title: Summating-potential/action-potential ratio in normal ears: effects of dehydration.
abstract: BACKGROUND
  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.
  
  
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Otolaryngology, Head and Neck Surgery, University of Washington School of Medicine, Seattle.
authors.forename: B D
authors.initials: BD
authors.lastname: Ridenour
authors.forename: W L
authors.initials: WL
authors.lastname: Meyerhoff
authors.forename: X
authors.initials: X
authors.lastname: Wang
authors.forename: G M
authors.initials: GM
authors.lastname: Gerken
mesh_terms: D000200:Action Potentials; D000328:Adult; D001300:Audiometry, Evoked Response; D003681:Dehydration / Q000139:chemically induced / Q000503:physiopathology*; D004423:Ear / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D012737:Sex Factors; D014508:Urea
chemical_list: D014508:Urea