One of the most important causes of blindness in many countries is diabetic retinopathy (DR). Due to ischemic retinal changes, the patients may progress into proliferative diabetic retinopathy (PDR) including retinal neovascularization, vitreous hemorrhage ending by tractional retinal detachment and neovascular glaucoma and hence loss of vision. Pan retinal photocoagulation (PRP) and intravitreal injection of anti-vascular endothelial growth factor (anti-VEGF) are considered mandatory treatment modalities in PDR   .
The process of progression of diabetic retinopathy may depend on different mechanisms including long duration of diabetes, prolonged hyperglycemia, microangiopathic changes, abnormal metabolic pathway, oxidative stress and subclinical inflammation. Early detection of PDR and follow up of patients by serum biomarkers beside the clinical picture may play a role in prevention and control of PDR   .
In 1930, C-reactive protein (CRP) was identified, it is a protein synthesized mainly by liver and adipose tissue, it is considered as acute phase protein increased in inflammatory and microbial invasion. In practice measurement of CRP is helpful in diagnosis of some inflammatory conditions. The role of CRP in DR is still not clear, but many studies reported that CRP increased in DR especially in PDR depending on the inflammatory process and abnormal metabolic and vascular pathways mechanisms involved in DR giving the importance for CRP in assessment of the DR state    . Also aging process and hyperglycemia activate multiple cellular pathways that play the important role in progression of diabetic retinopathy in a relation to the inflammation and endothelial injury that associated with the pathogenesis of diabetic microvascular complications    .
Some studies as Song et al. (2015)  , Yang et al. (2016)  and Zhou et al. (2016)  reported the relationship between elevated serum CRP and the progression of proliferative changes of diabetic retinopathy, they also stated that CRP not significantly increased in NPDR, but significantly increased in PDR, so that CRP is as considered a biomarker in the progression into PDR.
This study was limited in evaluation of serum CRP in cases with DR without attention to other contributing factors that may play a role in the progression of DR.
2. Patients and Methods
This observational case control study was performed in Zagazig University Hospitals in the period from January 2016 to September 2016. The study was performed according to WMA Declaration of Helsinki-Ethical Principles for Medical Research Involving Human Subjects.
The study included 90 persons who were selected to be free of any possible contributing factors that may affect serum CRP as inflammatory and degenerative conditions. They were divided into 4 groups, the group A of 30 healthy non diabetic persons as a control group, group B of 30 diabetic patients without retinopathy, group C of 30 patients with untreated bilateral PDR, and group D of the same patients in group C but after 3 months of laser treatment (PRP).
Criteria of selection of the control group (A): The age of persons was determined according to group B, C to be similar matched.
- Age from 30 - 70 years.
- Non diabetic persons via estimation of fasting blood glucose level below 110 mg%, postprandial level below 150 mg% and glycosylated hemoglobin (HbA1c) below 6%.
- Free of other degenerative or inflammatory systemic or ophthalmic diseases by systemic and ophthalmic examination.
Criteria of selection of group B:
- The same criteria in the control group A except they are diabetic patients (both type I and II) under medical treatment without retinopathy to detect normal retinal finding by indirect ophthalmoscope, slit-lamp biomicroscopy and fluorescein angiography.
Criteria of selection of group C:
- The same criteria in the control group A except they are diabetic patients (both type I and II) under medical treatment with PDR diagnosed by indirect ophthalmoscope, slit-lamp biomicroscopy and florescence angiography by detection of neovascular malformations at optic disc (NVD), elsewhere in the retina (NVE) or at iris (NVI). Cases with vitreous hemorrhage and tractional retinal detachment were excluded.
Criteria of group D:
- The patients in this group represented the same patients of group C but after 3 months of bilateral laser treatment by PRP, 1000 - 1400 shots of double frequency Yag laser of moderate power, spot size 200 - 400 μm, duration 0.1 second.
