During the 19 year period the number of cataract operations performed each year increased sevenfold from 1335 in 1980 to 9653 in 1998. We confirmed with the chart review validation 188 diagnoses of post-cataract surgery endophthalmitis and found that the prevalence of post-cataract endophthalmitis increased over that period (Fig 1A). Despite the transition from predominantly intracapsular cataract extraction in 1980 to extracapsular extraction (1982 onwards) then to predominantly phacoemulsification from 1990 onwards (which by 1998 represented over 80% of cataract procedures), there was no significant change in the incidence of endophthalmitis, which averaged 1.98 per 1000 procedures (Fig 1B).
Figure 1 (A) The yearly number of cataract procedures and cases of post-cataract endophthalmitis in Western Australia from 1980 to 1988. (B) The same data shown as incidence rate of post-cataract endophthalmitis (per 1000 cataract procedures) in Western Australia (more ...)
An intriguing feature of the incidence of endophthalmitis was how the rate varied from about one to three cases per 1000 procedures and peaked roughly every 3 years. This may simply represent a random temporal fluctuation in the small population of cases. However, the time series analysis results showed a highly significant negative serial correlation over the entire period of 1980–98 (the first order autocorrelation parameter ρ = −0.674, 95% CI −1.094 to −0.181, p=0.006), despite random fluctuation in some years. This suggests that the incidence rate of endophthalmitis in a year was negatively correlated with the incidence rate of a previous or a following year, as shown in Figure 1B. The reason for this pattern of regular fluctuation is obscure and warrants further investigation.
There was little difference in the incidence of endophthalmitis between those operations coded as extracapsular (1.64 per 1000, 46 298 operations) or phacoemulsification (1.98 per 1000, 32 355 operations) with a rate ratio (RR) of 0.887 (95% CI 0.601 to 1.308, p=0.544). However, the incidence was higher for intracapsular extraction (3.58 per 1000, 5024 operations) compared to extracapsular extraction (RR 0.454, 95% CI 0.272 to 0.760, p=0.003) or phacoemulsification (RR 0.513, 95% CI 0.297 to 0.886, p=0.017).
We also found that the incidence varied with the location of the surgery. There were cataract procedure codes recorded for patients treated at 66 hospitals; however, 14 had less than 10 cases in total and as these were considered coding errors they were eliminated from the analysis. Of these 52 hospitals, 31 had cases of endophthalmitis (mean 1.98 per 1000) as shown in Figure 2, the lowest incidence being 0.65 per 1000 cases—two cases from two hospitals, totalling 3067 cataract procedures, both at the lower prediction interval (not shown in Fig 2).
The incidence and the prediction interval for postoperative endophthalmitis by hospital of cataract surgery in Western Australia (1980 to 1998).
An above average incidence was found among those hospitals that performed less than 1000 cataract procedures (29 of the 52, totalling 5966 cataract procedures), eight of the 29 had 15 cases of endophthalmitis with the individual incidence varying from 3.1–16.4 per 1000, and five of the eight were at or above the upper prediction interval (Fig 2). Endophthalmitis did not occur at 21 hospitals where a total of 3854 cataract operations were performed; however, none of these hospitals individually performed more than 550 cases.
The number of days between the surgery and admission for endophthalmitis was 7 days or less for 82 cases (43.6%), 51 cases (27.1%) occurred after 30 days, which confirms the findings of another general population study of endophthalmitis.8
This suggests that for many patients the infection was of a more indolent nature perhaps related to biofilm formation on the lens implant.9
If remedial or modifiable risk factors at the time of surgery can be identified with further study, a more directed approach for prophylaxis may be formulated. For example once an “at-risk situation” is identified, oral prophylaxis with the highly efficacious combination of ciprofloxacin and rifampicin tablets may be instituted. The ocular pharmacokinetics of these antibiotics would allow sufficient time to effectively treat over half the potential cases of endophthalmitis.10,11