Meniere's disease: Structural considerations in early cochlea hydrops

Abstract

Objective: Structural features of the human cochlea may control early lesion formation in endolymphatic hydrops. This process may hinge on three structural features: the flattened spiral shape of the human cochlea, the toroidal configuration of the distended cochlea duct, and the distensibility characteristics of Reissner's membrane. An analytical method is presented to assess the variation in hydropic distention that may occur in the several turns of the cochlea due to these structural features.

Methods: A normal human cochlea is used to illustrate the method of analysis. Structural dimensions were taken from a mid-modiolar section. Reissner's membrane was projected to assume a spiral toroid shape as it distends. Peak membrane stress proclivities in each cochlea turn were calculated analytically. Membrane strain was assessed from a collagen model of Reissner's membrane. Sagittal membrane displacements were quantified geometrically.

Results: Stress levels in Reissner's membrane were projected to be the lowest in the lower basal turn and to increase progressively to a peak value in the apex. Strain in Reissner's membrane in the apical turn was projected to be substantially higher than in the lower turns. Sagittal displacement of Reissner's membrane was projected to be most pronounced in the apical turn in all the stages of early cochlea hydrops.

Conclusion: Structural features appear to underlie a differential susceptibility to hydrops in the human cochlea. The flattened spiral shape of the human cochlea coupled with the anticlastic configuration and the distensile characteristics of Reissner's membrane are projected to result in distinct histological stages as hydropic disease in the cochlea progresses.

Keywords: Meniere's disease; apical hydrops; low frequency hearing loss.

FIGURE 1

The human cochlea exhibits a flattened spiral shape with an increasing cochlea duct radius in each turn from the apex to base. The arrows indicate the radial distance of the medial boundary of Reissner's membrane from the modiolus center line.

FIGURE 2

Cochlea duct with a hemi‐cylindrical Reissner's membrane distended medially toward the cochlea modiolus.

FIGURE 3

Figure and formulation for assessing the distention and displacement of Reissner's membrane based on circle sector analysis.

FIGURE 4

Relative stress and strain values in a collagen model of Reissner's membrane.

FIGURE 5

Same model cochlea as shown in Figure 1 with projected relative degrees of endolymphatic hydrops in vivo. The low hydropic displacements of RM in the middle and lower turns are potentially elastic and reversible in vivo and would not be evident in the postmortem histology, whereas the plastic distention in the apical turn would persist in the histology as isolated apical hydrops. The numerical values illustrated for peak RM displacement are calculated values relative to the length of RM in its non‐displaced position and are not to scale.