What is endolymph fluid?

Endolymph Fluid: Composition and Function in the Inner Ear

Endolymph is a specialized potassium-rich fluid contained within the membranous labyrinth of the inner ear that plays a critical role in hearing and balance function through its unique electrochemical properties. 1, 2

Composition and Characteristics

• Unique ionic composition:

  • High potassium (K+) concentration
  • Low sodium (Na+) concentration
  • Low calcium (Ca2+) and magnesium (Mg2+) concentrations
  • Positive electrical potential (+80 mV endocochlear potential) 3

• Location: Found within the membranous labyrinth of the inner ear, including:

  • Cochlear duct (scala media)
  • Semicircular canals
  • Utricle and saccule

• Contrast with perilymph: Endolymph is separated from perilymph (which has composition similar to normal extracellular fluid) by the membranous labyrinth, creating two distinct fluid compartments with dramatically different compositions 1

Physiological Functions

• Mechanoelectrical transduction: The unique ionic composition of endolymph is essential for:

  • Transmission of mechanical stimuli to hair cells
  • Conversion of mechanical stimuli into nerve potentials 1, 2

• Hair cell function: The high K+ concentration and positive endocochlear potential create the electrochemical gradient necessary for hair cell depolarization during sound or movement stimulation 3

• Signal precision: The unique composition allows for precise ciliary mechanoelectrical transduction, enabling the detection of pressure events with diverse amplitudes and durations 2

Production and Regulation

• Production sites:

  • Cochlea: Stria vascularis (marginal cells)
  • Vestibular system: Dark cells 4

• Transport mechanisms: Specialized ion channels and transporters maintain the unique composition:

  • K+ channels and transporters actively move potassium into endolymph
  • Na+/K+-ATPase pumps in marginal and dark cells
  • Various chloride channels 4, 3

Clinical Significance

• Endolymphatic hydrops: Excessive accumulation of endolymph is associated with Ménière's disease, characterized by:

  • Episodic vertigo
  • Fluctuating sensorineural hearing loss
  • Tinnitus
  • Aural fullness 5, 6

• Diagnostic imaging: Advanced MRI techniques can visualize endolymphatic hydrops:

  • Delayed 3-D fluid-attenuated inversion recovery (FLAIR)
  • Inversion recovery turbo spin-echo with real reconstruction
  • Gadolinium-enhanced imaging 5

• Therapeutic implications: Several treatments for Ménière's disease target endolymph regulation:

  • Diuretics (to reduce endolymph volume)
  • Low-sodium diet (1500-2300 mg daily)
  • Endolymphatic sac procedures 5, 6

Pathophysiology in Disease

• Pressure abnormalities: Endolymphatic hydrops is associated with increased hydrostatic pressure, though direct measurement is challenging without risking hearing damage 7

• Ion transport disorders: Dysfunction in ion channels and transporters can disrupt endolymph homeostasis, leading to hearing disorders:

  • Jervell and Lange-Nielsen syndrome (K+ transport defects)
  • Bartter syndrome type IV (Cl- transport defects) 4

The maintenance of proper endolymph composition is essential for normal auditory and vestibular function, and disruptions in its homeostasis can result in significant hearing and balance disorders.