The clinical anatomy of the chorda tympani: a micro-CT study

Abstract

Background: Few studies have attempted to define the intraosseous course of the chorda tympani (CT) a bilateral nerve that conveys taste from the anterior two-thirds of the tongue and provides parasympathetic innervation to the submandibular and sublingual salivary glands. This is surprising given that iatrogenic injury of the CT is a well-known complication of middle ear surgery and results in taste disturbance, dry mouth, and unpleasant sensory symptoms.

Aims: The principle aim of the study was to accurately define the posterior canaliculus (PC) of the CT in the temporal bone to provide guidance for middle ear surgery. A secondary aim was to evaluate the proximity of the PC to a groove which is drilled in the temporal bone during the insertion of a subannular ventilation tube (an otosurgical procedure).

Methods: 40 cadaver temporal bones from 27 cadavers (15 male, mean age 75 years, 13 bilateral) were scanned using a SkyScan micro-CT scanner. 3-D multiplanar reconstructions were generated using the software platform Amira 4.1. Images were oriented to Reid’s axial, sagittal and coronal planes using data from the orientation of the semicircular canals and a mathematical algorithm. The PC of the CT was measured in relation to reproducible bony landmarks, the stylomastoid foramen, tympanic sulcus, external auditory meatus and facial nerve canal, using image processing programmes ImageJ and Fiji, and Amira. In the applied follow-up study, 10 of the 40 temporal bones were drilled by an ear surgeon as for subannular ventilation tube placement. The temporal bones were then reimaged and oriented to Reid’s planes as above and the proximity of the drilled groove to the PC analysed. The angulation of the tympanic membrane was also measured in all 40 cadaver temporal bones.

Results: Quantitative data showed that the CT originated from the facial nerve outside the skull in 6 specimens and from within the facial canal in 34. In the latter, the PC originated a mean of 3.2 ± 1.8mm above the stylomastoid foramen at an angle of 18 ± 9° to the sagittal plane and 13 ± 6° to the coronal plane. The posterior canaliculus was 12.3 ± 3.8mm long with maximum and minimum diameters of 1.2 ± 0.2mm and 0.4 ± 0.1mm, respectively. As the PC travelled cranially it became closer to the tympanic sulcus and external auditory meatus and more distant from the facial nerve canal. It entered the middle ear at a height 62 ± 10% of the height of the tympanic membrane. In the applied study, drilling the temporal bone for subannular ventilation tube placement had the potential to cause iatrogenic injury in two cases (20%). The tympanic membrane was found to be angled at a mean value of 34° (range 16 to 45°) to the parasagittal plane and 42° (range 22 to 58°) to the axial plane.

Conclusions: This novel micro-CT study defines the precise anatomy of the PC which houses the CT. These data and those from the applied study relating to subannular tube placement may help the surgeon protect the CT from iatrogenic injury. This thesis has also documented, more precisely than ever before, the angle of the human tympanic membrane which lies at a greater angle to the axial and parasagittal planes than previously reported.