Vestibular evoked myogenic potentials in term newborn infants with severe hyperbilirubinemia

Infant Cervical Vestibular Evoked Myogenic Potentials: A Scoping Review

OBJECTIVES

Children diagnosed with hearing loss typically demonstrate increased rates of vestibular loss as compared with their peers, with hearing within normal limits. Decreased vestibular function is linked with delays in gross motor development, acquisition of gross motor skills, and academic challenges. Timely development of sitting and walking gross motor skills aids in the progress of environmental exploratory activities, which have been tied to cognitive, language, and vocabulary development. Considering the time-sensitive development of gross motor skills and cognitive, language, and vocabulary development, identifying vestibular loss in infancy can support early intervention. This scoping review analyzes stimulus, recording, and participant factors relevant to assessing cervical vestibular evoked myogenic potentials (cVEMPs) in the infant population.

DESIGN

The scoping literature review was conducted on literature published between 2000 and 2023, focusing on articles assessing cVEMPs in infants. Two authors independently followed Preferred Reporting Items for Systematic and Meta-Analysis guidelines for title and abstract screening, full-text review, data extraction, and quality assessments. Sixteen articles meeting the inclusion criteria were included in the analysis.

RESULTS

The existing literature lacks consensus regarding stimulus and recording parameters for measuring infant cVEMPs. In addition, the review reveals a decrease in cVEMP response occurrence rates with the severity of hearing loss, especially in cases of severe to profound hearing loss, compared with mild to moderate sensorineural hearing loss in infants.

CONCLUSIONS

This scoping review demonstrates the increasing use of cVEMP as a reliable tool for objectively assessing infant vestibular function. The lack of consensus in stimulus and recording parameters emphasizes the need for systematic research to establish an evidence-based protocol for cVEMP measurements in infants. Such a protocol will ensure the reliable measurement of cVEMPs in infants and enhance the effectiveness of cVEMP as part of the infant vestibular test battery. In addition, there is a necessity for a comprehensive large-scale study to evaluate the practicality and feasibility of implementing vestibular screening protocols for infants diagnosed with sensorineural hearing loss in the United States.

Ca2+-dependent recruitment of voltage-gated sodium channels underlies bilirubin-induced overexcitation and neurotoxicity

Neonatal jaundice is prevalent among newborns and can lead to severe neurological deficits, particularly sensorimotor dysfunction. Previous studies have shown that bilirubin (BIL) enhances the intrinsic excitability of central neurons and this can potentially contribute to their overexcitation, Ca²⁺ overload, and neurotoxicity. However, the cellular mechanisms underlying elevated neuronal excitability remain unknown. By performing patch-clamp recordings from neonatal neurons in the rat medial vestibular nucleus (MVN), a crucial relay station for locomotor and balance control, we found that BIL (3 μM) drastically increases the spontaneous firing rates by upregulating the current-mediated voltage-gated sodium channels (VGSCs), while shifting their voltage-dependent activation toward more hyperpolarized potentials. Immunofluorescence labeling and western immunoblotting with an anti-NaV1.1 antibody, revealed that BIL elevates the expression of VGSCs by promoting their recruitment to the membrane. Furthermore, we found that this VGSC-trafficking process is Ca²⁺ dependent because preloading MVN neurons with the Ca²⁺ buffer BAPTA-AM, or exocytosis inhibitor TAT-NSF700, prevents the effects of BIL, indicating the upregulated activity and density of functional VGSCs as the core mechanism accountable for the BIL-induced overexcitation of neonatal neurons. Most importantly, rectification of such overexcitation with a low dose of VGSC blocker lidocaine significantly attenuates BIL-induced cell death. We suggest that this enhancement of VGSC currents directly contributes to the vulnerability of neonatal brain to hyperbilirubinemia, implicating the activity and trafficking of NaV1.1 channels as a potential target for neuroprotection in cases of severe jaundice.

Bilirubin-induced neurotoxic and ototoxic effects in rat cochlear and vestibular organotypic cultures

Exposure to high levels of bilirubin in hyperbilirubinemia patients and animal models can result in sensorineural deafness. However, the mechanisms underlying bilirubin-induced damage to the inner ear, including the cochlear and vestibular organs, remain unknown. The present analyses of cochlear and vestibular organotypic cultures obtained from postnatal day 3 rats exposed to bilirubin at varying concentrations (0, 10, 50, 100, or 250 μM) for 24 h revealed that auditory nerve fibers (ANFs) and vestibular nerve endings were destroyed even at low doses (10 and 50 μM). Additionally, as the bilirubin dose increased, spiral ganglion neurons (SGNs) and vestibular ganglion neurons (VGNs) exhibited gradual shrinkage in conjunction with nuclei condensation or fragmentation in a dose-dependent manner. The loss of cochlear and vestibular hair cells (HCs) was only evident in explants treated with the highest concentration of bilirubin (250 μM), and bilirubin-induced major apoptosis most likely occurred via the extrinsic apoptotic pathway. Thus, the present results indicate that inner ear neurons and fibers were more sensitive to, and exhibited more severe damage following, bilirubin-induced neurotoxicity than sensory HCs, which illustrates the underlying causes of auditory neuropathy and vestibulopathy in hyperbilirubinemia patients.