About the pathophysiology of acute unilateral vestibular deficit – vestibular neuritis (VN) or peripheral vestibulopathy (PVP)?

Abstract

OBJECTIVE: To determine whether patients with acute unilateral peripheral vestibulopathy (PVP), often called “vestibular neuritis/neuronitis or neuropathy” (VN) have a vestibular lesion pattern consistent with the distribution of the neurological afferents.

BACKGROUND: Much is known about the clinical nature of PVP, however less so about its etiology and pathogenesis. Due to the frequency with which VN is used to describe the syndrome, an inflammation of the vestibular nerve or of one of its branches is often assumed to be the cause of PVP, though there is insufficient data so far to support this assumption.

METHODS: We conducted a retrospective study of 25 patients who had presented to our clinic with PVP and had all vestibular receptor organs tested shortly after start of symptoms. We analysed their vestibular lesion patterns in order to determine whether they were consistent with the neuritis hypothesis (NH).

RESULTS: The lesion patterns varied conspicuously. 76% did not follow an innervation pattern, thereby contradicting the NH and only 24% had a lesion pattern that either definitely (16%) or probably (8%) supported the NH.

CONCLUSION: These results should remind us to be careful before jumping to quick conclusions about the pathogenetic nature of PVP. With any reason to question VN as the only cause of PVP, we should reconsider the treatment approach to PVP. If the cause probably or even possibly lies inside the vestibular labyrinth, an intratympanic steroid injection might prove to be a more effective measure, even in first-line treatment. If the etiology is unsure, a combination of systemic and intratympanic steroid treatment may be adequate.

Keywords

Vestibular neuritis peripheral vestibulopathy lesion pattern etiology receptor testing

1 Introduction

The symptoms of an acute vestibular syndrome are well described in the literature. They are mainly due to a unilateral peripheral vestibulopathy (PVP), if central lesions are excluded and are most commonly named vestibular neuritis, neuronitis or neuropathy- for a short historical review see Reker [34]. Since there are no formal diagnostic criteria there is some variation in the definition, which poses a challenge when trying to compare studies from different authors or clinics [3, 38]. PVP manifests itself as the acute onset of spinning vertigo lasting many hours or several days, often accompanied by nausea and vomiting [14, 20]. The patient usually presents with gait instability [14] and a falling tendency toward the affected side. The clinician normally detects a horizontal nystagmus beating toward the healthy side obeying Alexander’s law. Caloric and head impulse testing show hypofunction of the horizontal semicircular canal (hSCC) [38]. The frequently occurring ocular tilt reaction [22] toward the affected side, which can be observed directly through fundoscopy or be inferred from testing the subjective visual vertical (SVV) [24], is indicative of utricular hypofunction as is an asymmetry ratio (AR) in ocular vestibular evoked myogenic potentials (oVEMPs). The saccular afferents from the vestibular organ as tested through cervical VEMPs frequently remain unaffected [14], but can also be dysfunctional [30, 46]. The frequent sparing of the sacculus previously led to the conclusion that vestibular neuritis chiefly affects the superior vestibular nerve whilst sparing the inferior vestibular nerve in most cases [14]. Another explanation for these findings calls attention to the fact that patients who only have pathologies in their posterior SCC (pSCC) and/or sacculus are less likely to be diagnosed as having PVP due to the absence of a spontaneous nystagmus (SPN) or pathological caloric test results [21, 26].

At the time the etiological hypothesis of superior/inferior/total vestibular neuritis (henceforth: neuritis hypothesis, NH) was proposed it was reasonable to assume that the dysfunction in PVP follows a neuroanatomical pattern, since most routine testing included only one receptor per nerve branch (caloric for hSCC, i.e. superior vestibular nerve, cVEMP for sacculus, i.e. inferior vestibular nerve). However, such assumptions need to be re-evaluated in the advent of new diagnostic methods that might offer new insights into the pathogenesis of PVP. For about a decade, it has now been possible to assess both utricular and saccular function in clinical practice using oVEMPs (introduced in 2007 [35]) and cVEMPs (introduced in 1992 [36]). The video head impulse test (vHIT), introduced for hSCC testing in 2009 [5, 28] and for vertical SCC testing in 2013 [27], which serves as an alternative to the more expensive, time-consuming and more uncomfortable magnetic search-coil HIT first described for testing all SCC in 1998 [10], allows for a quick assessment of all SCC receptors separately. Given the availability of these new precise methods it is sensible to test the NH against all receptors. That is, if the NH is true, PVP should either show a hypofunction of all receptors of the superior vestibular nerve, all receptors of the inferior vestibular nerve or both. In other words, the lesion pattern of the vestibular receptors in PVP should be the same as the innervation pattern of the vestibular organ.

