This month’s post is by Amanda Davis. Amanda is a 3rd year audiology doctoral student at UNC Chapel Hill completing her vestibular clinical rotation with us. She has a particular interest in cochlear implants and a recent patient that we saw inspired her to write this.
There are several hypothesized reasons for postoperative vestibular dysfunction, including but not limited to:
- Anatomical trauma from electrode insertion
- Acute labyrinthitis
- Adverse reaction to the insertion of a foreign body
- Endolymphatic hydrops
- Electrical stimulation from the implant itself
Typically, disruption to that system can recover with time, but a small percentage of the population who receive a cochlear implant have resulting vestibular dysfunction that does not resolve, leaving them with an impaired vestibular function that warrants therapy and lifestyle changes.
Vestibular Dysfunction and Cochlear Implants
Occasionally, etiologies that impair hearing to the point of cochlear implantation can also be accompanied by underlying vestibulopathy. One study found approximately 15.2% of patients screened positive for unilateral vestibular hypofunction and 10.5% of patients screened positive for bilateral vestibular hypofunction prior to cochlear implantation.
More often than not, the individual has learned to compensate for this vestibular weakness, and there is limited functional impact in their day-to-day life. However, when the compensated system is disrupted by surgery, the brain’s interpretation of the incoming signals from the vestibular system goes awry with resultant dizziness and imbalance. This experience can make the recovery period after receiving a cochlear implant more difficult, and impact the individual’s safety, emotional well-being, and overall quality of life.
Prior knowledge of pre-existing vestibulopathy can only be accomplished by a pre-operative vestibular function screening.
This experience was exactly the case for one of our patients. Prior to cochlear implantation this patient was a young man whose identity revolved around being outdoors, climbing, tight-roping, and doing all sorts of other activities in the mountains. As his hearing loss progressed, he decided to pursue cochlear implantation in his left ear so that he may be able to continue to communicate with his young daughter. Initially after implantation, he experienced persistent sensations of vertigo and dizziness. His medical team assured him that this experience was normal, and that it would subside with time, as it does for most individuals.
After months of debilitating dizziness and fear, he received a comprehensive vestibular evaluation. This exam indicated a profound bilateral vestibular hypofunction, likely that had at least partially existed prior to surgery. Unfortunately, he never received a vestibular assessment prior to surgery, which may have provided insight into decision making regarding implantation.
Vestibular Assessment Prior to CI Implantation
According to the 2019 AAA Clinical Practice Guideline for Cochlear Implants, vestibular assessment is listed as a potential part of the test battery prior to implantation, as it “may affect the ear of choice for implantation, as well as identify patients who could be more susceptible to balance difficulties following cochlear implant surgery”, though its incorporation is not standardized.
If we suggest that the risk is high enough to warrant a vestibular screening prior to cochlear implantation, the next step would then be to determine what that test battery should consist of. Conducting a comprehensive vestibular evaluation prior to implantation, while helpful, may cause strain on resources and lead to increased wait periods to receive the implant.
Implementation of a vestibular function screening protocol may have to be tailored to best fit the resources of the clinic, taking a variety of forms. Some clinics defer to the surgeon to decide what tests, if any, should be performed. Other clinics utilize a two-step screening protocol, where subjective patient report and questionnaires are used to determine if vestibular evaluation is warranted prior to surgery. We are wary of this approach due to the aforementioned case. Even further, some clinics use vHIT as a screening measurement, and others use a protocol, including Romberg and Fukuda tests, assessment for spontaneous nystagmus, Head Impulse Test, evaluation for Head Shaking Nystagmus and caloric tests.
Minimally, we suggest that the screening protocol include vHIT and skull vibration testing (of which an overview can be found here).
At present, there is little to no research on the routineness of pre-implantation vestibular function screenings across the United States. This area would benefit from future investigation in order to standardize the approach for pre-operative vestibular evaluations.
Ultimately, a pre-surgical vestibular function screening would help better assess the risks of surgery for an individual considering cochlear implantation, more accurately determine the likelihood of long-lasting vestibular symptoms, and in some cases, may help dictate which side the patient should implant (in cases of bilateral, symmetric hearing loss).
A vestibular function screening would also serve as a baseline examination of function to compare to post-operatively if a patient has prolonged dizziness symptoms, giving more insight as to whether an underlying vestibulopathy existed prior to surgery or if dysfunction was related to the procedure.