Ankle-brachial Index, Peripheral Arterial Disease, and Diabetic Retinopathy

Dear Editor

We read with interest the article by Yun et al. [1], who reported that the frequency of diabetic retinopathy was associated with the presence of arterial stiffness as measured by the brachial ankle pulse wave velocity, but not with common carotid artery intima-media thickness, carotid plaque, or peripheral arterial disease (PAD), in a Korean sample of type 2 diabetic patients. They suggested that the association between macroangiopathy and microangiopathy may be due to a functional rather than structural process within the vascular system, although arterial stiffness is widely accepted as a structural phenomenon characterized by medial calcium accumulation in the vessel wall.

Yun et al. [1] found no relation of PAD as diagnosed by ankle brachial index (ABI) measurement to the presence of retinopathy in their diabetic participants. They successfully discussed this finding in light of the previous studies that had displayed conflicting data with respect to an association with microangiopathy and clinical or subclinical atherosclerotic diseases. They also linked their "negative condition" findings to several factors such as selective survival, discordant sample sizes in the tested groups, population characteristics, image magnification limitations, or the age distributions in their study population. At this point, we needed to make some more contribution by going through the diagnosis of PAD by ABI measurement in people with diabetes mellitus.

Use of ABI measurement in the diagnosis of PAD in subjects with diabetes, especially in those with long-standing disease, requires caution with regard to recent findings in large trials and recommendations inserted into the guidelines. Diabetes has been linked not only to accelerated atherosclerosis and plaque formation but also to earlier initiation of arterial stiffness [2], as was also shown in the study by Yun et al. [1]. However, it is well known that arterial stiffness often causes inversely increased ABI values due to incompressible vessels [3], masking the presence of occlusive disease in the lower extremities that could be identified by ABI testing. Consistent with this, diabetes was clearly found to be a risk factor for a high ABI [4]. Moreover, diabetic individuals with ABI <0.90 and ABI >1.40 were demonstrated to display similar patterns of abnormalities in different diagnostic tests for lower extremity ischemia, and a high ABI was suggested to be PAD-equivalent [4]. Therefore, it is possible that the true prevalence of PAD might have been underestimated by determining a low ABI in the study by Yun et al. [1], resulting in insufficient statistical power as well. Indeed, Table 1 of the study shows that the frequency of PAD was detected to be 20.45% lower in those subjects with retinopathy compared to those without, which is not consistent with the knowledge that the duration of diabetes is positively associated with the prevalence of cardiovascular diseases. In order to improve the understanding of our readers, it would be helpful if the authors could re-evaluate their data for high ABI (>1.4) or collectively with ABI ≤0.9 because, Papanas et al. [5] reported a significantly higher prevalence of a low ABI in diabetics with microvascular complications when the subjects with medial arterial calcification were specifically excluded.