Redefining Cochlear Implant Strategies: Auditory Nerve Excitability in Scala Vestibuli vs. Scala Tympani Placements

Hearing loss is a major health issue that affects millions of people around the world. It does more than just make conversations difficult—it can impact cognitive function, interfere with work and social interactions, and lower overall quality of life. For those with severe or complete hearing loss, cochlear implants (CIs) have been life-changing. These devices work by bypassing the damaged hair cells inside the cochlea and sending electrical signals directly to the auditory nerve which restores a sense of hearing. However, CIs have some limitations, especially for patients with structural abnormalities or blockages inside the cochlea. When standard electrode placement is not an option, alternative surgical techniques need to be explored to ensure the implant can still effectively stimulate the auditory nerve. The most common approach for cochlear implantation involves placing the electrode array in the scala tympani (ST), which is the lower duct of the cochlea and this location is preferred because it is easier for surgeons to access and sits close to the spiral ganglion neurons that relay sound signals to the brain. But not everyone has a clear path for ST electrode placement. Some patients develop ossification, fibrosis, otosclerosis, or cochlear obstructions due to meningitis or congenital abnormalities, all of which can make it impossible to insert the electrode properly. For these cases, an alternative option is to place the electrode in the scala vestibuli (SV), the upper duct of the cochlea. While SV placement has been used in some patients, it is not yet well understood how this approach compares to the standard ST placement in terms of hearing outcomes.

A major question surrounding SV implantation is whether it stimulates the auditory nerve as effectively as ST placement. For a CI to work well, it needs to precisely activate specific nerve fibers while avoiding unnecessary stimulation of neighboring fibers. This ensures that the brain interprets electrical signals clearly, allowing the person to recognize different sounds and speech patterns. However, several factors—such as differences in how electrical currents spread through cochlear fluids, variations in nerve fiber positioning, and the structural properties of surrounding tissues—could affect how well the SV electrode stimulates the nerve. To this account, a new research paper has been published in Journal of Neural Engineering authored by Dr. Andreas Fellner, Dr. Cornelia Wenger, and Professor Frank Rattay from the Vienna University of Technology alongside Dr. Amirreza Heshmat from University of Texas MD Anderson Cancer Center. They built a detailed three-dimensional finite element model (FEM) of the human cochlea, reconstructed from high-resolution micro-CT scans, to replicate the structure and electrical properties of the cochlea as accurately as possible in order to analyse how auditory nerve fibers respond to different electrode placements.   

The research team used monophasic anodic and cathodic pulses, each lasting 50 microseconds to compare nerve excitability between ST and SV placements. They were able to map out the extracellular voltage distributions along the nerve fibers using FEM calculations. These results were then fed into a multi-compartment neuron model, which helped predict where and how action potentials (APs) were triggered. Essentially, they were able to determine the minimum electrical charge needed to stimulate the nerves (threshold currents) and see how different parts of the cochlea responded to stimulation. One of the most eye-opening discoveries was the difference in stimulation efficiency between ST and SV electrodes, particularly when looking at pulse polarity. When using cathodic (negative) stimulation, SV electrodes required less current to activate the auditory nerve fibers than ST electrodes. That means SV placement could be an advantage for patients who cannot have an ST implant, since it takes less energy to get the nerve fibers firing. But the opposite happened with anodic (positive) pulses—SV electrodes needed much higher currents compared to ST electrodes. This difference shows that where an electrode is placed in the cochlea significantly impacts how it interacts with nerve fibers, making pulse polarity selection a crucial factor in implant programming. Afterward, the authors looked at how far the stimulation spread. This is a big deal for sound clarity because ideally, an electrode should only activate the intended nerve fibers and not spill over to neighboring ones. Their findings showed that SV electrodes were more selective. On the other hand, ST electrodes had a broader spread, especially when higher currents were used. The authors also found that moving an electrode by as little as 3 to 9 degrees could dramatically change the amount of current needed to trigger nerve activity. In some cases, shifting the electrode even slightly lowered the threshold significantly, while in others, it made stimulation much harder. This shows how important precise electrode placement is during surgery, as even minor adjustments can make a noticeable difference in the excitability of auditory nerve fibers.  

To ensure their model was as realistic as possible, the researchers compared their simulated intracochlear voltage data with real-world clinical measurements taken from actual CI users. Specifically, they looked at voltage decay patterns along implanted electrode arrays and found that their FEM simulations closely matched recorded patient data. This was an important validation step, proving that their model was not just a theoretical exercise, but could accurately reflect what happens inside a human cochlea. Because of this, their findings could have a direct impact on clinical decision-making, particularly when it comes to choosing the best electrode placement strategy for each patient. The team also investigated where the APs actually started in the auditory nerve fibers. In most cases, whether the electrode was in the ST or SV, the APs were triggered at the peripheral dendritic terminal of the nerve. This suggests that this region is the primary target for electrical stimulation by such lateral-wall electrodes, making it a critical area to consider when designing electrode arrays while also bearing the patient and frequency specific neural degeneration status in mind. However, when stimulation intensity was very high, some APs were triggered closer to the center of the nerve, which could reduce the precision of frequency discrimination at stronger stimulation levels. To take their findings a step further, the team also tested suprathreshold stimulation, meaning they increased the current beyond the minimum level needed to activate the nerve. This was an important part of the study because real-world listening environments involve a wide range of sound intensities, and CIs need to be able to adjust accordingly. What they found was that SV electrodes maintained better selectivity at higher currents. Meanwhile, ST electrodes had more widespread activation.

In conclusion, the research from the Vienna University of Technology could change how CIs are used and the new innovation can provide solutions that were once considered impossible. It demonstrated that SV implantation is more than just a backup plan—it is a real and effective alternative. In fact, cathodic stimulation in the SV required lower current thresholds, therefore, less energy is needed to activate the nerves. This has direct implications for both CI programming and surgical planning, allowing doctors to adjust stimulation settings to improve patient outcomes. Another major advantage of the new technology is related to how precisely the electrodes stimulate the nerve fibers, which is critical for clear and natural sound perception because it showed that SV electrodes are more selective than ST electrodes, especially at higher stimulation intensities. Therefore, patients may experience sharper pitch perception which is important in noisy environments where speech clarity is easily lost. Indeed, the results suggest that electrode positioning is more important than previously thought which could lead to new electrode designs that improve sound quality.