Several recent pieces of research have noted that the destructive changes in the retina noted in diabetics, known as diabetic retinopathy, have been occurring at lower blood sugar levels (both hemoglobin A1C and daily blood sugar level). Many patients who have never met the formal criteria for diabetes, but have higher than normal blood sugar levels, seem to be developing diabetes.
I have linked to several of these studies. In 2005 the Diabetes Prevention Project (reported at ADA 65th Annual Scientific Sessions) found that 8% of prediabetics, who never developed diabetes, nevertheless showed retinopathy:
ADA Scientific Sessions: Retinopathy Found in Pre-Diabetes. Elizabeth Thompson Beckley. DOC News August 1, 2005. Volume 2 Number 8 p. 1
Here is some more recent research along the same lines:
Relation between fasting glucose and retinopathy for diagnosis of diabetes: three population-based cross-sectional studies Wong TY, et al Lancet 2008; 371: 736-743.
Association of A1C and Fasting Plasma Glucose Levels With Diabetic Retinopathy Prevalence in the U.S. Population: Implications for diabetes diagnostic thresholds Yiling J. Cheng et al. Diabetes Care November 2009 vol. 32 no. 11 2027-2032. doi: 10.2337/dc09-0440
And Colagiuri and colleagues in 2010 metastudy reported in Diabetes Care noted that hemoglobin A1C in the mid- 6’s was associated with signs of retinopathy:
Glycemic Thresholds for Diabetes-Specific Retinopathy: Implications for Diagnostic Criteria for Diabetes:The DETECT-2 Collaboration Writing group. Stephen Colagiuri et al. Diabetes Care Published online before print October 26, 2010, doi: 10.2337/dc10-1206
Overall the messages of these articles suggest that prediabetics are about 1/3 as likely as diabetics to see retinopathy, but somewhere in the 8-11% range of prediabetics showed signs of the damage to the retina.
I also discussed the issue of prediabetes and retinopathy on my diabetes website.