Soft surgery with EVO® electrode array preserves residual hearing

An ongoing challenge for CI manufacturers involves optimising protocols for hearing preservation following cochlear implant surgery so that they take into account the design and dynamics of different electrode arrays. The objectives of the present study were to evaluate a soft surgical procedure involving the use of dexamethasone and hyaluronic acid and to assess the efficacy of preserving residual hearing with a specifically designed straight soft electrode array—the Oticon Medical EVO®. Significant preservation of residual hearing was achieved in five patients for whom the soft surgery protocol could be followed. These results demonstrate the feasibility of hearing preservation surgeries with the EVO® electrode array1.

Professor Ricardo F. Bento and his colleague from the Department of Otolaryngology at the University of São Paulo in Brazil followed a soft surgery protocol in 7 candidates for cochlear implantation who were implanted with a Digisonic SP® implant with the EVO® straight soft electrode array1.
The objective of the study was to measure the preservation of residual hearing after a soft surgery protocol combining round-window insertion of an EVO® electrode array and intra-operative application of dexamethasone in the middle ear cavity, and to evaluate an anti-inflammatory steroid as a means of potentially minimising the intra-cochlear damage associated with electrode insertion. 

The EVO® electrode array

The EVO® electrode array 

The EVO® electrode array was designed in order to meet the constraints of hearing preservation surgeries. It is a long (24-mm), thin (proximal diameter = 0.5 mm; distal diameter = 0.4 mm), flexible array with a smooth silicone surface carrying 20 micro-machined titanium-iridium electrodes (Figure 1).


Figure 1. The Oticon Medical straight soft EVO® electrode array

Measuring residual hearing preservation

In order to evaluate the preservation of functional hearing, pre- and post-surgical pure-tone audiometry was evaluated using a calibrated clinical AC33 audiometer (Interacoustics, Assens, Denmark) in a double-walled sound booth. Post-surgical thresholds were then compared to pre-surgical measurements to evaluate any changes caused by cochlear implant surgery.

Average Pre vs. Post-op hearing thresholds


The soft surgery approach was shown to be successful in five patients. Individual results for the five patients who underwent a successful soft surgery and round window insertion protocol are presented first. 
In the soft surgery group, the different pre- and post-operative thresholds measured are shown in Figure 2. In this group, when comparing post-operative hearing thresholds (unaided) to the preoperative values, we found an average increase in thresholds of 18.7 dB HL of up to 500 Hz and up to 1 kHz in thresholds of 15.7 dB HL. These values are consistent with industry reported ranges for hearing preservation with soft surgeries. The averaged residual thresholds after surgery in this group of patients were 73 dB HL up to 500 Hz, values suitable for direct acoustic stimulation using a hybrid (AES) processor. 

Figure 2. Average pre vs. post-op hearing thresholds in the five soft surgery patients

When considering the data from the full cohort of patients represented in Figure 3, this diagram allows for direct visual comparison of any change between preoperative thresholds on the horizontal axis and post-operative thresholds on the vertical axis. The diagonal represents the equivalence line, and the points on the diagonal represent no pre- or post-operative variations in thresholds +/- 0 dB HL. Patients who experienced a decrement in post-operative thresholds are shown above the diagonal. The further away the point is from the diagonal, the greater the difference between pre- and post-operative measurements. 

The EVO® electrode array was designed in order to meet the constraints of hearing preservation surgeries

Results showed that two patients (P2 & P3, blue triangles on Figure 3) could not have a full electrode insertion through the round window and had to be excluded from the soft surgery group, as they subsequently underwent electrode insertion using a cochleostomy approach. These patients showed worse results compared to the patients from the soft surgery group (orange circles on Figure 3), with individual threshold decrements at 500 Hz of 60 and 65 dB HL, respectively. 
In the soft surgery group, however, four patients had threshold changes at 500 Hz of 0, 10, 10 and 15 dB HL, respectively, indicative of very good hearing preservation outcomes.

Figure 3. Individual (n = 7) pre- vs. post-operative thresholds chart

This preliminary experiment on hearing preservation with the EVO® electrode array showed that residual hearing was preserved in patients for whom a soft surgery protocol could be followed These results highlight the importance of surgical approach combined with electrode array specifications for successful hearing preservation in CI patients.

About the study

Soft surgery protocol for hearing preservation 
The proposed soft surgery protocol was developed by Professor Bento’s team2. In this protocol, first, a wide posterior tympanotomy is performed in order to expose the round window membrane. The middle ear cavity is then filled with a dexamethasone solution (4 mg/mL). The steroid solution is only removed after the implant receiver has been fixed with two titanium screws. The anterior border of the round window membrane is then delicately perforated, and hyaluronic acid is applied on the membrane and along the electrode array before slow insertion of the EVO® electrode array. 

Reference List:

1. Bento, R.F., Danieli, F., Magalhães, A.T., Gnansia, D., Hoen, M. (2016) Residual Hearing Preservation with the EVO® Cochlear Implant Electrode Array: Preliminary Results. International Archives Otorhinolaryngology. 16 Feb 2016. 
2. Ramos, B.F., Tsuji, R.K., Bento, R.F., Goffi-Gomez, M.V., Ramos, H.F., Samuel, P.A. and Brito, R. Hearing preservation using topical dexamethasone alone and together with hyaluronic acid in cochlear implantation. Acta Otolaryngologica. May 2015.