Pathophysiology Sensorineural hearing loss

1 pathophysiology

1.1 cochlear dead regions in sensory hearing loss

1.1.1 cochlear hair cells

1.1.1.1 hair cell damage




1.2 neural tuning curves

1.2.1 frequency selectivity
1.2.2 ihc vs ohc hearing loss


1.3 dead region audiometry

1.3.1 pure tone audiometry (pta)
1.3.2 psychoacoustic tuning curves (ptc) , threshold equalizing noise (ten) tests
1.3.3 perceptual consequences of dead region


1.4 vestibulocochlear nerve pathology

pathophysiology

sensory hearing loss caused abnormal structure or function of hair cells of organ of corti in cochlea. neural hearing impairments consequent upon damage eighth cranial nerve (the vestibulocochlear nerve) or auditory tracts of brainstem. if higher levels of auditory tract affected known central deafness. central deafness may present sensorineural deafness should distinguishable history , audiological testing.

cochlear dead regions in sensory hearing loss

hearing impairment may associated damage hair cells in cochlea. there may complete loss of function of inner hair cells (ihcs) on region of cochlea; called dead region . region can defined in terms of range of characteristic frequencies (cfs) of ihcs and/or neurons adjacent dead region.

cochlear hair cells

figure 3: cross-section of cochlea.

outer hair cells (ohcs) contribute structure of organ of corti, situated between basilar membrane , tectorial membrane within cochlea (see figure 3). tunnel of corti, runs through organ of corti, divides ohcs , inner hair cells (ihcs). ohcs connected reticular laminar , deiters’ cells. there twelve thousand ohcs in each human ear, , these arranged in 5 rows. each ohc has small tufts of hairs , or cilia, on upper surface known stereocilia, , these arranged rows graded in height. there approximately 140 stereocilia on each ohc.

the fundamental role of ohcs , ihcs function sensory receptors. main function of ihcs transmit sound information via afferent neurons. transducing mechanical movements or signals neural activity. when stimulated, stereocilia on ihcs move, causing flow of electric current pass through hair cells. electric current creates action potentials within connected afferent neurons.

ohcs different in contribute active mechanism of cochlea. receiving mechanical signals or vibrations along basilar membrane, , transducing them electrochemical signals. stereocilia found on ohcs in contact tectorial membrane. therefore, when basilar membrane moves due vibrations, stereocilia bend. direction in bend, dictates firing rate of auditory neurons connected ohcs.

the bending of stereocilia towards basal body of ohc causes excitation of hair cell. thus, increase in firing rate of auditory neurons connected hair cell occurs. on other hand, bending of stereocilia away basal body of ohc causes inhibition of hair cell. thus, decrease in firing rate of auditory neurons connected hair cell occurs. ohcs unique in able contract , expand (electromotility). therefore, in response electrical stimulations provided efferent nerve supply, can alter in length, shape , stiffness. these changes influence response of basilar membrane sound. therefore clear ohcs play major role in active processes of cochlea. main function of active mechanism finely tune basilar membrane, , provide high sensitivity quiet sounds. active mechanism dependent on cochlea being in physiological condition. however, cochlea susceptible damage.

hair cell damage

snhl commonly caused damage ohcs , ihcs. there 2 methods might become damaged. firstly, entire hair cell might die. secondly, stereocilia might become distorted or destroyed. damage cochlea can occur in several ways, example viral infection, exposure ototoxic chemicals, , intense noise exposure. damage ohcs results in either less effective active mechanism, or may not function @ all. ohcs contribute providing high sensitivity quiet sounds @ specific range of frequencies (approximately 2–4 khz). thus, damage ohcs results in reduction of sensitivity of basilar membrane weak sounds. amplification these sounds therefore required, in order basilar membrane respond efficiently. ihcs less susceptible damage in comparison ohcs. however, if become damaged, result in overall loss of sensitivity.

neural tuning curves
frequency selectivity

figure 4: neural tuning curve normal hearing.

the traveling wave along basilar membrane peaks @ different places along it, depending on whether sound low or high frequency. due mass , stiffness of basilar membrane, low frequency waves peak in apex, while high frequency sounds peak in basal end of cochlea. therefore, each position along basilar membrane finely tuned particular frequency. these tuned frequencies referred characteristic frequencies (cf).

if sound entering ear displaced characteristic frequency, strength of response basilar membrane progressively lessen. fine tuning of basilar membrane created input of 2 separate mechanisms. first mechanism being linear passive mechanism, dependent on mechanical structure of basilar membrane , surrounding structures. second mechanism non-linear active mechanism, dependent on functioning of ohcs, , general physiological condition of cochlea itself. base , apex of basilar membrane differ in stiffness , width, cause basilar membrane respond varying frequencies differently along length. base of basilar membrane narrow , stiff, resulting in responding best high frequency sounds. apex of basilar membrane wider , less stiff in comparison base, causing respond best low frequencies.