All groups were subjected to estimation of serum level of C-reactive protein, by collection of fasting 12 hours venous blood samples, estimation of serum (CRP) using C-Reactive Protein ELISA Kit, Human (CYT298) (Merck).
Data was collected and statistical analysis was performed using One Way ANOVA, determination of P value, >0.05 was insignificant, <0.005 was significant and <0.001 was highly significant.
The demographic data of the study revealed that in the control group A, there were 16 male (53.3%) and 14 female (46.7%), in group B, C there were 15 male (50%) and 15 female (50%), The mean age in the control group A was (52.55 ± 6.28), in group B was (51.35 ± 7.06) years, in group C, D was (50.15 ± 3.59) years.
The type of diabetes in group B was type I diabetes in 11 patients (36.6%) and type II in 19 patients (64.4%) and in group C, D Type I in 10 patients (33.3%) and type II in 20 patients (66.6%).
The level of serum CRP in the control group A was (8.23 ± 4.12) mg-L not differed significantly from group B of diabetic patients without retinopathy (7.89 ± 3.02) mg-L (P > 0.05), but differ significantly from group C of patients with untreated PDR (14.03 ± 6.19) mg-L (P < 0.001) and in group D (13.9 ± 4.43) mg-L which not differed significantly from group C (P > 0.05) (Table 1).
Also, the level of serum CRP among persons above 50 years old in the control group A was (11.03 ± 1.52) mg-L not differed significantly from the same age
Table 1. Demographic data and serum CRP levels in all groups.
P (>0.05) insignificant between * and ** (group A, B). P (<0.001) highly significant between ** and *** (group B, C). P (>0.05) insignificant between *** and **** (group C, D).
Group in group B of diabetic patients without retinopathy (12.1 ± 2.17) mg-L (P > 0.05), but differed significantly from group C with untreated PDR (15.12 ± 1.09) mg-L (P < 0.001) and group D (14.84 ± 2.52) mg-L which also not differed significantly from group C (P > 0.05).
Micro-vascular changes in diabetic patients may complicate into serious conditions such as PDR, the mechanisms of these changes may be related to different pathological factors. Monitoring of these factors may have a role in follow up of these patients. CRP is considered one of the factors that added a famous question about the relation to DR and the possibility to monitor this factor in the progression and regression of that process. The present study asked this question, which was previously studied by Heng et al. (2013)  and Song et al. (2015)  .
The present study reported that CRP was significantly higher in diabetic patients with PDR in comparison to the control persons and diabetic patients without retinopathy, in agree with the study of Song et al. (2015)  and Gorska-Ciebiada et al. (2015)  , denoting that the vascular and inflammatory pathogenesis in diabetic patients giving the risk for diabetic complications especially in elderly patients as CRP normally increases in elderly persons, and the present study also revealed that it was higher in persons above 50 years than others, but also in elderly diabetic patients with PRD was significantly higher than elderly control persons and diabetic patients without retinopathy (P < 0.001), so that , an explanation was obtained by Kislinger et al. (1999)  and Wautier et al. (2001)  who stated that hyperglycemia may be responsible for a change in the immune inflammatory pathway and hence the production of non-enzymatic glycation on lysine and arginine residues accumulate in diabetic tissues-plays the important role in pathogenesis of diabetic complications.
As regards to the serum level of CRP after the laser treatment by PRP, the present study stated that there was no significant difference between the level of CRP in patients with PDR before and after treatment denoting that it not be the important biomarker for follow up of patients after treatment. Also the mechanism of regression of level of the inflammatory biomarkers in relation to treatment may depend on other different factors that were incorporated in the study of Sharma et al. (2015)  .
C-reactive protein was considered a biomarker for PDR, but not a good indictor used for follow up patients after laser treatment. Regular estimation of serum level of CRP especially in elderly diabetic patients may be a good biomarker for diabetic micro-vascular complications. More studies are needed for evaluation of other inflammatory markers that may be contributed in the progression of DR.
Conflict of Interest
The authors declare no conflict of interest.