Permeability of the Round Window Membrane Is Influenced by the Composition of Applied Drug Solutions and by Common Surgical Procedures

Introduction: Intratympanic drug delivery has become widely used in the clinic, but little is known regarding how clinically used drug preparations affect round window membrane (RWM) permeability or how much drug is actually delivered to the cochlea. This study evaluated the effect of clinically relevant carrier solutions and of suction near the RWM on the permeability properties of the RWM. Methods: RWM permeability was assessed by perfusion of the marker trimethylphenylammonium into the round window niche while monitoring entry into perilymph using trimethylphenylammonium-selective electrodes sealed into scala tympani. Results: High-osmolarity solution increased RWM permeability by a factor of 2 to 3, benzyl alcohol (a preservative used in some drug formulations) increased permeability by a factor of 3 to 5, and suctioning near the RWM increased permeability by a factor of 10 to 15. Conclusion: Variations in available drug formulations can potentially alter RWM permeability properties and affect the amount of drug delivered to the inner ear. Drug solution osmolarity, benzyl alcohol content, and possible drying of the RWM during suctioning the middle ear can all have a substantial influence of the perilymph levels of drug achieved.