To our knowledge an analysis of the lesion patterns in PVP with the aim of testing the NH has not been performed so far. The goal of our study was to map the lesion patterns of our PVP patients in order to test whether they are the same as the innervation patterns.

This retrospective study was performed according to the declaration of Helsinki and approved by the local ethics committee. We analysed the lesion patterns of the patients who had presented and been treated for PVP in our clinic from 2006 till 2014.

2 Etiology and pathogenesis of PVP: The evidence so far

The research on the etiology and pathogenesis of PVP is varied. There are many contradictory findings, some of which support the NH while others do not or even contradict it. The many terms – vestibular neuritis/neuronitis/neuropathy/neuronopathy, acute vestibulopathy, acute vestibular neurolabyrinthitis, acute labyrinthitis 1 – often used interchangeably to describe PVP, testify to the fact that there is still much to be discovered about the nature of the disease. A less disputed issue is the fact that PVP is often preceded by a viral infection of the upper respiratory tract [38] or the higher prevalence (compared to controls) of Herpes viruses in the eighth and other cranial nerves and their neurons [1 , 42] as well as in the saliva of patients with PVP [33]. One animal study also supports the etiopathogenetic role of Herpes viruses, but points to histopathological changes in both the vestibular nerve and labyrinth [13]. HSV-1 has also be detected in the labyrinth [2]

The anatomical evidence of a vestibulo-facial nerve anastomosis is one way of explaining the presence of those viruses in vestibular neurons [32]. However, the prevalence as described in those studies is insufficient to support Herpes viruses as the only etiological factor. More likely, those findings point to the fact that PVP may have a multifactorial etiology. Alternatively, they raise the question whether PVP might be, rather than a single pathological entity, a catch-all basket for a group of diseases with similar symptoms that do not match another known diagnostic entity.

Often, the case for the NH is made by presenting PVP as analogous to Bell’s palsy, which is also thought to be caused by a reactivation of viruses that subsequently leads to nerve edema and a loss of function [2, 15]. However reasonable this analogy may seem, there are two important differences: Firstly, corticosteroids have been found to be reasonably effective in the treatment of Bell’s palsy [15], while their effect in treating PVP is less established [16, 44]. This might be suggestive of a distinctive pathogenetic mechanism rather than a similarity. Secondly, while the Bell’s palsy analogy is a justified model to consider, other equally plausible analogies should also be taken into account. This includes, for instance, Herpes labialis, in which the reactivation of viruses – often proposed as a possible pathomechanism in PVP – leads to pathological changes in the sensory end organ (skin). This analogy would support a pathogenetic model with intralabyrinthine changes, rather than an isolated neuritis.

Other evidence cited in favor of the NH includes MRI studies and histopathological analyses. There is one MRI double case report that mentions changes in the vestibular nerve (but not in the labyrinth) in two patients, 7 and 11 days after symptom onset [25]. Besides being a case report, findings made many days after the first symptoms need to be considered with much care. Moreover, this study contradicts two others that looked at MRI’s of 8 and 60 patients with PVP and found no signs of a neuritis [23, 41]. Histopathological studies have found that there was evidence of viral infections and subsequent loss of neurons in the vestibular nerve. However, they also found significant changes in the labyrinth: There was a kind of “epithelialization” on the otolith organs i.e. maculae as well as in the cristae of the SCCs [4]. An animal study confirms the evidence of a viral infection of both the vestibular nerve and labyrinth [13].

The fact that benign paroxysmal positional vertigo (BPPV) is more prevalent in patients with a history of PVP (some 20% of patients with a history of PVP will develop BPPV in their lifetime [20]) also supports a pathogenetic model with involvement of an inthralabyrinthine lesion.

As a whole, these findings do not strongly support the NH. It would therefore at least be prudent to consider the involvement of the vestibular labyrinth in the pathogenesis of PVP.

