Summating-potential/action-potential ratio in normal ears: Effects of dehydration

Introduction: Studies using electrocochleography (ECoG) in patients with Meniere's disease and in patients with perilymph fistula have demonstrated abnormally large summating-potential (SP) components compared with the amplitude of the action-potential (AP). The possible influence of normal physiological variation, methodological differences, and test-retest variations are often difficult to interpret. The purpose of this study is to specify what constitutes a clinically important change in the SP AP ratio in normal hearing subjects under the condition of dehydration induced by urea. Materials and Methods: Baseline ECoG was performed on 20 normal hearing volunteer subjects after an 8 hour fast. Urea, 20 g, was administered orally, following which recordings were made at 60 and 90 minutes. The majority of subjects were found to have maximal SP AP changes at 60 minutes post dehydration; therefore, only these results were included in the final analysis. Subsets of data were statistically evaluated. Results: In all ears but one, the preingestion SP AP ratio was less than or equal to 0.37. Male and female data were shown to comprise separate populations both before and following dehydration. The 95% upper limit for baseline SP AP ratio was 0.39 for males and was 0.25 for females. The gender-related difference remained after dehydration. The mean male SP AP ratio showed a statistically significant but slight decrease whereas the female SP AP ratio was unaltered. Conclusion: These data demonstrate a statistically significant gender-related difference in SP AP ratio in normals. The upper limit of normal is 0.39 for males and 0.25 for females. A basis for the gender difference is puzzling and requires further investigation. Dehydration produces a statistically significant reduction in SP AP ratio in men. This change was not observed in women.