Dilutional Factors

Some topical products that are ototoxic if used as supplied may not be ototoxic at lower concentrations. Acetic acid at 5% seems to cause more problems than acetic acid at 2%. However, simply diluting a product may not render it safe.

In a series of studies done in cats, Chlorhexidine gluconate was shown to cause degeneration of the hair cells in the labyrinthine vestibule of the vestibular apparatus. At a 2% concentration, chlorhexidine caused profound degeneration of these cells, but at a concentration of 0.05% there was less degeneration of these cells.¹⁰ However, even in the dilute chlorhexidine group there were still clinical vestibular signs. Subsequent studies in cats showed loss of hair cells in the organ of Corti over a very wide range at both concentrations. This indicates a cause for hearing loss after the use of chlorhexidine gluconate.¹¹ Chlorhexidine gluconate also caused the loss of the mucociliary clearance function in the mucosa of the tympanic bulla as it produced subsequent cell destruction.¹²

The use of systemic drugs such as salicylates and furosemide causes increased concentrations of gentamicin in the endolymph in the cochlea within the inner ear, resulting in clinical ototoxicity.¹³ Salicylates may actually increase the membrane conductance of the outer cochlear hair cells, increasing the amplification of sound. In human medicine, this results in tinnitus, a high-pitched ringing in the ears.¹⁴ Other systemic drugs such as erythromycin, streptomycin, and cisplatin are also known ototoxins.