Severe retinopathy in type 1 diabetic patients is not related to the level of plasma homocysteine

The vascular-injuring amino acid homocysteine was previously shown to be increased in plasma in type 1 diabetic patients with clinical signs of nephropathy. Previous studies have also shown an inconsistent relationship between the development of diabetic nephropathy and retinopathy, indicating different pathogenetic mechanisms. In this study, plasma homocysteine was measured in 25 type 1 diabetic patients with a well-characterized form of severe retinopathy. Furthermore, a group of 24 type 1 diabetic patients with similar age at onset of diabetes and diabetes duration with no or minimal background retinopathy were investigated, in order to determine whether plasma homocysteine levels are different from those in patients with severe retinopathy. Patients with severe retinopathy did not have higher plasma levels of homocysteine (13.9 mu moI/L; 5.9-30.7, median and range) than those without retinopathy (10.4 mu mol/L; 5.7-18.9). Within the group of patients with severe retinopathy, increased homocysteine levels were confined to the patients (19.9 mu mol/L; 10.0-30.7, n = 9) with serum creatinine. levels > 100 mu mol/L, compared to those patients (9.6; 5.9-14.3 mu mol/L, n = 15) with a serum creatinine below 100 mu mol/L. None of the patients without or with minimal background retinopathy had serum creatinine levels > 100 mu mol/L. We conclude that diabetic retinopathy is not associated with increased plasma homocysteine levels, but plasma homocysteine accumulates, probably owing to reduced glomerular filtration, in diabetic patients with signs of nephropathy. In these patients, the promoting effect of nephropathy on the development of retinopathy does not seem to be mediated through homocysteine.