Pterygium is a benign yet incorrigible degeneration associated with elevated lesions of the conjunctiva and contiguous cornea. Despite highly effective surgical interventions, it may still affect life quality of patients due to the local depth of invasion and recurrence. Therefore, the search for the exact etiopathogenesis that could pave the way to find the best treatment option for CP with minimal risk of recurrence and/or complications, remains to be an interest of modern ophthalmology [1, 3]. On the other hand, TDH is a new topic for many researchers to investigate the underlying etiology of several irrelevant systemic or ocular conditions.
Pterygium is characterized by inflammatory infiltrates, fibrosis, proliferation, angiogenesis and extracellular matrix breakdown and its pathogenesis is mainly associated with long-term exposure to ultraviolet radiation [1]. Moreover, apoptotic and oncogenic proteins, DNA methylation, lymphangiogenesis, viral infection, extracellular matrix modulators, inflammatory mediators, loss of heterozygosity, microsatellite instability, alterations in cholesterol metabolism and epithelial-mesenchymal cell transition have been identified as other causes [2].
Several studies attempted to discover the underlying molecular mechanisms affecting the course of pterygium as this may help in exploring new therapeutic targets. In this context, the effects of oxidative stress on CP have been well investigated to date [16].
ROS has an important role in the pathological process of pterygium formation, progression and recurrence through suppression of local/systemic immune responses, mutations in certain genes, pyroptosis (a highly inflammatory form of programmed cell death) and glucose-6-phosphate-dehydrogenase (G6PD) deficiency with the help of oxidative damage [17–19].
Oxidative stress and free radicals are closely related to pathophysiological processes in many ophthalmological diseases including but not limited to glaucoma, keratoconus, age-related macular degeneration, and cataractogenesis [20]. It is also claimed to take part in pathogenesis of ocular surface inflammation [21]. Photooxidative products of this network can be revealed immunohistochemically at systemic and/or tissue level. Free radicals are oxygen-containing molecules which can cause large chain chemical reactions in human body as they react so easily with other molecules thanks to the uneven number of electrons they contain. These reactions are called oxidation. Free radicals may arise either naturally in our body through physical activity or inflammation or by environmental elements such as exposure to free radicals by factors such as UV light, ozone, chemicals (certain pesticides/cleaners, metals, solvents, etc.), smoking, radiation, air pollution and infectious organisms. Oxidation is an ordinary and necessary procedure of the body’s intricate mechanism of protecting its health. On the contrary, oxidative stress is a result of the imbalance between antioxidant and free radical activities. Antioxidants can transfer an electron to a free radical while keeping their stability which in turn causes stabilization and reduced reactivity of the free radical getting the electron. Free radicals can be of help in case they function properly. Excess free radicals may cause imbalance against antioxidants and in turn parts of the body including DNA, proteins and fatty tissues. This damage may cause various diseases over time as the aforementioned body parts constitutes a big part of our body [22].
In fact, the molecular pathogenesis of pterygium and the role of oxidative stress in this process have been investigated by several studies before. The diminished activities of antioxidant enzymes such as glutathione peroxidase, superoxide dismutase, and catalase (CAT); higher levels of malondialdehyde and nitric oxide (NO); increased 8-hydroxydeoxyguanosine (8-OHdG); altered p53 protein, and augmented inflammatory elements were found in pterygium tissue [20, 23]. However, they were mainly based on pterygium tissue analysis and it is not clear if pterygium pathogenesis is caused by oxidative stress. Furthermore, to the best of our knowledge, dynamic TDH in CP patients was not studied before.
There are only three articles in the literature analyzing oxidative stress in primary pterygium patients systemically. In their study with 61 subjects suffering from pterygium, Kurtul BE et al. [24] showed that distribution width of red cells was significantly larger in patients with pterygium than control group. Based on the results, they concluded that oxidative stress and inflammation cytokines could play a major role in the pathogenesis of pterygium. In another study, Rasool M. Et al. [25] pointed to a higher level of oxidative stress and inflammatory clinical parameters such as nitric oxide, malondialdehyde, tumor necrosis factor-α and matrix metalloproteinases-9 and a lower level of non-enzymatic small molecules and antioxidant enzymes in cancer patients following systemic chemotherapy. The authors of this study argued that these parameters could also take part in the pathogenesis of ocular complications related to systemic chemotherapy such as retinal degeneration, glaucoma, cataract, blepharitis, pterygium, macular degeneration and retinitis pigmentosa. In the third study evaluating serum total oxidant status (TOS) and total antioxidant status (TAS), Kilic-Toprak et al. [26] claimed that suitable environment for pterygium progress might be created by surged systemic oxidative state and offsetting antioxidant feedback.
