Transtympanic electrode positioning: manubrium is a poor guide.
BACKGROUND
Although a transtympanic electrode is commonly used for electrocochleography and electrically evoked auditory brainstem response, the variability of responses among healthy subjects is wide. The manubrium is the typical guidepost for electrode placement. This study addresses the hypothesis that electrode position relative to the round window niche is widely variable.


METHODS
: Postmortem anatomic dissection of 41 bequeathed adult crania (82 temporal bones).


METHODS
Drill marks were made on the medial wall of the mesotympanum from 2 manubrium-based positions: 1.5 to 2 mm posterior to the umbo, and halfway between the umbo and the annulus posteroinferior.


RESULTS
Distances to the lip of the round window niche ranged from 0.8 to 3.5 mm and from 1.8 to 4.5 mm for the 2 electrode sites, respectively. The posteriorly determined site was uniform on the promontory, but the posteroinferior site was into hypotympanic trabeculations in 81% of ears and into the jugular plate in 6%. Distances from the electrode sites to round window niche were not obviously associated with either the orientation of the manubrium in the head or the mastoid size.


CONCLUSIONS
The location of a transtympanic positioned electrode using the manubrium as guidepost is not accurately predictable relative to the round window niche.
Todd
Norman Wendell
NW
Department of Otolaryngology-Head and Neck Surgery, Temporal Bone Laboratory, Emory University, Atlanta, Georgia 30322, USA. ntodd@emory.edu
D000328:Adult; D001300:Audiometry, Evoked Response; D002102:Cadaver; D004432:Ear, Middle / Q000033:anatomy & histology / Q000502:physiology*; D004567:Electrodes, Implanted; D007839:Functional Laterality; D006257:Head / Q000033:anatomy & histology; D006801:Humans; D008371:Manubrium / Q000033:anatomy & histology*; D008416:Mastoid / Q000033:anatomy & histology; D012405:Round Window, Ear / Q000033:anatomy & histology; D012886:Skull / Q000033:anatomy & histology; D013701:Temporal Bone / Q000033:anatomy & histology