Optimizing Cochlear Implant Laterality in Patients With Unilateral Vestibular Weakness

As the global population continues to age, there is a growing prevalence rate of hearing loss associated with increased rates of physical disability and falls. After the Centers for Medicare and Medicaid Service (CMS) recently expanded the candidacy profile for cochlear implantation to include a lower threshold for hearing test scores, more patients with hearing loss now have the option to pursue cochlear implantation. Cochlear implantation is widely regarded to be a relatively safe procedure due to its minimally invasive nature and low rate of complications. However, surgical manipulation of the labyrinth presents a risk of potential damage to the vestibular structures of the inner ear. One of the most common complications of cochlear implantation is post-operative vestibular weakness, measured to be 16-30%. Vestibular weakness presenting as vertigo or dizziness is of greater risk to the aging population as it is a contributor to falls as well as poor quality of life.

In planning for cochlear implantation in patients with bilateral symmetric hearing loss, the choice of which ear to implant is based on physician and patient discretion. Apart from relative contraindications such as the presence of pathology within the ear canal or middle ear on one side, there are few delineating factors that provide clear decision-making with regard to laterality selection. At present, many surgeons simply let the patient decide which ear they prefer. Reducing potential post-operative complications such as vestibular symptoms presents one potential avenue for guidance on ear selection. In patients who have pre-existing unilateral vestibular weakness, it may be important to preserve vestibular function on the contralateral side in hopes of avoiding bilateral vestibular hypofunction after implantation; however, there is scant knowledge to inform surgeons on how pre-existing vestibular weakness could or should affect laterality selection in cochlear implantation and whether it has measurable impact on patients' course after surgery.

There is a scarcity of prospective research assessing how to optimize patients with vestibular weakness who are undergoing cochlear implantation. West et al. conducted a prospective observational trial assessing the impact of cochlear implantation on vestibular function through pre- and post-operative vestibular testing. They found that the ear implanted demonstrated worsened vestibular function after the procedure without an association with self-reported symptoms. A separate prospective observational study led by Rasmussen et al. also investigated the relationship between vestibular function and cochlear implantation and found decreased vestibular function on the implanted side. Although both studies considered patients with bilateral hearing loss, they did not introduce control for surgical laterality in patients with unilateral weakness. A retrospective study conducted by Lovin et al. focused on patients who had undergone videonystagmography testing in helping to select laterality. Of the 27 patients who were bilateral candidates for cochlear implantation, decision-making involving the impact of pre-existing vestibular hypofunction resulted in the alteration of treatment plans for four patients. Parmar et al. retrospectively evaluated patients who received cochlear implants and had pre-operative VNG testing. They did not find a difference in post-operative Dizziness Handicap Inventory (DHI) scores between those who had implantation in the ear with better vestibular function compared to those who were implanted on the contralateral side; however, this was limited by retrospective design, lack of pre-operative DHI scores, and, most importantly, that not all patients had to demonstrate unilateral vestibular weakness for inclusion. Finally, Nayak et al. published subset data on patients with vestibular hypofunction who received implantation ipsilateral and contralateral to the hypofunction. They did not find a difference in rates of subjective dizziness at 1 month post-operatively but again were limited by lack of pre-operative dizzy information and use of validated questionnaires.

The investigators hope to expand on the works of these studies with a prospective randomized control trial to better elucidate how to risk-stratify similar patients. Determining an accurate sample size through power analysis is not possible for this study at this time due to the lack of prior usable literature or data in determining effect size. The results from this pilot study will be used to generate a comprehensive power analysis for future studies dedicated to this topic.

Inclusion criteria

• Patients over the age of 50 years
• Bilateral and symmetric moderate to profound sensorineural hearing loss (SNHL). Symmetric SNHL will be defined objectively as no more than 20dB difference at any 2 consecutive frequencies or no more than 15dB difference at any 3 consecutive frequencies on standard audiometry and as no significant difference between the ears to the patient.
• Unilateral vestibular weakness. Unilateral vestibular weakness will be defined by caloric testing of a greater than 22% difference between ears using bithermal water irrigation.
• Candidates for cochlear implantation by meeting traditional cochlear implant audiometric criteria.
• Willingness and ability to comply with scheduled visits and study procedures.

Exclusion criteria

• Patients with prior unilateral cochlear implantation
• Asymmetric SNHL
• Absence of unilateral vestibular weakness
• Contraindication for caloric testing (including epilepsy, dysconjugate eye movements, and history of ear or eye surgery less than 2 months prior)
• Alternative reasons for selecting cochlear implant laterality, including external. middle, or inner ear disease, anatomical abnormalities, or retrocochlear pathology
• Pregnant women
• Fetuses, neonates, children
• Prisoners
• Cognitively impaired adults