ability with words (WRS)
- speech audiometry provides data to assess the validity of pure-tone thresholds and obtain diagnostic information regarding the site of lesion
- validation of pure-tone data is accomplished by comparing thresholds for speech materials, the SRT, to thresholds for pure tones in the 500 to 2000 Hz range (PTA - pure-tone average). such comparison should be within 6-8 dB. whenever a difference exists between the SRT and PTA the validity of the results are in question and the equipment, procedures used and patient cooperation must be checked.
- diagnostic information from speech audiometry can be obtained by comparing word and speech intelligibility scores at different intensities
- the shape of the performance intensity (PI) function and the difference in the functions for words and sentences provides diagnostic information
- normal - 90-100%
- flat - identical PI functions
- sloping high-frequency losses, PI function is reduced
- rising PI is elevated
- eighth nerve site
- direction and magnitude of PI fucntion is not predicatable but significant PI rollover occurs. rollover is defined as a significant reduction in PI scores that occurs at high intensities. That is, scores reach a maximum in the midrange intensities; when the intensity is increased a sharp decrease in performance occurs.
- Middle ear measurement is performed with immittance instruments.
- Immittance instruments do not assess hearing, they provide objective information on the mechanical transfer function of sound in the outer and middle ear and depending on the status of the outer and middle ear and cochlear function provide data on the integrity of auditory nerve function at the level of the brainstem
- with the immittance test battery it is possible to obtain diagnostic information on the status of the outer and middle ear, cochlear function and the integrity of the eight nerve and brainstem by obtaining a tynmpanogram as well as static measures of middle ear function and acoustic reflexes to ipsilateral and contralateral stimulation
- technological advances have automated the middle ear assessment technique to the point that microprocessor immittance equipment can now complete a middle ear assessment test battery for both ears in a matter of minutes
Auditory Evoked Potentials 19-21
- Auditory stimuli are capable of evoking responses in the electrical activity of the central nervous system and with the use of computer technology can be recorded from electrodes placed on the head.