8The Electrode and the Labyrinth: How Implantation Affects Balance

FOne labyrinth, two senses

The cochlea and the semicircular canals, utricle and saccule are a single perilymph- and endolymph-filled space; opening the cochlea inevitably perturbs the fluid system the balance organs depend on. The saccule sits immediately above the basal turn of the cochlea, which is why otolith function (measured by cVEMP) is among the most vulnerable to an inserted array. Because the two senses share blood supply, fluid and membranes, a deaf ear is frequently a vestibulopathic ear before any surgery, so loss must be read against baseline, not assumed to be all surgical. The full anatomy and physiology of the balance organs are developed in the Balance chapter; here the point is simply proximity equals risk.[2008][2009]

THow much function is actually lost

Meta-analysis puts overall vertigo after implantation near 9-10%, new-onset vertigo around 17%, and persisting symptoms around 7%, so symptomatic disturbance is uncommon but not rare. Objective testing tells a stronger story than symptoms: otolith and canal abnormalities are found in up to 40-44% of implanted ears, far exceeding the number who complain. Cervical VEMP (saccular) loss is the most frequent objective change, while the video head-impulse test of the canals is more often preserved, mirroring the saccule's anatomical proximity. Most recipients compensate centrally over weeks because the contralateral labyrinth and other senses take over, which is why a large objective loss can leave the patient asymptomatic.[2018][2017][2008]

TMechanisms of vestibular injury

Direct trauma from opening the cochlea and advancing the array can disrupt the basilar membrane, displace perilymph and damage adjacent vestibular structures, especially with a cochleostomy versus a round-window approach. Endolymphatic hydrops and fibrosis can develop after insertion, producing delayed, fluctuating dizziness rather than an immediate deficit. Inflammation and a foreign-body reaction around the array, plus possible loss of inner-ear fluid pressure, contribute to ongoing receptor dysfunction. Electrical current can spread from the cochlear electrode to vestibular afferents, occasionally producing stimulation-evoked dizziness that tracks with switching the device on.[2018][2011]

CWhy this shapes ear choice

Because implantation can cost vestibular function, the worse-hearing and worse-vestibular ear is generally the safer one to operate, sparing the better balance organ. Bilateral implantation raises the stakes: putting an array in both ears risks bilateral vestibular loss, which compensates far less well than a unilateral deficit and can leave the patient oscillopsic and unsteady in the dark. A patient who relies on one functioning labyrinth (a 'only-balancing ear', as with a prior vestibulopathy) needs particular caution about implanting that side. These trade-offs are summarised here and worked through with the full test battery in the next module and in the Balance chapter, which carries the complete work-up.[2006][2017]