Analysis of histopathological features of the vestibular sensory epithelium in Meniere's disease and clinical implications
It is noteworthy that despite the heterogeneity of ages of the patients (ages ranging from 29 to 83 years of age) and the length of time with symptoms of Meniere's disease prior to surgery (ranging from 1 to 20 years), there was a remarkably similarity of the endorgan histopathology between all subjects. This is suggestive that there may be a final common pathophysiological pathway for intractable stage IV Meniere's disease.
A common pattern of degeneration was partial or complete replacement of the normal epithelial cytoarchitecture by a monolayer of cells with a differential frequency of occurrence: 92% of the horizontal cristae and 75% of the saccular macula, compared with 24% of the utricular macula. These findings contrast with prior studies reporting normal cytoarchitecture of the vestibular sensory epithelium under light microscopy in Meniere's disease [
11,
16,
23]. A potential explanation is the relative infrequency of epithelial monolayer degeneration in the utricular macula, although we cannot exclude the possibility of more severe disease in our patients. Given the difficulties inherent in harvesting and preserving other endorgans, most prior studies have systematically analyzed only the utricular maculae [
12‐
14,
16]. Ylikoski et al. [
11] reported surgical acquisition of the utricular macula and horizontal canal cristae from 11 patients with Meniere's disease; although, it is not clear how many canal cristae ampullares were successfully harvested and analyzed. The present study represents the first systematic analysis of endorgan specimens from a large number of patients with intractable Meniere's disease, and is the first to document histopathological differences dependent on the endorgan.
Horner [
24] analyzed the post-mortem utricular and saccular macula from one patient with Meniere's disease with a history of vestibular nerve section ten years prior. In that case study, the saccular macula exhibited severe degenerative changes with only a few normal appearing sensory cells, and a denuding of ciliary tufts. The utricular macula of the same patient was relatively spared. We also noted relative sparing of the utricle, and in our study since we excluded patients with a history of prior surgery, there is no confounding effect of prior surgical or chemical denervation.
With regard to clinical implications, the consistent finding of monolayer degeneration of the horizontal cristae ampullares in intractable Meniere's disease is the likely explanation for the frequent occurrence of caloric paresis. Notably, the presence of monolayer degeneration was significantly associated with the presence of caloric paresis (p < 0.001), and monolayer formation was significantly associated with BM thickening (p < 0.001). There was not a significant correlation between the degree of BM thickening and the degree of caloric paresis. This is likely secondary to individual differences of the effect of hair cell loss on the degree of caloric paresis, or the differential effect of BM thickening on epithelial changes. Unfortunately, vestibular evoked myogenic potentials (VEMPs), which reflect saccular function, were not available at the time that these specimens were acquired.
The relative sparing of the utricle to degenerative changes in Meniere's disease warrants further investigation. The utricular macula was relatively spared in age-related hair cell loss compared with the cristae ampullares [
25,
26] and our laboratory applied unbiased stereology to demonstrate an age-related loss of type I and type II hair cells in the human horizontal cristae [
19], and in a subset of the same subjects, a lack of an age-related decline in type I or type II hair cell counts in the utricular macula [
18]. The utricular macula was also relatively spared in intratympanic gentamicin for intractable Meniere's disease [
27]. There are likely biochemical, anatomical, and vascular differences between the endorgans as the human horizontal crista ampullaris has a type I: type II ratio of near unity [
19], and the human utricular macula has ratio of 1.7 [
18]. Tsuji et al. [
28] reported a significant reduction of type II hair cell density, but not a significant reduction of type I hair cell density in Meniere's disease. In the present study, there appeared to be a relatively equal loss of type I and type II hair cells in endorgans which exhibited hair cell loss. However, type I and type II hair cells cannot be discerned in the endorgans with monolayer degeneration. The process may initially begin with hair cell loss and loss of BM homeostasis and then progress to an epithelial monolayer with severe BM thickening.
