The round window is one of two openings in the middle ear, localized at the level of the cochlea, allowing communication between the mesotympanum of the middle ear and the inner ear. It vibrates with opposite phase to vibrations from the inner ear, producing movement of perilymph in the cochlea.
It is located at the bottom of a funnel-shaped depression called the round window niche and measures approximately 2-3 mm long and 1.5 mm wide.
It is sealed by a membrane called the secondary tympanic membrane (round window membrane), which can sometimes be seen on high-resolution CT.
Absence or rigidity of the round window is associated with hearing loss.
Rupture of the secondary tympanic membrane (trauma, iatrogenic, etc.) can result in perilymph leakage.