Pure tone audiometry (PTA) Sensorineural hearing loss

figure 7: response of basilar membrane pure tone.



figure 8: response of basilar membrane pure tone, when there dead region.

however, appears not case. dead regions cannot found via pta audiograms. may because although neurons innervating dead region, cannot react vibration @ characteristic frequency. if basilar membrane vibration large enough, neurons tuned different characteristic frequencies such adjacent dead region, stimulated due spread of excitation. therefore, response patient @ test frequency obtained. referred “off-place listening”, , known ‘off-frequency listening’. lead false threshold being found. thus, appears person has better hearing do, resulting in dead region being missed. therefore, using pta alone, impossible identify extent of dead region (see figure 7 , 8).

consequently, how audiometric threshold affected tone frequency within dead region? depends on location of dead region. thresholds @ low frequency dead regions, more inaccurate @ higher frequency dead regions. has been attributed fact excitation due vibration of basilar membrane spreads upwards apical regions of basilar membrane, more excitation spreads downwards higher frequency basal regions of cochlea. pattern of spread of excitation similar ‘upward spread of masking’ phenomenon. if tone sufficiently loud produce enough excitation @ functioning area of cochlea, above areas threshold. tone detected, due off-frequency listening results in misleading threshold.

to overcome issue of pta producing inaccurate thresholds within dead regions, masking of area beyond dead region being stimulated can used. means threshold of responding area sufficiently raised, cannot detect spread of excitation tone. technique has led suggestion low frequency dead region may related loss of 40-50 db. however, 1 of aims of pta determine whether or not there dead region, may difficult assess frequencies mask without use of other tests.

based on research has been suggested low frequency dead region may produce relatively flat loss, or gradually sloping loss towards higher frequencies. dead region less detectable due upward spread of excitation. whereas, there may more obvious steeply sloping loss @ high frequencies high frequency dead region. although slope represents less pronounced downward spread of excitation, rather accurate thresholds frequencies non-functioning hair cells. mid-frequency dead regions, small range, appear have less effect on patient’s ability hear in everyday life, , may produce notch in pta thresholds. although clear pta not best test identify dead region.