The chorda tympani nerve (CTN) is frequently damaged in various inflammatory middle ear disorders. To determine the validity of electrogustometry in the diagnosis of preoperative severity of a middle ear lesion of the posteosuperior portion of the mesotympanum, the threshold determined by electrogustometry was examined preoperatively and was correlated with the intraoperative findings.
One hundred and six ears of 97 patients with various middle ear diseases were examined. The patients ranged in age from 5 to 70 years (average, 39.4): 55 of them were male and 42 were female. In 80 of them the appearance of the tympanic membrane on the opposite side was normal. The disorders were classified as follows: A: Non-inflammatory diseases including middle ear anomalies, posttraumatic perforation of the tympanic membranes and congenital cholesteatoma without infection (n=10); B: chronic otitis media without cholesteatoma (n=14); C: pars flaccida retraction type cholesteatoma (n=34); D: pars tensa retraction type cholesteatoma and adhesive otitis media (n=15); E: surgically treated ears (n=33). The average threshold±S.D. (dB) of each group according to the above classification was as follows: A: -1.0±4.4, B: -1.5±6.2; C: 7.4±13.7; D: 9.6±18.2; E: 20.9±16.8. The rates of abnormal threshold of electrogustometry (more than 8dB) of A, B, C, D and Ewere 0 %, 7.1%, 44.1%, 46.7% and 72.7%, respectively. The intraoperative findings around the CTN were retrospectively classified according to the following 5groups: I: No pathological tissue was attached to the CTN (n=44); II: the Granulation tissue or matrix of the cholesteatoma was attached to the CTN (n=19); III: the CTN was surrounded by granulation or a matrix of cholesteatoma (n=3); IV: The CTN was not identifiable due to the severe pathological condition of the middle ear cavity, in either surgically treated or untreated ears (n=16), V: Surgically treated ears in which the CTN was identifiable (n=24). The average threshold ±S.D. (dB) for I, II, III, IV and V were 1.9±9.6, 2.0±11.7, 16.0±20.3, 32.0±7.4, 16.0 ±17.0, respectively. The rates of abnormal threshold of electrogustometry (more than 8dB) of I, II, III, IV and V were 27.3%. 31.6%. 66.7%, 100% and 62.5%, respectively. In both the above classification by the disorders and that by the intraoperative findings around the CTN, differences in the average thresholds between the surgically treated sides and the opposite sides with normally appearing tympanic membranes showed a tendency similar to that of the thresholds of the treated sides. In the present study, the threshold of electrogustometry was correlated with the disease and the intraoperative findings during middle ear surgery, and it was higher in adhesive otitis media and cholesteatoma of the pars tensa type. Electrogustometry is a useful preoperative examination to predict the condition of the posterosuperior portion of the mesotympanum.