||The histopathologic hallmark of Meniere's disease is a generalized distension of the cochlear and saccular portions of the endolymphatic system (endolymphatic hydrops) due to an increase in the pressure and volume of the endolymph. It is the mechanical effects of the endolymphatic hydrops that appear to be responsible for the production of the symptoms in Meniere's disease.
The earliest changes occur in the scala media (the cochlear portion of the endolymphatic system), where there is a displacement or ballooning of the thin Reissner's membrane into the scala vestibuli. The apical turn is the first area in which this dilatation of Reissner's membrane occurs. In the more advanced stages of the disease the distension is greater in the middle and basal turns.
The cochlear duct has the capability of great dilatation, and the thin Reissner's membrane may distend to such an extent that it lies against the bony walls of the scala vestibuli. If this happens the cochlear duct is in effect occupying the entire scala vestibuli. The cochlear duct may even herniate through the helicotrema into the scala tympani. In the early stages of this disease ("compensated" Meniere's disease), Reissner's membrane appears to maintain its elasticity so that it can expand during each attack and return to its normal position when the attack is over. In chronic or "decompensated" Meniere's disease, Reissner's membrane permanently loses its elasticity and remains in the distended position.
The increased area of the cochlear duct has been statistically correlated with both the duration of the disease and with the total average hearing loss. A correlation has also been found between the increased area in each turn of the cochlea and the average hearing loss in the corresponding frequencies.