Examination of patients with eye problems often requires topical ophthalmic anesthesia. Tetracaine hydrochloride solution commonly is used because it easily penetrates all three layers of the cornea. Instillation of ophthalmic analgesics can be painful; some experts think that the acidity of the solution causes corneal swelling and edema, resulting in pain. Tetracaine hydrochloride 0.5 percent ophthalmic solution has an acidic pH level of 4.54. Bicarbonate has been used as a buffer to decrease the pain of local anesthetics and ophthalmic solutions. Weaver and associates compared pain on instillation of plain tetracaine hydrochloride ophthalmic solution with pain caused by a solution buffered with sodium bicarbonate to a pH level of 7.4.
Using a prospective, randomized, double-blinded, crossover research design, volunteers 18 years and older received two drops of 0.5 percent tetracaine hydrochloride or two drops of 0.5 percent tetracaine hydrochloride buffered with sodium bicarbonate to a pH level of 7.4. After at least seven days, each participant crossed over to receive a similar dose of the other study drug. Pain severity on instillation was measured using a 100-mm visual analog scale (VAS). Assessments were done at one and five minutes after instillation.
Fifty-one (85 percent) of the 60 subjects reported pain immediately after administration of buffered tetracaine. Fewer, but not significantly fewer, patients reported pain immediately following administration of the plain tetracaine. Based on specific pain scores at all measurements, the VAS for buffered tetracaine was significantly higher than that for the plain solution. The results did not vary by the patients' gender or use of contact lenses, and no adverse events were documented.
The authors conclude that a buffered solution for ophthalmic anesthesia was more painful than a plain solution. Possible reasons for the problem with buffered solutions could be the presence of a precipitate or the increased penetrability of a pH-adjusted ophthalmic solution that may result in an increased perception of pain. Because these results were noted in normal control patients, they should be confirmed in patients with painful eye conditions.