this selectivity frequencies can illustrated neural tuning curves. these demonstrate frequencies fiber responds to, showing threshold levels (db spl) of auditory nerve fibers function of different frequencies. demonstrates auditory nerve fibers respond best, , hence have better thresholds @ fiber s characteristic frequency , frequencies surrounding it. basilar membrane said ‘sharply tuned’ due sharp ‘v’ shaped curve, ‘tip’ centered @ auditory fibers characteristic frequency. shape shows how few frequencies fiber responds to. if broader ‘v’ shape, responding more frequencies (see figure 4).

ihc vs ohc hearing loss

a normal neural tuning curve characterised broadly tuned low frequency ‘tail’, finely tuned middle frequency ‘tip’. however, there partial or complete damage ohcs, unharmed ihcs, resulting tuning curve show elimination of sensitivity @ quiet sounds. i.e. neural tuning curve sensitive (at ‘tip’) (see figure 5).

where both ohcs , ihcs damaged, resulting neural tuning curve show elimination of sensitivity @ ‘tip . however, due ihc damage, whole tuning curve becomes raised, giving loss of sensitivity across frequencies (see figure 6). necessary first row of ohcs damaged elimination of finely tuned ‘tip’ occur. supports idea incidence of ohc damage , loss of sensitivity quiet sounds, occurs more ihc loss.

when ihcs or part of basilar membrane damaged or destroyed, no longer function transducers, result ‘dead region’. dead regions can defined in terms of characteristic frequencies of ihc, related specific place along basilar membrane dead region occurs. assuming there has been no shift in characteristic frequencies relating regions of basilar membrane, due damage of ohcs. occurs ihc damage. dead regions can defined anatomical place of non-functioning ihc (such “apical dead region”), or characteristic frequencies of ihc adjacent dead region.

dead region audiometry
pure tone audiometry (pta)

dead regions affect audiometric results, perhaps not in way expected. example, may expected thresholds not obtained @ frequencies within dead region, obtained @ frequencies adjacent dead region. therefore, assuming normal hearing exists around dead region, produce audiogram has dramatically steep slope between frequency threshold obtained, , frequency threshold cannot obtained due dead region.

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.

psychoacoustic tuning curves (ptc) , threshold equalizing noise (ten) tests

although debate continues regarding reliability of such tests, has been suggested psychoacoustic tuning curves (ptcs) , threshold-equalising noise (ten) results may useful in detecting dead regions, rather pta. ptcs similar neural tuning curves. illustrate level of masker (db spl) tone @ threshold, function of deviation center frequency (hz). measured presenting fixed low intensity pure tone while presenting narrow-band masker, varying center frequency. masker level varied, level of masker needed mask test signal found masker @ each center frequency. tip of ptc masker level needed mask test signal lowest. normal hearing people when masker center frequency closest frequency of test signal (see figure 9).

in case of dead regions, when test signal lies within boundaries of dead region, tip of ptc shifted edge of dead region, area still functioning , detecting spread of excitation signal. in case of low frequency dead region, tip shifted upwards indicating low frequency dead region starting @ tip of curve. high frequency dead region, tip shifted downwards signal frequency functioning area below dead region. however, traditional method of obtaining ptcs not practical clinical use, , has been argued tens not accurate enough. fast method finding ptcs has been developed , may provide solution. however, more research validate method required, before can accepted clinically.

perceptual consequences of dead region

audiogram configurations not indicators of how dead region affect person functionally, due individual differences. example, sloping audiogram present dead region, due spread of excitation. however, individual may affected differently corresponding sloped audiogram caused partial damage hair cells rather dead region. perceive sounds differently, yet audiogram suggests have same degree of loss. huss , moore investigated how hearing impaired patients perceive pure tones, , found perceive tones noisy , distorted, more (on average) person without hearing impairment. however, found perception of tones being noise, not directly related frequencies within dead regions, , therefore not indicator of dead region. therefore suggests audiograms, , poor representation of dead regions, inaccurate predictors of patient’s perception of pure tone quality.

research kluk , moore has shown dead regions may affect patient’s perception of frequencies beyond dead regions. there enhancement in ability distinguish between tones differ in frequency, in regions beyond dead regions compared tones further away. explanation may cortical re-mapping has occurred. whereby, neurons stimulated dead region, have been reassigned respond functioning areas near it. leads over-representation of these areas, resulting in increased perceptual sensitivity small frequency differences in tones.

vestibulocochlear nerve pathology

congenital deformity of internal auditory canal,
neoplastic , pseudo-neoplastic lesions, special detailed emphasis on schwannoma of eighth cranial nerve (acoustic neuroma),
non-neoplastic internal auditory canal/cerebellopontine angle pathology, including vascular loops,




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