Other sensory epithelial changes, including the loss and shortening of hair cell stereocilia and cellular vacuolization, were observed equally frequently among the vestibular endorgans. These non-specific changes have been reported in prior studies, which for the most part studied the utricular maculae [
11,
13,
14,
16]. Our study demonstrates that these findings occur amongst all of the individual vestibular endorgans. Loss of hair cell stereocilia may be directly related to Meniere's disease pathology, surgical manipulation or tissue processing, as stereocilial loss has been reported in surgically acquired acoustic neuroma specimens; however, in acoustic neuroma specimens the remaining stereocilial roots were noted [
29]. In our specimens, stereocilial roots were not present. In studies of the endolymphatic sac from patients undergoing surgical resection, stereocilia-like structures have been found in the all of the endolymphatic sacs of surgically acquired specimens for Meniere's disease, but were found in only one out of 18 specimens from acoustic neuroma [
30], similarly to animal models [
31]. Stereocilial loss has been reported in aging as well [
14]. In our study, increasing age was not correlated with loss of stereocilia, and the one patient whose endorgans exhibited stereocilia was 81 years old (patient #16); the two younger patients (29 and 32 years old) had stereocilia loss.
In the present study, supporting cell vesicles and hair cell vacuoles were observed in the majority of all endorgans from subjects with Meniere's disease. Previous histopathological studies of surgically acquired vestibular endorgans from Meniere's disease have noted vesicle formation within the supporting cell cytoplasm, particularly in the apical and middle regions, described as a "foamy appearance of the cell" [
11,
12,
32]. Vacuoles have also been reported in both vestibular hair cells and supporting cells in acoustic neuroma specimens, more numerous in aging, without correlation with tumor size or caloric paresis [
29]. Sensory hair cell and supporting cell vacuoles have also been reported in Meniere's specimens, with reports of increased vacuolization with aging [
13,
14,
16].
Analysis of histopathological features of the basement membrane and stroma underlying the vestibular sensory epithelium in Meniere's disease
A consistently observed striking histopathology was thickening of the BM underlying the sensory and non-sensory epithelium, a finding which was highly significantly correlated with epithelial degeneration. In a total of 24 endorgans which exhibited monolayer degeneration, only one endorgan did not have an associated BM thickening. However, not all endorgans exhibiting BM thickening had degeneration to a monolayer. Neuroepithelial BM thickening occurred less frequently in the utricular macula which also exhibited less frequently epithelial degeneration to a monolayer. Basal lamina thickening of the non-sensory utricle and endolymphatic duct was noted in surgical specimens from a patient with Meniere's disease [
33]. However, their study did not comment on BM pathology underlying the sensory portion of the vestibular endorgans. One of the reasons that prior studies may not have observed BM thickening is that human temporal bones were traditionally embedded in celloidin and sectioned at 20 microns. Likely because of the thickness of the sections or the difficulty to visualize the BM under hematoxylin and eosin, fine alterations such as BM thickening had not previously been described.
Renal BM thickening may occur after a preceding vascular event with subsequent scarring, secondary to a form of deposition disease, or as a reparative process [
34]. A focal thickening of the BM, increased deposition of collagen fibers, and lymphoid infiltrates in the endolymphatic sac epithelium from a patient with Meniere's disease has been reported, however prior ultrasound treatment may have caused an altered cellular response [
35]. Immunological challenge to the endolymphatic sac produces endolymphatic hydrops and cochleo-vestibular disturbances in animal models [
36,
37]. However, in all of the endorgans in the present study, there were no lymphoid cells or inflammatory changes. In the kidney, the BM plays a critical role in solute and ion transport regulation [
38], and thus the thickened BM in the vestibular endorgan may exhibit dysfunctional regulation of endolymphatic fluid and ionic composition.
In the majority of the endorgans, the vestibular stroma underlying the sensory epithelium was notable for increased intercellular stromal spaces. However, the increased intercellular spaces did not correlate with either BM thickening or epithelial degeneration to a monolayer. Surgical manipulation or tissue processing artifact cannot be ruled out. In the present study, ultrastructural pathological vacuolization was noted in the stromal fibrocytes in endorgans from Meniere's patients. Fibrocytic pathology in the spiral ligament of the aged gerbil has been studied extensively [
39]. The spiral ligament contains specialized fibrocytes, and both the spiral ligament and the stria vascularis exhibited volume loss in Meniere's disease compared with age-matched normatives [
40], indicative of spiral ligamental pathology in Meniere's disease. In agreement with a previous study of the utricular maculae from patients with Meniere's disease [
11], stromal nerve fibers appeared to be normal in all endorgans. The endothelial cells of the stromal blood vessels also exhibited vacuolization, which may indicate microvascular pathology in the vestibular endorgans in Meniere's disease.