Mechanical explanation of the on-off effect (diphasic impedance change) in otospongiosis.

Impedancemetry gives important information in otospongiosis, both for relative compliance and impedance audiometry. The detection of the stapedius reflex is facilitated by recruitment in some audiometric types of otospongiosis and enables us to solve many of the masking problems which are particularly difficult in this disease due to the very important cross-hearing (or cross-over). Diphasic impedance changes (on-off effect) once detected must be systematically investigated since they allow us to detect otospongiosis early on. This sign is absolutely characteristic of the onset of a stapediovestibular involvement and is of primary interest in this disease because of its early appearance - before any classical audiometric change and any air-bone gap, i.e. before deafness. Diphasic impedance changes must thus be systematically investigated in case of so-called unilateral otospongiosis, in the audiometrically normal ear, and for the detection of otospongiosis in otospongiotic families. It is also helpful in confirming the otospongiotic origin of sensorineural losses before an air-bone gap appears. Our experience with routine stapedectomies and impedance audiometry measurements have led us to a purely mechanical explanation of the on-off effect, i.e. the response of an elastic system to a deformation with a different mechanism of the two inverse peaks, with a different mass and thus a different inertia. This mechanical explanation fits perfectly with anatomical findings. Everything occurs as if the evolutive stages of the onset of the stapediovestibular involvement were defined in impedancemetry, starting from the normal stapedius reflex, first by mixed forms, then by a pure on-off effect, finally by the disappearance of the stapedius reflex.