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Focus On: Complete Cochlear Coverage

 

The Benefits of Complete
Cochlear Coverage (CCC)

 MED-EL’s Standard Electrode Array is the first and the only array in the industry to provide stimulation of the entire cochlea. This degree of stimulation includes the apical region which is considered to be the last five millimetres of the scala tympani. This region contains a dense network of peripheral processes spiralling from the tip of the modiolus located in the second turn towards the Organ of Corti. Studies demonstrate that by including the apex in the range of stimulation improves speech discrimination and sound quality among CI recipients.
Complete Cochlear Coverage
Neural structures in the apex
In the human cochlea, the highest density of neural innervation is located 18 to 24 mm from the base1. Between 24 and 31 mm, a high concentration of neural tissue is found in the form of dendrites. The insertion depth and angle of an electrode array determine the amount of neural tissue that will be stimulated. An electrode array of only 20 mm stimulates a restricted frequency range. In contrast, a long electrode array provides Complete Cochlear Coverage, and thus, stimulation of all neural tissues. Implant candidates frequently demonstrate some limited degree of residual hearing in the low frequency region, which suggests that excitable neural tissue is probably present in this region. A long electrode array allows stimulation of these neural structures. 

Matching place to pitch
A long electrode array, reaching the apex, offers the opportunity to provide a better match between the incoming signal and the physical location of the stimulating electrode. A more shallow insertion results in a mismatch between the low frequency output filter (pitch) and the location of stimulation (place).2

In a unique group of subjects who had normal or near-normal hearing in the contralateral ear, Vermeire et. al. showed that MED-EL implanted subjects demonstrated the pitch sensations produced by electrical stimulation that on average closely resembled Greenwood’s frequency-place function.2 They conclude that the results do not deviate significantly from acoustic frequency maps, i.e., the pitch produced by electric stimulation at a certain location in the cochlea is equal to the pitch produced by acoustic stimulation in the same location.

In simulation studies, Dorman et al. found that the insertion depth of the most apical electrode made a difference in speech perception abilities.They postulated that a shallow insertion depth could be partially responsible for reports of “high pitched“ speech or poor intelligibility at initial stimulation. They conclude that insertion depth may affect the maximum level of performance achieved and the time required to reach that level of performance. 
Benefits of CCC
Results from a study underway at the University of Würzburg in Germany demonstrate that not only do a vast majority of patients prefer a map which includes activated apical channels, but in fact, their performance is superior in this condition. In this prospective study, 20 subjects who had a fully inserted MED-EL standard array, were given 2 maps at the time of initial stimulation: a basal 8 map intended to mimic the restricted cochlear coverage seen with most commercially available devices, and a spread 8 condition allowing for apical stimulation.

After 6 weeks of using each map for equal periods of time, subjects were tested and asked to declare their preferred map. Patients were then allowed to compare their preferred map to a map containing all 12 channels active for another 6 weeks. They were then given one of three 10 channel maps. At 18 weeks, 17 of the 19 subjects preferred and performed better in a condition that included all 4 apical channels or 2 of the 4 apical channels. Of particular interest is that this preference is closely correlated with performance.

These results have also been demonstrated in a similarly designed study conducted at the Medical University of Hannover. This study demonstrated that after several months of using an 8-channel map (Figure 1), newly implanted subjects with a MED-EL standard electrode array of 31.5 mm length improved in speech perception with stimulation over the entire length of the cochlea, including the apical region. At the end of the study, 11 out of 14 subjects chose to use all 12 channels; 3 subjects chose a 10 channel map where two of the apical channels were activated. These data suggests that the vast majority of cochlear implant recipients benefit from stimulation of the entire length of the cochlea.

Figure 1: Improved speech perception with
stimulation over the entire length of the cochlea

In order to obtain a single measure per subject, test score differences for every subject were averaged and plotted as one single data point per subject, with an ellipsoid around the mean value, calculated from the two standard deviations. The standard deviation of the differences spread-8 minus basal-8 gives the horizontal radius of the ellipsoid; the standard deviation of the difference full-12 minus best 8-channel condition gives the vertical radius. The three red ellipsoids indicate the three subjects that chose a 10-channel configuration at the end of the study, i.e. with two of the apical channels switched off. The other 11 subjects left the study with full-12.
Future considerations
Special attention to apical stimulation should also be given to the paediatric population. Infants and very young patients in need of an implant are most likely to have intact peripheral processes along the whole cochlear length. Electrical stimulation of peripheral processes throughout the life of the patient, and from base to apex, including the most apical region, could maintain the spiral ganglion cell population. Unstimulated peripheral processes may degenerate and accelerate the loss of spiral ganglion cells. It can also be hypothesised that stimulation of larger portions of the auditory nerve, as is achieved in deep electrode insertions, would activate a larger area of the auditory cortex. Full cochlear stimulation would therefore provide a better starting point for cortical development in prelingually deafened children.
  1. Spoendlin H and Schrott A. (1990) Quantitative evaluation of the human cochlea nerve. Acta Oto-Laryngol; 470: 61-70.
  2. Vermeire et al. (2008) Neural tonotopy in cochlear implants: An evaluation in unilateral cochlear implant patients with unilateral deafness and tinnitus. Hearing Research 245; 98-106.
  3. Dorman et al. (1997) Simulating the effect of cochlear-implant electrode insertion depth on speech understanding. J. Acoust. Soc. Am. 102 (5) Pt. 1 Nov 1997.
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