Aqueous contamination during small incision cataract surgery: a lesson in study design.

Endophthalmitis remains a small but definite risk during all forms of intraocular surgery and considerable effort has been and still is being expended by ophthalmologists throughout the world to minimise this risk because of the serious visual morbidity associated with such infection. The origin of the infecting organisms is predominantly from commensals on the ocular surfacel 2 and, to a lesser extent, from airborne micro-organisms3 or other endo-genous sources such as the genitourinary tract. It has been clearly demonstrated that viable organisms are introduced into the eye during cataract surgery, and bacteria may be isolated from aqueous in a quarter or more eyes undergoing extracapsular procedures,-8 even when there has been surface disinfection with povidone iodine or preoper-ative topical antibiotics have been administered.89 The reasons for the high rate of anterior chamber contamination during extracapsular surgery include the relatively large size of the wound, which is not self sealing, and generation of negative intraocular pressure, which lead to an influx of the surrounding, potentially contaminated irrigating fluids. The introduction of small incision cataract surgery should theoretically reduce these opportunities for intraocular contamination, but is this really the case in practice? In this issue of the BJO, Manners et al (p 878) report a 20% incidence of aqueous contamination in small incision cataract surgery, which is similar to their previous study of extracapsular surgery.8 The authors have, therefore, concluded that small incision surgery confers 'no proved advantage' over extracapsular surgery in reducing bacterial contamination of the aqueous. However, in two larger studies of 200 and 230 patients respectively, Egger et a!6 7 found that aqueous contamination was significantly less with phacoemulsification than with conventional extracap-sular surgery. It is very likely, therefore, that the study described here did not have the power to detect a small but real difference in contamination rates with the different surgical techniques. A 60% reduction in contamination-that is, only five positive cultures, would have been required in this study to achieve statistical significance at the 5% level. How important are these levels of aqueous contamination and are they clinically relevant? In the series described by Egger et a16 7 and Manners et al, the numbers of bacteria detected were very small. In only one of the 10 positive cases in the current series reported by Manners et al was more than a single colony of bacteria grown on a culture plate, and in their previous study, only …