Typical developmental changes in vestibular-evoked myogenic potentials (c-VMEPs) in normal infant and children
11.2 VEMPs in Infants with Congenital Profound Hearing Loss
Colebatch et al.  showed that VEMPs were evident at a high incidence in patients with profound sensorineural hearing loss, but that they were abolished in all of their patients who underwent unilateral vestibular neurectomy. These authors also reported the VEMPs were abolished in some but not all patients with unilateral loss of a caloric response after vestibular neuritis. They hypothesized that VEMPs are of vestibular origin and that the saccule is probably an acoustically sensitive organ.
In our study, 67 % of the 54 ears of 33 children with congenital profound hearing loss showed normal VEMPs (Fig. 11.2), but 5 % of ears of the children showed abnormal VEMPs with low amplitude. This is a surprising finding because they cannot hear air-conducted loud click stimuli at all. However, they have VEMPs, suggesting that VEMP testing can illuminate vestibular activity in deaf infants and children.
Click-evoked normal VEMPs (right) and audiogram (left) of a child with congenital profound hearing loss
Sheykholeslami et al. [4, 5] confirmed that saccular origin of this short-latency acoustic response and that a saccular acoustic response persist in the human ear and has a well-defined frequency tuning curve. Currently, recorded VEMPs are induced using various stimuli including clicks , tone bursts , electrical stimuli , bone-conducted sounds , and head taps .
11.3 VEMPs in Children with No Inner Ear Malformation After Cochlear Implantation
The cochlear function of both ears is markedly impaired in infants and children who are candidates for cochlear implantation. However, vestibular function is also impaired in 10–20 % of such infants. After cochlear implantation, patients can hear speech sounds, which are converted to electrical signals in a speech processor; these signals are transmitted to the internal receiver under the scalp ad conducted to the electrodes in the cochlea. Thus, cochlea nerves that are stimulated electrically convey information to the central auditory brainstem pathway and auditory cortex.
There are two problems for vestibular end organs after cochlear implantation. One is traumatic damage of vestibular end organs incurred following insertion of the electrodes of the cochlear implant into the scala tympani. Tien and Linthicum histopathologically analyzed the vestibular apparatus from human temporal bones after cochlear implantation was carried out . The other problem is that electrical stimulation may spread not only the cochlear nerve but also the facial nerve or the vestibular nerve in patients with a multichannel cochlear implant because of current spread. Based on these findings, VEMPs are considered useful for evaluating electrical current spread to the inferior vestibular nerve.
Jin et al. compared VEMPs before and after surgery . Before surgery, 6 of the 12 children showed normal VEMPs, 1 showed a decrease in the amplitude of VEMPs, and 5 showed no VEMPs. After surgery, with the cochlear implant switched off, 11 showed no VEMPs, and 1 showed decreased VEMPs. These results reveal that even normal VEMPS disappear owing to trauma following electrode insertion. With the cochlear implant switched on, four children showed normal VEMPs, but eight did not (Fig. 11.3). This can be explained by the fact that these four children’s inferior vestibular nerves were stimulated by the spread of electrical current from the cochlear implant. We questioned why one-third of these children with cochlear implants showed VEMPs, but others did not. Later, Jin et al. demonstrated that VEMPs evoked by cochlear implants may be related to an electrical current intensity at a comfortable level (C level), particularly in channels that are closer to the apical turn of the cochlea .
Changes in VEMPs before and after cochlear implantation. (a) Before surgery. (b) Switched-off cochlear implant (CI) after surgery and switched-on CI after surgery
The patients who showed no VEMPs with the cochlear implant switched on may require higher current intensities to elicit clear VEMPs (if they need to be recorded). However, it is difficult to increase current intensity in such children because they feel pain or facial nerve stimulation when the current intensity is higher than the C level.
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11.4 VEMPs in Children with Inner Ear Malformation After Cochlear Implantation
Inner ear malformations and cochlear nerve deficiency (CND) present a major inner ear disorder in approximately 20 % of children with congenital sensorineural hearing loss . They are usually characterized by profound hearing loss, and their development delays gross motor functions such as head control or independent walking, because such functions are related to abnormal inner ear structures . However, it is not easy to unequivocally determine whether vestibular sensory cells of semicircular canals and otolith organs or primary vestibular afferent neurons are present in patients with inner ear malformations, particularly common cavity deformity. In an embryological study, it has been found that, in the human fetal developmental stage, the vestibular system develops earlier than the cochlear system . Thus, it is speculated that sensory cells of vestibular end organs and vestibular afferent neurons may be present in patient with inner ear malformations, which is similar to early-stage inner ear development.
In our study, we reported that VEMPs could be elicited with the cochlear implant switched on and suggested that the electrical stimulation of a cochlear implant may directly stimulate the inferior vestibular nerve [10, 11]. If VEMPs are evoked with the cochlear implant switched on, it suggests that some of the inferior vestibular neurons are present. In contrast, if VEMPs are absent with the cochlear implant switched on, it suggests that the inferior vestibular neurons may be absent.
Seven children with inner ear malformation and CND who underwent cochlear implantation participated in this study (Table 11.1). The patients had common cavity deformity (n = 2), incomplete partition type I (n = 2), incomplete partition type II (n = 1), and CND. After surgery, VEMPs were recorded with the cochlear implant device switched both off and on. All the patients showed VEMPs with the cochlear implant switched on (Figs. 11.4 and 11.5).
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