I read with interest an exceptional study by Shimada and associates regarding the use of intraoperative irrigation with povidone–iodine. The study improves our understanding of povidone–iodine and heightens our awareness of its importance at a time when intraocular injections and surgery volumes are on the rise.
For ophthalmic surgeons and those involved in povidone–iodine research, I have a few questions and comments. Shimada and associates point out the important relationship between the concentration of povidone–iodine and its bactericidal effect. The free iodine released from the povidone–iodine is bactericidal, and the povidone–iodine complex serves only as a reservoir for free iodine, which is in equilibrium with the complex. Paradoxically, the lower the percentile of povidone–iodine, the greater the parts per million of free iodine and the greater the bactericidal effect. For instance, 1% has a faster bacteria kill time compared with 10%. What is the lowest percentile that can be used effectively? That depends on the bacteria load. If you use too low of a percentile, then the available iodine from the povidone–iodine complex may become depleted and it may run out of “ammunition” or free iodine before sufficient bacteria kill. Thus, using a lower percentile for irrigation with an increased frequency, insuring enough fresh supply of povidone–iodine complex, makes good sense. However, it does beg the question, why use 10% on the periorbital skin? Would a lower percentile be less toxic and still be effective against the bacteria load?
Further, can topically applied povidone–iodine kill bacteria that find their way into the anterior chamber? In other words, does free iodine pass through the cornea and remain there as free iodine, and if so, for how long? Simply stated, measuring free iodine is difficult. In this study, anterior chamber fluid iodide concentration was measured, so we can not be sure how much free iodine was present. However, if there were free iodine present, that would be a very significant finding that could rival that of topically applied antibiotics. The cultures of the anterior chamber suggest such. If present, the use of povidone–iodine eyedrops before and after surgery could replace the use of topical antibiotics for cataract surgery patients and other ophthalmic procedures.
Like the early publication by Isenberg and associates, this article helps to guide our knowledge of povidone–iodine and its clinical usefulness in ophthalmology. It inspires us to continue in the research of optimizing formulations for ophthalmic use that are more effective, less toxic, less costly, and applied intelligently.