Electrocochleographic recordings in chronic and healed perilymphatic fistula

This study examined electrocochleographic recordings in 15 guinea pigs with perilymphatic fistulas. A chronic round window fistula was induced in each animal by insertion of a sigmoid stainless steel needle with the lumen intact. In the contralateral control ear, a similar needle was inserted with the lumen crushed. In this way the model compared acute (control group) versus chronic (experimental group) perilymphatic fisulae. Electrocochleographic recordings were obtained before, immediately after needle insertion, in a subset at two to three weeks after needle insertion, and again at 17-22 weeks when the fistulas had healed with the needles still in place. Minimal threshold changes were noted from the pre- to the acute post-condition. More marked elevation of threshold was noted for the 17-22 weeks condition. Threshold elevation was greater in the experimental ears but was present in the control ears. The SP/AP amplitude ratio was generally stable for the pre- to post-conditions in both ears. For the 17-22 week epoch the SP/AP amplitude ratio was still fairly stable for the click stimulus but the ratio diminished for the toneburst stimuli. For the subset analyzed at 7-21 days the SP/AP amplitude ratio tended to increase for both groups for all stimuli although AP thresholds were normal.