Zum diagnostischen Wert der ipsilateral ausgelösten Impedanzänderung des Trommelfells
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Diagnostic reliability of ipsilateral evoked impedance changes of the tympanic membrane
The ipsilateral registration of impedance change is necessary in case of unilateral cochlear or retrocochlear disease with middle ear impairment of the other ear and, therefore, impossibility of contralateral evocation of the reflex. If contralateral evocation fails, moreover, it will be useful in order to clarify, whether the evocation did not succeed or the registration at the contralateral ear; thus enabling a differentiation between lesions of the reflexe's afferent and efferent way.
In ipsilateral impedance-change-measurements it should be distinguished between an increase of the acoustic impedance (— as searched for — turn to the right at the balancemeter), and a decrease of it (turn to the left at the balancemeter).
The “turns of the left” at ipsilateral evocation occur without latency and only at SPL 100 db or more. They can be observed in patients suffering from otosclerosis, complete deafness and in dissected temporal bones as well. This acoustic-mechanical effect rises at high sound pressures from the non-linearity of the sound-field, which produces an extra pressure unbalancing the pressure before and behind the tympanic membrane. This effect may interfere with the real reflex-response, but only at more than 100 db SPL; however, it must be distinguished from the proper response in order not to find the reflex falsely positiv.
In three clinical examples (two patients with multiple sclerosis, one patient with cerebello-pontine angle lesion), the absence of ipsilateral impedance reflexes (isolated or additional) is demonstrated.
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