Various tests namely TOS, TAS, oxidative stress index, lipid hydroperoxide, paraoxonase, arylesterase and thiols can be employed in measuring the level of antioxidant and oxidant enzymes as along with molecules to determine state of oxidative stress in organisms. Thiols consist of 52.9 % of the total serum antioxidant capacity containing sulfhydryl (-SH) groups and are responsible for modulating glutathione related antioxidant enzymes. Given the presence of ROS in the environment, –SH groups form disulphide bonds by oxidizing, and which implies a prospective protein oxidation mediated by radicals. The disulphide bonds can be reduced back to thiol groups thereby maintaining TDH. Weakened thiol disulphide homeostatic state and shifted balance towards disulphide or thiol side are the two main abnormalities found in TDH tests.
Although lower molecular weight thiol compounds such as cysteine disulfide (CySS), Cys (cysteine), cysteinylglycine (CysGly), reduced glutathione (GSH), oxidized glutathione (GSSG), homocysteine (HCysSS) and glutamylcysteine (GluCys) could usually be measured in the past, only a small portion of the total thiol in the organism is made up by lower molecular weight thiols. The remaining large fraction, on the contrary, is fundamentally consisted of albumin and thiols in other proteins. When analyzing oxidative stress, only the level of thiol could be measured as level of disulphide could not be gauged since 1979. Nonetheless, the new technique developed by Erel and Neselioglu [15] allows simultaneous measurement of both the disulfide and thiol levels resulting in a thorough assessment. In addition to that, the method is superior to other sophisticated methods such as bioluminescent systems, fluorescence capillary electrophoresis and high-performance liquid chromatography (HPLC), in terms of simplicity, ease of use, practicality, sensitivity, cost-efficiency, speed, reliability and the possibility of remeasurement [27].
Among above mentioned studies, the research conducted by Kilic-Toprak et al. [26] is similar to our study. However, our work is more comprehensive compared to it because thiols are the most important reductant compounds in organisms and TDH enables dynamic regulation, performs redox signaling, and sits in a key position as a spot of oxidative stress, it is a useful marker as a clinical measure of oxidative stress in blood serum than TAS and TOS. Moreover, although our sample size was similar to theirs, contradicting with their results, we could not reach statistical significance potentially because our patients’ pterygium grades (grade 1–2) and hence cumulative oxidative load, were lower than their study (grade 2–3). Nevertheless, we found a slight yet insignificant deviation in thiol disulfide balance towards oxidation and a statistically significant negative correlation between both native thiol grade of pterygium and total thiol levels and grade of pterygium in PG. Erel and Neşelioğlu noted that, under oxidative stress conditions, increase in reduced thiol concentration is accompanied by a concurrent decrease in native (non-reduced) thiol concentrations and raise in disulfide amounts. Additionally, they argued that level of plasma disulphide were higher in subjects with degenerative disorders whereas lower in others with proliferative conditions. Moreover, a shift in the balance towards either disulphide or thiol side is at least a sign of weakened TDH even if it is not statistically significant [28]. In the light of this information, though not significant, our results cast doubt on the possibility of a predisposing systemic oxidative background for pterygium formation suggested by above-mentioned studies.
The sole limitation in our study is that it was carried out in a single center with a relatively small group of patients. Yet, it is an invaluable study as it is the first one in its kind to investigate the potential relationship between TDH and CP.
In conclusion, our study is remarkable since it is the first study investigating TDH in patients with conjunctival pterygium. A negative correlation hinting that slightly weakened dynamic TDH in CP patients was showed in this study. Although statistical significance could not be achieved in all the evaluated parameters, we showed that thiol oxidation slightly increased in CP patients implying that this topic must be further examined. Should this relationship be supported by studies with larger samples, systemic antioxidant therapies can be a strong treatment candidate for CP patients.