Measuring cochlear duct length in Asian population: worth giving a thought!

The anatomy of the cochlea forms the basis for a successful cochlear implantation. Cochlear duct length (CDL) is defined as the length of the scala media as measured from the middle of the round window to helicotrema. Preoperative measurement of CDL is particularly important when precise intracochlear electrode array placement is desired. It can be done both histologically and radiologically. Preoperative high-resolution computed tomography (HRCT) scan which forms an integral part of cochlear implant workup is a useful tool to calculate CDL using 3D reconstructions.

This study was done in SMS Medical College and Hospital, Jaipur, India, which is a tertiary care hospital and referral centre for cochlear implants. HRCT temporal bones of all children less than 6 years of age, with congenital bilateral severe-to-profound SNHL who were being worked up for cochlear implant were studied and analysed. 124 patients (56 females and 68 males) with hearing loss were evaluated for cochlear implantation. HRCT temporal bone of these patients was analysed and a variable A was measured which is defined as the linear measurement from the round window to the farthest point on the opposite wall of the cochlea on a reformatted CT scan slice.

Mean of distance A for right ear of these patients was 8.10 mm (range 7.7-9.2 mm). Mean for the same in left ear of these patients was 8.14 mm (range 7.7-9.0 mm), giving an overall average of 8.12 mm. Using the formula, CDL = 4.16A-3.98, we calculated the length of cochlear duct. Mean cochlear duct length was 29.8 mm with a range from 28 to 34.3 mm.

To the best of our knowledge, this is the first large sample study of cochlear length in population of this part of the world. A smaller cochlear length in this part of the world as compared to the Caucasian cochlear duct is a significant finding in understanding of the cochlear anatomy and physiology. It would also have great implications on the insertion depth in cochlear implantation.