LogMAR gain: Insufficient parameter to conclude HbA1c levels do not affect cataract surgery outcomes

Abstract

In their recent paper, Liu et al.1 reported that patients gained an equal amount of lines of corrected distance visual acuity (CDVA) after cataract surgery regardless of the presence of diabetes and diabetic retinopathy (DR) status. Furthermore, the gain in CDVA was independent of the patients’ preoperative serum hemoglobin A1c (HbA1c) levels. These conclusions were drawn by univariate and multivariate analysis of the following CDVA parameters: (1) the mean change in logarithm of the minimum angle of resolution (logMAR) units and (2) the proportion of eyes achieving a Snellen CDVA of 20/20. In our opinion, the authors’ conclusion regarding the effect of DR and HbA1c levels on cataract surgery outcomes in diabetic patients requires analysis and discussion beyond what was presented in the paper. Previous studies report an association between the degree of DR and foveal thickening after cataract surgery and that these factors are inversely associated with visual outcomes. In a study by Ylinen et al.,2 the HbA1c levels were inversely associated with the increase in central subfield macular thickness, even after adjustment for confounding factors. Furthermore, poor glycemic control has been associated with high intraocular vascular endothelial growth factor levels, which predicted a risk for postoperative exacerbation of macular edema.3,4 Typically, cataract surgery significantly improves the patient’s vision. Only a small proportion of eyes develop clinically significant pseudophakic cystoid macular edema, which poorly reflects to the mean change in logMAR units. Also, the baseline CDVA might not be comparable between the different DR or HbA1c subgroups, which makes the gain in logMAR an inappropriate parameter to compare the outcomes of cataract surgery between groups. It would be interesting to determine how the CDVA is distributed among the minority of patients with a poor clinical outcome. For example, analysis of the proportion of eyes with 20/40 CDVA, with 20/32 better reflecting with unwanted clinical outcomes, might be more relevant than the proportion of eyes with 20/20 CDVA or even the mean change in logMAR units. Based on the above, we believe that the evidence is not adequate to conclude there is no relationship between HbA1c control and CDVA after cataract surgery. Moreover, CDVA is only a partial measure of the patient’s visual function. For example, a recent study of patients with diabetic macular edema who switched from long-term ranibizumab treatment to aflibercept5 found a stronger relationship between the changes in central retinal thickness and contrast sensitivity than between the changes in central retinal thickness and CDVA. Changes in macular thickness might affect many daily visual tasks that are highly dependent on contrast sensitivity (eg, reading, driving, and face recognition); however, the impairment might not be apparent from a typical high-contrast visual acuity assessment. In Table 1, we present a univariate post hoc analysis of cataract surgery outcomes in diabetic patients by HbA1c groups (ie, HbA1c <6.5%, 6.5% to 8.9%, ≥9.0%).2Table 1: Cataract surgery outcomes at 28 days by preoperative HbA1c levels in diabetic patients.