Large vestibular aqueduct and congenital sensorineural hearing loss.

The inner ear is composed of the membranous labyrinth and the osseous labyrinth (1). The membranous labyrinth has two major subdivisions, a sensory portion called the sensory labyrinth and a nonsensory portion designated the nonsensory labyrinth. The sensory labyrinth lies within the petrous portion of the temporal bone. It contains two intercommunicating portions: 1) the cochlear labyrinth that consists of the cochlea and is concerned with hearing, and 2) the vestibular labyrinth that contains the utricle, saccule, and semicircular canals, all of which are concerned with equilibrium. These hollow chambers are filled with fluid, known as endolymph, that resembles intracellular fluid. The endolymph is rich in potassium ions but poor in sodium ions. The various structures considered to be involved in the production of endolymph include the specialized epithelial cells and related blood vessels of the stria vascularis of the cochlear duct, and the dark cells of the utricle and semicircular canals (1). The nonsensoryelement of the membranous labyrinth is formed by the endolymphatic duct and sac whose main function is believed to be the degradation and absorption of endolymph produced in the sensory labyrinth, ie, the cochlea and vestibule.