The main goal of this study was to determine how many of the patients with PVP present with an intralabyrintine lesion pattern. In other words: In what percentage of patients is the vestibular lesion pattern at odds with the vestibular innervation anatomy?

3 Materials and methods

The patient database of our clinic was searched for patients aged 18–80 who had presented between January 2006 and April 2014 and for whom a diagnosis of PVP was considered due to their clinical symptoms (n = 202). After applying the exclusion criteria, 25 patients qualified for our study.

Exclusion criteria were as follows (see alsoFig. 1): patients with a different final diagnosis (n = 60, 7 Menière or migraine; 42 with additional hearing loss, thus labyrinthitis; 11 with vestibular schwannoma), patients on whom relevant diagnostic tests were not performed within 10 days of symptom onset and within a maximum of 3 days after the beginning of treatment with corticosteroids (n = 117). There were no patients in this sample with vertigo of central cause, who we would also have excluded from our study.

We included only patients who had tests performed on all their SCC receptors and the utricle. There were 8 patients without cVEMP’s, thus without saccular diagnostics.

Our approach to classifying patients as having PVP is one of a diagnosis of exclusion [40]. This means we included all patients with prolonged acute vertigo and nystagmus without hearing loss, for which no other diagnosis or more likely cause was found.

In order to understand whether receptors innervated by the same nerve partition (superior, inferior or both) were similarly affected – which would support the NH – we introduced a scale of 4 different dysfunction levels (DL) for each receptor (see Table 1). Those levels are to be understood as follows: 0 = no dysfunction, 1 = slight dysfunction or possible false positive, 2 = definite dysfunction, 3 = strong dysfunction or total loss of function.

The dysfunction levels for the vertical SCC’s were calculated on the basis of their respective HIT gains. If for a specific receptor there were clear corrective saccades despite the presence of a normal gain (3 cases, i.e. covert saccades), they were classified as strongly dysfunctional. The DL of the hSCC was calculated from either the HIT alone or the average of HIT, DVA and caloric CP, in case those latter tests had been performed. The DL’s for the utricle were similarly calculated by using either one or an average of the test results from the SVV [43], the ocular torsion (OT) [9, 11] and oVEMP. Finally, the saccular DL was calculated on the basis of cVEMP’s[37, 45].

We then classified the patients according to whether their lesion pattern supported the NH (i.e. the lesion pattern was the same as the innervation pattern), using a second set of groups: definite neuritis pattern (NP, probable NP, probable ILP (intralabyrinthine lesion pattern) and definite ILP (see Table 2). Strictly speaking, the occurrence of an ILP in a patient would contradict the NH. An intralabyrinthine lesion though, could theoretically present as a NP, which is why the occasional occurrence of a NP does not, by itself, contradict the intralabyrinthine lesion thesis. Therefore, if the NH is true, we would expect an overwhelming majority of the patients to present with a NP, allowing for the few remaining cases to be explained as measurement errors.

4 Results

Among our 25 patients, 19 (76%) had a lesion pattern that looked more like an ILP than a neuritis pattern (NP) (see Fig. 2). 9 of those had a definite ILP, while another 10 likely did not fit the neuritis pattern and thus did not support the neuritis hypothesis (NH). Only 6 patients (24%) had a neuritis pattern, potentially supporting the NH, 4 (16%) of which had a definite, the remaining 2 a likely neuritis pattern. Roughly half of the patients with a definite ILP had major dysfunction differences in the receptors of the superior vestibular nerve, the other half had dysfunction differences in the receptors of the inferior vestibular nerve.

4.1 Inter-method-validity of diagnostic methods for the same vestibular receptor

In order to validly use different measurement methods for the same receptor, it is important that those measurements deliver similar results. Below are the numbers of inter-measurement-discrepancies for the different methods to assess the hSCC and utricular function. Two measurements for the same receptor were seen as significantly discrepant when the resulting DL scores differed by more than 1 DL point.

4.1.1 hSCC

In 2 out of 23 cases where more than the HIT was performed those tests significantly differed from one another (by more than 1 DL point).

4.1.2 Utricle

In 3 out of 15 cases where more than one test for the utricle was performed those results significantly differed from one another (twice by 2 DL points, once by 3 DL points).

