By Bre Myers
Electrocochleography (ECochG) has been used in clinical practices since the 1960s. Historically, it has been touted as helpful in the identification of Meniere’s disease (MD). However, differences in recording strategies significantly reduce its efficiency in this endeavor (Ho Oh et al, 2014). Moreover, consensus guidelines put forth by Lopez-Escamez et al (2015) use a set of specific symptomatic findings to classify MD as either definite or probable, leaving ECochG results out of the diagnostic picture.
As our understanding and access to new tests of the vestibular system grow, is there still a need to keep ECochGs in our toolbox? Perhaps, but we may need to repurpose the ECochG. Auditory neuropathy spectrum disorder (ANSD), compared to MD, is a fairly “young,” less familiar disorder. The ability of the ECochG to enhance the cochlear microphonic of the auditory brainstem response may be the right tool for the job when it comes to ANSD’s clinical diagnosis.
Historical Uses of ECochGs
You know the saying, “It’s hard to teach an old dog new tricks,” but that just may be what needs to happen to transition ECochG out of the vestibular toolbox and into the auditory one. Historically, ECochGs have been used in the hopes of identifying MD. The rationale for this assumption is derived from the suspected increased levels of endolymph in the MD-affected ear. This elevated level produces a larger-than-typical summating potential (SP), which may be visible in an ECochG recording (FIGURE 1).
Results have been mixed at best, though their legend persists. Part of this may lie in what the literature says versus how ECochGs are actually performed in an outpatient setting. Much of the supportive studies for ECochG in MD identification use a transtympanic approach. This requires either the placement of pressure equalization (PE) tubes and electrodes surgically fished through them into the middle-ear space or a transtympanic needle electrode pushed through the tympanic membrane. Both methods require skills beyond the scope of clinical audiologists, who are the ones routinely tasked with performing the studies. Because neither of these two methods are practical for an audiologist to perform, extratympanic methods have been adopted in the outpatient clinical setting.
A less invasive approach is to use a wick “tymptrode” that is placed on the external surface of the tympanic membrane. However, even this can be uncomfortable, at best, for the patient. Another more common approach uses gold foil “tiptrodes” that are inserted deep in the ear canal, similar to earphones inserted for audiometric testing.
However, what we gain in comfort and safety we lose in recording capabilities. While tiptrodes offer a better view of the cochlear microphonic, summating potential (SP), and action potential (AP) compared to mastoid or earlobe placement, serious concern over the decrease and variability in SP amplitude has been reported, making the widely used recording strategy fairly useless in consistent identification of MD (Ho Oh et al, 2014).
So How Is MD Properly Identified?
The latest consensus put forth by the Classification Committee of the Bárány Society, the Equilibrium Committee of the American Academy of Otolaryngology-Head and Neck Surgery (AAO-HNS), the European Academy of Otology and Neurology (EAONO), the Japan Society for Equilibrium Research, and the Korean Balance Society, in 2015, includes two categories: Definite, and Probable Meniere’s (Lopez-Escamez et al, 2015). The diagnosis is based on clinical symptomatic criteria (TABLE 1), not on specific test results, according to Lopez-Escamez et al (2015).
|DEFINITE MD||PROBABLE MD|
However, the early identification of MD remains a goal in the clinic setting. Because the early stages of MD share similar attributes to other peripheral vestibular disorders, well-meaning practitioners may request ECochGs as part of their initial vestibular work-up. The hope is that performing an ECochG on these patients in the early stages will reveal a large SP/AP area ratio, but according to Oh et al (2014), this is not consistently the case.
In this study, using extratympanic recording methods, no significant differences were found in the SP/AP area ratio between the control group and patients who were categorically identified as definite, probable, or possible MD using the previous AAO-HSN guidelines. The authors concluded that neither a positive nor a negative finding significantly contributes to the differential diagnosis.
So What’s Left for the ECochG?
As a clinician, I love the increased visibility of the cochlear microphonic and Wave I using tiptrodes compared to an electrode placed on the mastoid. But, as someone interested in identifying functional capabilities and the affected areas of the vestibular system, performing vestibular evoked myogenic potentials (VEMP), video head impulse testing (vHIT), rotary chair, or posturography measures will provide more useful information. There is hope, however, as ECochG has found utility in the identification of ANSD.
The ANSD spectrum is still being defined. Audiometric configurations are varied in ANSD. However, most cases have seemingly normal, or near normal, cochlear function evidenced by present otoacoustic emissions and present cochlear microphonics with otherwise absent Waves I-V during auditory brainstem response evaluations (Colucci, 2020).
In a Frontiers in Neuroscience editorial by Pienkowski et al (2018), the authors review the merits of the ECochG’s abilities to capture cochlear microphonics and potentially become an integral differentiator between inner and outer hair-cell function or auditory nerve fiber (ANF) loss. More investigation is necessary to determine how precise this measurement can become. However, ECochGs are emerging as an effective tool in the diagnostic battery used in the diagnosis of ANSD.
Additionally, ECochGs may also find a new application for evaluating noise effects within the cochlea. As the research bank grows surrounding hidden hearing loss and cochlear synaptopathy, an easily accessible clinical measure, such as the ECochG, may see its stock rise in this patient community, as well. The editorial also highlights findings of several studies (Liberman et al, 2016; Bramhall et al, 2017) that found differences in the compound action potential (CAP) in those with a history of noise exposure compared to control subjects (Pienkowski et al, 2018).
Perhaps it is not the ECochG that needs to learn some new tricks after all. Rather, it is the clinicians and providers who need to repurpose the ECochG for use in more suitable patient populations.
This article is a part of the July/August 2021 Audiology Today issue.
Bramhall NF, Konrad-Martin D, McMillan GP, Griest SE. (2017) Auditory brainstem response altered in humans with noise exposure despite normal outer hair cell function. Ear Hear 38:e1–e12.
Colucci D. (2020) Updates on auditory neuropathy spectrum disorder. Hear J 73(6):32–34.
Ho Oh K, Kim KW, Chang J, et al. (2014) Can we use electrocochleography as a clinical tool in the diagnosis of Meniere’s disease during the early symptomatic period? Acta Oto-Laryngol 134(8):771–775.
Liberman MC, Epstein MJ, Cleveland SS, Wang H, Maison SF. (2016) Toward a differential diagnosis of hidden hearing loss in humans. PLoS ONE 11:e0162726.
Lopez-Escamez JA, Carey J, Chung WH, et al. (2015) Diagnostic criteria for Meniere’s disease. J Vestib Res 25(1):1–7.
Pienkowski M, Adunka OF, Lichtenhan JT. (2018) Editorial: New advances in electrocochleography for clinical and basic investigation. Front Neurosci 12:310.