Perilymph
Perilympha
Also known as: perilymphatic fluid, cochlear aqueduct
The sodium-rich, extracellular-like fluid filling the bony labyrinth around the membranous labyrinth — including the scala vestibuli and scala tympani — and bathing the basolateral surfaces of the hair cells.
Overview
Perilymph resembles cerebrospinal fluid and plasma ultrafiltrate: Na⁺ ≈140 mmol/L, K⁺ ≈5 mmol/L, Ca²⁺ ≈1.2 mmol/L, with a protein content higher than CSF. It fills the space between the bony and membranous labyrinths, the scala vestibuli and scala tympani (which communicate at the helicotrema), and the spaces around hair-cell bodies (via the basilar membrane and the fluid spaces of the organ of Corti, the "cortilymph").
It is thought to be formed by filtration from local capillaries (perilymph of the scala tympani) and partly from CSF via the cochlear aqueduct (perilymphatic duct), which connects the basal scala tympani with the subarachnoid space near the jugular foramen; the aqueduct is often narrow or occluded in adults.
Perilymph transmits the pressure waves set up by the stapes at the oval window, and its low K⁺ provides the "exit sink" into which hair cells release the K⁺ that entered from endolymph.
- Location
Bony labyrinth: vestibule (around utricle and saccule), semicircular canals (around ducts), scala vestibuli and scala tympani of the cochlea.
Function
- Transmit pressure waves from the oval window through the cochlea to the round window
- Bathe the basolateral membranes of hair cells and nerve endings in a low-K⁺ medium
- Cushion the membranous labyrinth
- Physiology
Composition (approx.): Na⁺ 140, K⁺ 5–7, Cl⁻ 120 mmol/L; potential ≈0 mV. Continuity with CSF explains why meningitis can spread to the labyrinth (labyrinthitis ossificans) and why CSF pressure changes may affect hearing.
Clinical significance
Post-meningitic deafness
Bacterial (especially pneumococcal) meningitis can spread via the cochlear aqueduct, causing labyrinthitis and sensorineural deafness; the cochlea may ossify within weeks, so urgent audiology and early cochlear implantation are needed.
Perilymph fistula
Abnormal leak of perilymph through the oval or round window (barotrauma, stapes surgery, head injury) causes fluctuating hearing and vertigo induced by pressure (Hennebert sign) or loud noise.
Compiled from standard anatomy and physiology references — see the Special Senses sources. For education only; not medical advice.