Round window
Fenestra cochleae
Also known as: fenestra rotunda, cochlear window, secondary tympanic membrane
An opening below and behind the oval window, closed by a flexible membrane (secondary tympanic membrane), at the base of the scala tympani; it bulges outwards to release pressure when the stapes moves inwards.
Overview
The round window lies in a niche posteroinferior to the promontory and is sealed by the secondary tympanic membrane — a three-layered membrane (outer mucosa, middle fibrous layer, inner endothelial layer facing perilymph). It terminates the scala tympani at the base of the cochlea.
Because the cochlear fluids are incompressible, each inward movement of the stapes at the oval window must be matched by an outward movement of the round window membrane (and vice versa); this allows the fluid and the basilar membrane to move. The round window is also a route for drugs into the inner ear (intratympanic gentamicin or steroids) and the usual site of electrode insertion in cochlear implantation.
- Location
Medial wall of the middle ear, posteroinferior to the promontory, below the oval window.
Function
- Pressure relief for the cochlear fluids, allowing the travelling wave
- Access route for cochlear implant electrodes and intratympanic drugs
Clinical significance
Cochlear implants
In severe-to-profound sensorineural hearing loss, an electrode array inserted through the round window into the scala tympani directly stimulates spiral ganglion neurons at tonotopic sites, bypassing absent hair cells.
Round window rupture
Barotrauma (diving, straining) can rupture the membrane (perilymph fistula), causing sudden hearing loss and vertigo.
Compiled from standard anatomy and physiology references — see the Special Senses sources. For education only; not medical advice.