We explored the ototoxic effects of formic acid (molecular weight 46 Da) at different osmotic pressures.
Materials and Methods
The osmotic pressures tested were 120, 300, and 430 mOsm. The acidities of these solutions were adjusted to pH 4. 0 by addition of small amounts of boric acid. Ototoxicity was evaluated in guinea pigs by measuring eights nerve compound action potentials (CAPs). Clicks and tone bursts at 4 and 8 kHz served as stimuli. After baseline CAP data were obtained, the middle ears were filled with formic acid (pH 4. 0) and CAPs relative to baseline were measured 30 min or 24 h later. To prevent infection of the middle ear cavity, all animals received 30 mg/kg body weight erythromycin via injection. To control pain, all animals were intraperitoneally injected with 30 mg/kg body weight pentobarbital sodium, and also received topical injections of 1% (w/v) xylocaine solution.
At 24 h, the formic acid solution with the lowest osmotic pressure (120 mOsm) had minimal adverse effects on the CAPs, but the other solutions significantly reduced the CAPs.
Formic acid solutions (pH 4) of three different osmolarities (120, 300, and 430 mOsm) were evaluated in terms of ototoxicity. The solution of lowest osmolarity was not associated with any ototoxicity whereas the other two solutions caused significant ototoxicity. Thus, the extent of ototoxicity increased when the osmolarity rose.