All in all, when more than one method was used to assess a receptor function in our study population, it can be said that these methods lead to fairly similar results, meaning that they were sufficiently conclusive to draw further conclusions from them.

5 Discussion

The results of our study do not support the NH since about 3/4 of our patients had an intralabyrinthine lesion pattern inconsistent with an isolated nerve lesion.

Our study does not prove that the only location there can be a lesion causing PVP is the vestibular labyrinth, neither does it reject the NH altogether. However, our results do suggest that there could be other loci affected by PVP in addition to the vestibular nerve.

Another model to consider is that PVP consists of a combined vestibular neuro-labyrinthitis sparing the cochlea. This theory would account for the tendency among certain patients to present with a NP while the ILP found in the others can be explained by an intra-labyrinthine lesion that does not follow the innervation pattern.

Consequently, it raises the question whethersystemic corticosteroids are the best approach to treat PVP. It is well documented, that even high-dosage i.v. corticosteroids only barely reach the perilymph at very low concentrations, while intratympanic delivery would be much more effective [6–8 , 31], not to mention the reduction of systemic adverse effects. And meta-analyses as well as recent studies do not yet show clear evidence for the efficacy of systemic steroid treatment [16 , 19]. In sudden sensorineural hearing loss, where a neuritis is rarely assumed to be the cause, intratympanic corticosteroid application is widely accepted for salvage treatment with sufficient evidence in several studies including meta-analyses [39].

Furthermore, an intralabyrinthine lesion theory would give a more appealing explanation as to why patients with a history of PVP are more likely to suffer from BPPV: It is conceivable that an inflammation of the utricle may cause otoliths to dissociate from the receptor, while the NH fails to address this issue.

A recent study by Magliulo et al. [29] looked at 1-year follow-up results of PVP patients in their oVEMP, cVEMP and vHIT measurements showing acute-phase findings similar to ours: Half of their patients had incomplete involvement of vestibular receptors of their superior and/or inferior vestibular nerve. However, the authors remained true to the NH and argue for the existence of even more subgroups of vestibular neuritis than the two (three) described so far, superior and inferior vestibular neuritis (or the combination thereof). If one were to apply the principle of parsimony though, it might be more accurate to consider an intralabyrinthine lesion as a possible cause of PVP.

6 Limitations

There are several limitations to our study, the main ones being the small sample size and the retrospective study design.

Other criticism may concern the introduction of our classification system of vestibular receptor dysfunction with four different DL’s. We think though that the use of such semi-quantifiable values is justifiable: The idea of introducing DL’s was developed to apply enough spacing between normal vestibular receptor function and a clear dysfunction in order to judge whether the patient’s lesion pattern is in accordance with the NH or not.

Another limitation is the use of different receptor function measurements independent of their availability for each patient. Since our investigation was of a qualitative rather than a quantitative nature, we think that this does not negatively impact the conclusions of our study. However, this certainly is an issue, which we would address in a prospective study design.

Another criticism that might emerge is that neuritis does not need to affect all receptors similarly. In light of other known neuritic lesions of cranial nerves that lead to a more or less homogenous dysfunction of the affected sensory or motor organ (i.e. Bell’s Palsy, optic neuritis, oculomotor neuritis), such criticism does not strike us as justified.

7 Conclusions

To our knowledge, this is the first study to address the issue of lesion patterns in patients with PVP by using tests of all 5 vestibular receptors aiming at proving or disproving the neuritis hypothesis. Our findings, although limited due to the study design and small sample size, indicate that it might be premature to assume that all PVP is caused by neuritis. Further research into lesion patterns in PVP is needed to clarify this issue.

If our suspicion that a significant number of PVP patients, if not all of them, have an intralabyrinthine lesion or inflammation. This may have important implications for the treatment approach to PVP. Since even high dose i.v. corticosteroids are known to insufficiently penetrate the blood-labyrinth barrier, administration of intratympanic steroids could be a more effective choice to treat affected vestibular endorgan lesions. In uncertain lesions, a combination of systemic and intratympanic treatment could be an alternative approach with maybe lower doses of systemic steroids. A large European multicentre study is in preparation.

Footnotes

1

This term is misleading, because it is usually used to describe a combined vestibulo-cochlear deficit. (Baloh, Clinical Practice 2003)

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