STRC Research

Perilymph Flow and Solute Diffusion

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Perilymph Flow and Solute Diffusion

Drugs and gene therapy vectors enter the cochlea through the round window and spread by two competing processes: passive diffusion (slow, moves substance toward the apex) and active clearance from scala tympani (removes substance, keeps apical concentrations low).

Salt & Ma 2001 quantified both in guinea pigs using ion-selective microelectrodes sealed without perilymph leakage:

  • Longitudinal perilymph flow rate: 4.4 nl/min base-to-apex
  • Clearance half-time from scala tympani: ~60 min
  • Diffusion coefficient for TMPA (small ion, MW ~150): 1.01 × 10⁻⁵ cm²/s

At these numbers, diffusion and clearance reach steady state within ~4-6 hours. After that, prolonging the application doesn’t help — the gradient is stable and apical regions stay ~40× more dilute than the base.

Flow vs diffusion: longitudinal perilymph flow at 4.4 nl/min is tiny. It takes 18+ hours to displace the full volume of scala tympani (4.76 µl) by flow alone. Flow matters for apical delivery only when it’s dramatically increased — which happens when the otic capsule is perforated (CSF floods in at 0.8 µl/min+). Perforation immediately disrupts native gradients.

Mouse vs human cochlea: mouse ST is ~4.5 mm, human ~28.5 mm. With identical diffusion parameters, a human cochlea reaches apical steady state with 6× lower apical concentration and takes much longer to equilibrate. Mouse rescue results systematically overestimate apical delivery in humans.

Practical implications:

  • Cochleostomy or posterior semicircular canal injection reaches the apex more efficiently
  • Hydrogel depots can slow clearance rate and improve distribution
  • Large molecules (AAV ~25 nm) diffuse much slower than ions — their gradients are steeper than the TMPA numbers suggest
  • Any fluid sampling after cochlear perforation measures displaced perilymph mixed with CSF, not native perilymph — scientifically meaningless

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