Predicting the effect of post-implant cochlear fibrosis on residual hearing

Abstract

Intracochlear scarring is a well-described sequela of cochlear implantation. We developed a mathematical model of passive cochlear mechanics to predict the impact that this might have upon residual acoustical hearing after implantation. The cochlea was modeled using lumped impedance terms for scala vestibuli (SV), scala tympani (ST), and the cochlear partition (CP). The damping of ST and CP was increased in the basal one half of the cochlea to simulate the effect of scar tissue. We found that increasing the damping of the ST predominantly reduced basilar membrane vibrations in the apex of the cochlea while increasing the damping of the CP predominantly reduced basilar membrane vibrations in the base of the cochlea. As long as intracochlear scarring continues to occur with cochlear implantation, there will be limitations on hearing preservation. Newer surgical techniques and electrode technologies that do not result in as much scar tissue formation will permit improved hearing preservation.

Fig. 1

An example of post-implantation intracochlear scarring. This is plastic-embedded cross-section of the basal turn of a cat cochlea after 1 year of implantation. The ST is scala tympani and CP is cochlear partition.

Fig. 2

(A) A schematic diagram of the unrolled cochlear duct. For the purposes of our model, the electrode was considered to only cause scar tissue in the basal one-half of the cochlea. (B) A passive model of cochlear mechanics. K, M, and R are stiffness, mass, and damping, respectively. n is the number of the cochlear segments (n = 500). P_stapes(x) is the input pressure at the stapes attached to the oval window of the SV. The total impedance at a cochlear segment [Z(x)] is the sum of the impedances of SV, ST, and CP.

Fig. 3

Predicted changes in basilar membrane velocity after an increase in ST damping. Basilar membrane velocity is plotted at two different cochlear segments (the base and the apex) with normal, 100 times, and 1000 times increased ST damping. Note the large decrease in basilar membrane velocity at the apex of the cochlea.

Fig. 4

Predicted changes in basilar membrane velocity after an increase in CP damping. Basilar membrane velocity is plotted at two different cochlear segments (the base and the apex) with normal, 100 times, and 1000 times increased CP damping. Note the large decrease in basilar membrane velocity at the base of the cochlea.

Fig. 5

Contour plots of predicted changes in the magnitude of basilar membrane velocity at the characteristic frequency (CF). Damping was increased from 1 to 1000 times normal for the ST (A) and the CP (B). Note the y-axis is plotted on a linear scale for (A) and a logarithmic scale for (B) for a clearer presentation of the data. The change in basilar membrane velocity was calculated in dB by the following equation: $$\text{dB}\text{change}=20\ast {log}_{10}\left(\frac{\text{basilar}\text{membrane}\text{velocity}\text{at}\text{CF}\text{with}\text{increased}\text{damping}}{\text{basilar}\text{membrane}\text{velocity}\text{at}\text{CF}\text{with}\text{normal}\text{damping}}\right).$$