[Anatomical course of the chorda tympani nerve in middle ear surgery: clinical classification and relevance for...

The anatomical course of the Chorda tympani nerve (CTN) in the middle ear is known to be very variable, which becomes most obvious during middle ear operations. As well, postoperative gustatory dysfunction belongs to the most frequent complications after otosurgical interventions. The aim of this study was to develop a clinical classification of the intraoperatively visible course of the CTN and to demonstrate a possible correlation of its anatomic course with the prevalence of actual postoperative taste disorders.

Only cases of first-time ear surgery all operated by one experienced otosurgeon were assessed. Preoperatively, gustatory testing with standardised chemical solutions (sweet, sour, salty, bitter) was performed. During surgery the course of the CTN was classified at first sight by 3 topografical criteria: The angle (A), the height (H) and the depth (D) of the nerve's emergence from the lateral tympanic wall. In cases of postoperative taste dysfunction the patients were followed-up on a regular basis.

103 patients were included in the study. The angle of the nerve;s emergence from the tympanic bone was below 30 degrees in 59 (A1), between 30 degrees and 60 degrees in 41 (A2) and within 60 degrees and 90 degrees in 3 cases (A3). The height of emergence was 33 times in the superior (H1), 61 times in the middle (H2) and 9 times in the inferior third (H3) of the posterior meatal wall. The depth of emergence lay within the plain of the tympanic sulcus in 26 individuals (D1), 44 times medially to this plain without overlapping bone (D2) and 33 times medially but covered by a bony spur (D3). The CTN was preserved in 92 procedures. Postoperatively, gustatory function was diminished in 7% of the cases.

This study confirms the high diversity of the surgical anatomy of the CTN. According to the proposed classification the A1-H1-D2/3-situation belongs to the CTN-constellations observed most frequently. It is this very anatomical situation, that apparently tends to be associated with postoperative taste deficits quite regularly due to inevitable intraoperative manipulations. Taking into account the results of this study an otosurgeon might be able to anticipate the potential risk for the CTN by judging the AHD-status intraoperatively and - therefore - reduce taste disorders postoperatively. Additionally, the probability of a postoperative gustatory dysfunction depends upon the kind of underlying middle ear disease as well.