Our single-centre study has shown that in people with diabetes, peri-operative glycaemic control as measured by HbA1c was not associated with the risk of developing post-operative ophthalmic complications (as defined by the Royal College of Anaesthetists—Table 3) [17] after cataract surgery (adjusted OR 1.00; 95% CI: 0.99–1.02; p = 0.85). However, for those under secondary care follow-up for their diabetes, there was an increased risk (adjusted OR 1.55, 95% CI: 1.06–2.27, p = 0.03). For the overall cohort, the eGFR was a stronger predictor of the risk of developing complications than HbA1c (adjusted OR 0.99, 95% CI: 0.98–1.00, p = 0.03). We accept, however, that the small difference in value is likely to be of little clinical significance. We have also shown that a higher than previously reported number of people with diabetes underwent cataract surgery—34.6% of the whole cohort.
Table 3 Possible post-operative complications of cataract surgery Our data are in line with recently published data from a UK Biobank study looking at the relationship between HbA1c and the risk of post-operative complications in all forms of surgery. Those authors showed that, whilst the unadjusted risk of an adverse post-operative event was significant (OR [95% CI] 1.43 [1.02–2.02]; p = 0.04) in individuals who underwent any form of surgery within 1 year of an abnormal HbA1c measurement, the risk of poor pre-operative glycaemic control no longer remained significant after adjustment for hyperglycaemia-related comorbidity (OR [95% CI] 1.37 [0.97–1.93]; p = 0.07) [18].
Diabetes causes macrovascular and microvascular complications, namely myocardial infarction, stroke, and renal, nerve and eye disease. Discussions of eye disease in diabetes most often centre on retinopathy, but cataract formation is a recognised ophthalmic complication of diabetes and a leading cause of blindness worldwide [19]. In people with diabetes, cataracts occur more frequently and at an earlier age than in those without diabetes [20, 21]. Age is the major risk factor for those with type 2 diabetes, whilst the presence of other microvascular diseases is the main risk in those with type 1 diabetes [22]. It is believed that diabetes causes cataract formation by causing thickening of the posterior capsule of the lens. The molecular mechanism is believed to be a combination of the non-enzymatic glycation of lens proteins and the accumulation of sorbitol together with polyol pathway activation leading to oxidative stress and apoptosis of lens epithelium [23]. The cascade is similar to that described as one of the unifying mechanisms for the development of other microvascular complications in diabetes [24].
Cataracts are usually treated surgically, with the preferred method being phacoemulsification, which was first introduced almost 50 years ago but became more widely accepted in the mid-1990s [25, 26]. Phacoemulsification is preferred partly because it has better outcomes for visual acuity and has a lower cost compared to extracapsular cataract extraction [27]. As with all forms of surgery, it has a complication rate.
The most recent audit from the Royal College of Ophthalmologists showed that a complication occurred in 2.8% of all cataract operations carried out in 2018–2019 [2]. These complications included uveitis, keratopathy, worsened visual acuity, retinopathy and maculopathy [28,29,30]. Although the overall outcome of phacoemulsification is excellent, results may differ in people with diabetes [31].
There are data to show that inadequate glycaemic control, measured as high peri-operative glucose [9,10,11,12] or HbA1c [13,14,15], is associated with poor post-operative outcomes. These include increased risk of surgical site infection, the development of acute kidney injury, risk of myocardial infarction, time in the intensive care unit, or death [32]. For cataract surgery, several studies have compared the outcomes with and without diabetes. A review from 1998 showed that people with diabetes had a number of structural and functional corneal abnormalities that significantly increased the risk of post-operative harm [33]. However, there are no data on the outcomes of cataract surgery in people with high pre-operative HbA1c [16]. A small study compared the outcomes of cataract surgery in 56 eyes in people with either type 1 or type 2 diabetes to the outcomes of cataract surgery in 220 eyes of people without diabetes [34]. The glycaemic control of those with diabetes was generally excellent, with a median HbA1c of 44 mmol/mol (6.2%; the threshold HbA1c for diagnosing diabetes is 48 mmol/mol, and below 43 mmol/mol is considered normal). If, as the Royal College of Ophthalmologists report suggested, only 2.8% of people develop complications, this equates to 2 eyes vs 6 eyes in this study, a number that would suggest that the study was significantly underpowered to detect any differences [2]. It is therefore perhaps unsurprising that because of this lack of data, the most recent guideline on eye care for people with diabetes from the International Council of Ophthalmology makes no comment on peri-operative glycaemic control [35].
We found that a far higher proportion of those undergoing cataract surgery had diabetes than was reported over 25 years ago [3]. Those authors quoted a prevalence of approximately 20%, whilst our prevalence was 34.6%. This may reflect the increased prevalence of diabetes in the general population and the ageing population.
We also found that the outcome of cataract surgery is worse for those under secondary care follow-up for their diabetes, with a greater percentage of those people having post-operative complications. This is perhaps unsurprising because the vast majority of diabetes is looked after in primary care, and it is only those who have more difficult-to-control diabetes who would be referred to secondary care for ongoing diabetes management. Despite that, the diabetes control of those with and without complications may be deemed very good: 52 and 50 mmol/mol, respectively. These values are well below the target of 69 mmol/mol set by the UK national guideline for the management of people with diabetes undergoing surgery [36].
Our study has several strengths. We had a large sample size and, because our institution is rather geographically isolated, almost everyone operated on at our hospital is followed up here, and it is their predominant diabetes care provider if they are looked after in secondary care. We also use only one laboratory, meaning that we have access to the biochemical and haematological results for almost everyone.
We acknowledge that there are some limitations to our work. This is a single-centre retrospective cohort study, and as such may not be generalisable. We believe that we have captured everyone who was operated on because we have a specified database that is used for all surgical procedures undertaken at our hospital. We also have a single provider for diabetes-related eye screening across primary and secondary care for the majority of our geographical catchment area. However, there are alternative providers of diabetes eye screening at the peripheral margins of our catchment area. In those regions, cataract operations are carried out at local hospitals, and thus the number of people that travelled to our unit and were thus missed out in our database was likely to have been very small. Furthermore, we do not do ‘community clinics’, and so everyone listed on the diabetes register would be under secondary-care diabetes follow-up. People with long-standing type 2 diabetes, i.e. mainly the elderly population, may have other pre-existing comorbidities that could increase the risk of post-operative complications and are thus cofounding factors that were not adjusted for in our analysis. Furthermore, because cataract surgery is an elective procedure, it may have been that individuals were encouraged to improve their pre-operative glycaemic control in line with national guidance for general surgery [36]. This would have reduced the impact of chronic hyperglycaemia. Because of the retrospective note-based nature of this study, we were also unable to look for other factors that may have further confounded our results, such as smoking, alcohol, body mass index, anaemia, duration of diabetes and blood pressure. Several ophthalmic variables were also not determined because that was not the primary focus of the study. These included pre-operative intraocular pressure, pre-operative spherical equivalent, anterior chamber depth, mean keratometry, intraocular lens power and the presence of previous laser treatment or surgery. Furthermore, because this was a retrospective analysis interrogating clinic letters generated from eye clinic visits to see whether complications were mentioned, we were unable to determine if a complication occurred at the time of surgery or in the post-operative period.