The effects of cochlear implantation on vestibular function

Abstract

Objective: Determine the risk posed by cochlear implantation (CI) to the labyrinth.

Study design: Prospective cohort study.

Setting: Academic tertiary referral center.

Patients: Thirty-six ears belonging to 35 adult CI candidates (mean age, 46 yr; range, 23-69 yr).

Intervention: Cochlear implantation.

Main outcome measures: Vestibular function was assessed using the quantitative 3-dimensional head impulse test (qHIT), clinical head impulse test (cHIT), post-head shake nystagmus, caloric electronystagmography, vestibular-evoked myogenic potentials, dynamic visual acuity, and Dizziness Handicap Inventory.

Results: All 36 ears were tested using qHIT before CI, and 28 ears were tested 4 to 8 weeks after CI. qHIT showed that 1 of 28 ears had reduced function. cHIT was 44% sensitive and 94% specific for identification of severe-to-profound vestibular hypofunction confirmed by qHIT. Post-head shake nystagmus was unchanged in 11 of 11 subjects. New hyporeflexia was found in 1 of 16 electronystagmography-tested ears. Vestibular-evoked myogenic potential showed either a disappearance of response or an increase in threshold by greater than 10 dB in 5 of 16 ears. Passive dynamic visual acuity showed no change in 16 of 16 ears. Dizziness Handicap Inventory scores worsened in 3 of 28 and improved in 4 of 28 subjects.

Conclusion: Although small, the observed rate of labyrinthine injury was comparable to that for other risks of CI. Thus, it is important to educate CI candidates regarding possible risk to balance function, particularly when CI of an "only balancing ear" is contemplated. cHIT is useful for detecting severe high-frequency vestibular hypofunction and should be part of the pre-CI physical examination.

Figure 1

A) Pre-implant and B) post-implant qHIT data for Subject #5. Eye velocities are inverted. Comparison of before and after implantation measured VOR gains shows that the right ear remained the same whereas the left (implanted) side had a post-implant drop across all 3 semicircular canals.

Figure 2

Individual and mean ± standard deviation VOR gains for qHIT data for 28 subjects. These data are compared to normal, gentamicin-treated (Post-Gent), and surgically deafferented (SUVD) ears’ qHIT results for each SCC from prior studies (26, 34). The bolded lines represent the only subject with significantly decreased vestibular function in all 3 SCCs of the implanted ear.

Figure 3

Individual and mean ± standard deviation DVA for 15 subjects. These data are compared to that for normal and surgically deafferented (SUVD) ears’ results for each SCC from a prior study (31). None of the tested implanted ears had a significant decrease in all 3 SCCs. LogMAR=log10(minimum angle resolved).