Comparison of Serum Apolipoproteins and Traditional Lipids in Eyes with Diabetic Retinopathy in Indian Population: A Case Series

Abstract

Purpose:

To review the associations of serum lipoproteins and apolipoproteins with diabetic retinopathy (DR).

Materials and Methods:

This was a cross-sectional study of patients of DR. DR was graded according to modified Airlie House Classification system. Traditional serum lipids (total, low-density lipoprotein [LDL], and high-density lipoprotein [HDL] cholesterol and triglycerides [TGs]), apolipoprotein AI (apoAI), apolipoprotein B (apoB), and apoB-to-apoAI ratios were assessed. The analysis of variance test was applied. P < 0.05 was considered statistically significant, and P < 0.001 was considered highly significant.

Results:

Twenty-four patients with DR were evaluated. Serum apoAI was statistically significantly associated with a reduced likelihood of having more severe DR levels (P < 0.001). Increasing levels of apoB (P < 0.001) and apoB-to-apoAI ratio (P < 0.001) were statistically significantly associated with increasing DR severity levels. The association of total (P > 0.05) and LDL cholesterol (P > 0.05) with severity of DR was not statistically significant. HDL cholesterol levels were inversely associated with the severity of DR (P < 0.05). TGs were also statistically significantly associated with the severity of DR (P < 0.05).

Conclusion:

Serum apolipoprotein levels may be stronger biomarkers of DR than traditional lipids.

INTRODUCTION

Data from the Diabetes Control and Complications Trial showed that traditional measures of serum lipids were positively associated with the risk of diabetic retinopathy (DR) in type 1 diabetics.¹ However, other studies have not consistently shown similar associations.²,³ ApoAI better reflects lipid accumulation in peripheral tissues⁴ and apoB is present in the retina of the patients with DR.⁵ They may be more directly relevant to the bio-physiological changes associated with DR than the traditional lipids.⁶ However, other studies did not report similar associations.⁷,⁸ The current study compares the association of DR to apolipoproteins and traditional lipid profiles.

MATERIALS AND METHODS

This was a clinic-based observational study that evaluated the association of DR to apolipoproteins and traditional lipid profiles. Twenty-four consecutive subjects attending the DR clinic from April 2014 to April 2015 were enrolled. The subjects were aged between 30 and 70 years. Participants were included if they had diabetes mellitus with signs of DR. All patients consented to the study before enrolled, and the details of procedure were explained to them in local language. Participants with severe hypertension, acute infections, known cardiovascular and renal diseases, liver dysfunction, severe anemia, thyroid disorders, history of glaucoma, previous vitreo-retinal surgery, and/or had a dense cataract were excluded on the basis of history, examination, and routine investigations such as blood test, chest X-ray, electrocardiogram, and urine analysis. Retinal photographs were taken for all participants using standard fundus photography, which captures 30° of the posterior pole of the eye, including the macula and the optic nerve.⁹ The advantages of this type of color photography include ease of use, easily available, and it can be utilized to assist in documentation. Certain morphologic features such as hard exudates are easy to identify on color photographs.¹⁰

DR was graded according to the modified Airlie House Classification system.¹¹ The Early Treatment Diabetic Retinopathy Study (ETDRS) defined diabetic macular edema (DME) as retinal thickening or presence of hard exudates within 1 disc diameter of the center of the macula. To characterize the severity of macular edema and for treatment guidelines, the term clinically significant macular edema (CSME) is used. Macular edema is clinically significant if one of the following conditions are present: Retinal thickening at or within 500 μm of the center of the macula, hard exudates at or within 500 μm of the center of the macula if associated with thickening of the adjacent retina, and a zone or zones of retinal thickening 1 disc area in size, at least part of which is within 1 disc diameter of the macular center.¹²,¹³

Macular edema was documented using optical coherence tomography, which has now become a critical tool in the diagnosis and management of DME.¹⁰ Vision-threatening DR (VTDR) was defined to include severe nonproliferative DR (NPDR), proliferative DR (PDR), and cystoid macular edema (CME). An individual's DR level was based on the DR level of the worse eye.

