|cVEMP in person with complete bilateral vestibular loss due to gentamicin, but with some hearing. The potentials shown are replicable but they lack the characteristic below-baseline displacement for P1 (see below).|
Some comments about terminology
First, let us distinguish between "bilateral vestibulopathy", and "bilateral vestibular loss". Bilateral "vestibulopathy" means that there is something wrong with both ears. It doesn't mean that both ears are "dead". An example of a "bilateral vestibulopathy", might be bilateral vestibular neuritis, or even bilateral BPPV. Or perhaps BPPV on the right and vestibular neuritis on the left. It is a vague term.
Bilateral vestibular loss means that there is no vestibular function at all, on either side. An example of this might be severe gentamicin ototoxicity. Or someone with bilateral vestibular nerve sections (almost never occurs), or someone with bilateral acoustic neuromas such as in neurofibromatosis -- again exceedingly rare. Furthermore, persons with gentamicin ototoxicity vary in how much vestibular function remains -- roughly from 0 to 50%. If more than 50% function remains, they are generally not symptomatic enough to require much medical care.
So, what we are dealing with here with the term "bilateral vestibulopathy" is a wide assortment of illnesses and severity of impairment.
What would you expect ?
cVEMPs would be expected to be reduced or absent in persons with any kind of bilateral vestibular loss, such as due to aminoglycoside ototoxicity. As there are many other causes of so-called "bilateral vestibulopathy" than aminoglycoside ototoxicity, one cannot necessarily expect that results from one group will extend to the others. One would expect that patients with bilateral loss due to vestibular neuritis, might have preserved VEMPs, as this is generally the case for unilateral vestibular neuritis, but those with aminoglycosides might not. On the other hand, one would expect that oVEMPs might be obliterated in bilateral vestibular neuritis. Thus, one would expect that cVEMPs would be reduced in conditions that affect the entire ear (such as aminoglycosides), and oVEMPs would be reduced in conditions that are selective for the utricle (such as bilateral vestibular neuritis).
Zingera et al (2008) concluded in a study of "bilateral vestibulopathy" that "Thus, in our study population saccular function appeared to be less affected than horizontal semicircular canal function." They sudied patients diagnosed with bilateral vestibular loss, not necessarily from aminoglycosides. So their patient population presumably included at least patients with aminoglycosides (expect everything out), bilateral vestibular neuritis (expect cVEMP preserved), and idiopathic (who knows). Zingera et al, as do many authors, made a logical error in suggesting that cVEMP responses are the same as saccule function. Actually, the saccule might be entirely normal as the cVEMP reflects function of a reflex loop, not a single sensory organ. This paper also points out another use of language problem -- one really cannot have "bilateral vestibular loss", if VEMPs are present. More accurate use of language would be to say that they studied patients with "bilateral loss of horizontal canal function".
Gentamicin obliterates cVEMPs in guinea pigs. (Cheng et al, 2010)
Agrawal et al (2013), again in a study of "bilateral vestibulopathy", reported that "Utricular function differed significantly between patients by etiologic group: the poorest function was found in patients with BV due to aminoglycoside toxicity, and the best function in Meniere's disease patients. " Again, we presume Agrawal was presuming that "utricular function" could be inferred from oVEMP responses, although again the oVEMP reflects the function of a reflex loop, not necessarily the utricle. Again, the term "bilateral vestibulopathy" appears to be used rather than "bilateral vestibular loss".
In our practice, we have tested many patients with bilateral vestibular loss and normal hearing, and find that the cVEMP is a good test (Hain et al, 2006). We have also tested two deaf patients with bilateral loss, one due to Cogan's syndrome and another due to a Mondini malformation, and found absent or nearly absent responses in both. This is as would be expected if one believes that the saccule is affected in these conditions. Nevertheless, this conclusion can be questioned as a problem intrinsic to testing persons with bilateral hearing loss is that one does not know if they might also have a conductive hearing disturbance superimposed on the sensorineural loss. Because conductive hearing loss obliterates sound-induced cVEMPs, one cannot clearly relate an absent cVEMP to absent saccule function in this situation.
cVEMP's also appear to be absent after unilateral gentamicin treatment used for Meniere's disease (Helling et al, 2007). They may be useful in deciding whether or not more drug is indicated. A possible confounding problem is that there may be middle ear disease after the injection due to the perforation required for transtympanic gentamicin.