The extracellular fluid that fills the bony labyrinth of the inner ear, surrounding the membranous labyrinth. High sodium, low potassium — the opposite profile to endolymph.
Medical definition
Perilymph is the fluid that occupies the scala vestibuli and scala tympani of the cochlea, as well as the space between the bony and membranous labyrinths of the vestibular apparatus. Its ionic composition is high sodium, low potassium, similar to extracellular fluid and cerebrospinal fluid. It is distinct from endolymph, which fills the membranous labyrinth and has the opposite ionic profile. The difference in ionic concentration between perilymph and endolymph generates the endocochlear potential — the electrical gradient that powers hair cell transduction. Disruption of this gradient impairs both hearing and balance.
Why it matters for vertigo
Perilymph fistula is a tear in the oval or round window membrane, allowing perilymph to leak into the middle ear. It produces vertigo triggered by pressure changes, sneezing, straining, or nose-blowing. It is often caused by trauma, heavy lifting, or barotrauma. The condition is underdiagnosed because there is no reliable non-invasive test. In Meniere disease, the problem is not perilymph but endolymph: excess endolymph (endolymphatic hydrops) distends the membranous labyrinth and disrupts the perilymph-endolymph boundary.
Where I see this in clinic
Patients who present with vertigo after a forceful Valsalva maneuver, heavy lifting, or violent coughing, and then develop pressure-sensitive dizziness, I investigate for perilymph fistula. The Hennebert sign (vertigo or nystagmus on tragal pressure or pneumatic otoscopy) can suggest fistula. Management ranges from bed rest with head elevation for minor cases to surgical patching for persistent leaks. I am also careful about perilymph in patients with prior ear surgery, as a dehiscent oval window from previous stapes surgery can produce similar symptoms years later.
Related terms
Endolymph — the separate inner-ear fluid with opposite ionic composition. Aural fullness — pressure sensation that can accompany perilymph fistula. Cochlea — the hearing structure containing perilymph-filled compartments. Caloric test — used to assess vestibular function when fistula is suspected.
Medical Disclaimer: This glossary entry is for educational purposes only and is not a substitute for in-person clinical assessment. Consult Dr. Prateek Porwal directly. WhatsApp: 7393062200.