Fasting (>8 h) blood samples were drawn from participants at local pathology centers to assess fasting blood glucose level, serum lipids (total, high-density lipoprotein [HDL], and low-density lipoprotein [LDL] cholesterol and triglycerides [TGs]), apolipoprotein (apolipoprotein AI [apoAI] and apolipoprotein B [apoB]) levels, and A1C. Serum TG and cholesterol were measured by enzymatic methods, glycerol 3 phosphate oxidase N-ethyl sulfopropyl anisidine and cholesterol oxidase-peroxidase end point methods, respectively. Serum HDL was estimated by the polyethylene glycol precipitation method. Serum LDL was calculated using Friedewald formula (LDL = TC-HDL-TG/5). Serum apoAI and apoB were measured with rate immunonephelometry.

All participants underwent a standardized clinical examination and interview using a detailed questionnaire to obtain information including medical history, current cigarette smoking status, and the use of antihypertensive medications, lipid-lowering medications, and oral hypoglycemic agents. Hypertension was defined as systolic blood pressure (SBP) >140 mmHg, diastolic blood pressure (DBP) >90 mmHg or current use of antihypertensive medications.

Data analysis was performed using SPSS statistical software (version 22, SPSS, Inc., Chicago, Illinois, USA). The analysis of variance test was used to evaluate the differences between means. P < 0.05 was considered statistically significant, and P < 0.001 was considered highly significant.

RESULTS

Of 24 participants, 16.66% (4) had mild NPDR, 33.32% (8) had moderate NPDR, and 49.98% (12) had VTDR. Participants with severe DR had longer duration of diabetes and higher A1C. There was no association between DR severity and blood sugar levels for both fasting and postprandial [Table 1].

Table 1.

Association of diabetes duration, HbA1c, blood sugar, serum lipids, and serum apolipoproteins with the severity of DR

Intraocular pressure was within the normal limits in all subjects. SBP and DBP were elevated in three subjects. Serum apoAI was statistically significantly associated with a reduced likelihood of having more severe DR (P < 0.001) [Table 1]. Increasing levels of apoB and the apoB-to-apoAI ratio were statistically significantly associated with increased severity of DR (P < 0.001, both associations) [Table 1]. There was no statistical association of total cholesterol and LDL cholesterol with the severity of DR (P > 0.05, both associations) [Table 1]. There was a statistically significant inverse association of HDL cholesterol levels with the severity of DR (P < 0.05) [Table 1]. TG levels were statistically significantly elevated as the severity of DR increased (P < 0.005) [Table 1].

Supplementary analyses stratified VTDR into severe NPDR (n = 6), PDR (n = 4), and CSME (n = 2). The association between apoAI and DR severity remained statistically significant (P < 0.001); the association between apoB (P = 0.059) or the apoB-to-apoAI ratio (P = 0.055) and severity of DR severity were not statistically significant.

DISCUSSION

Our study demonstrated that the traditional measures of lipids, higher TG, and lower HDL cholesterol showed a significant association (P < 0.05) with the severity of DR. We also found the associations between serum apoAI, apoB, and apoB-to-apoAI ratio and the severity of DR were highly significant (P < 0.001, all associations). Our findings support previous studies that traditional serum lipids are not strongly or consistently associated with the severity of DR.

Although apolipoprotein measurements have not been widely used in clinical practice, these measures seem to have a distinct and more obvious association with DR compared to traditional lipids. ApoAI, apoB, and the apoB-to-apoAI ratio cover both damaging and protective lipoprotein pathways.

Despite the small sample size, the subjects are typical of other diabetic population with DR, showing strong associations with A1C level and the duration of diabetes.

There were some limitations in our study. First, this was a cross-sectional study; hence, it could not provide information as to whether apoAI and apoB predict progression of DR. Progression of DR has to be evaluated with a prospective study to assess a temporal association. Second, the grading of DR in our study was based on two fields, the optic disc and macula rather on seven fields as used in ETDRS. Third, the sample size for CSME was very small, which precluded specific analysis for CSME alone.

In summary, we report the associations among serum apolipoproteins (apoAI, apoB, and the apoB-to-apoAI ratio) and severity of DR. Concurrently, we showed that apart from HDL cholesterol and TG, conventional serum lipid levels were not significantly associated with DR.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.