# Classification and Segmentation of Diabetic Retinopathy: A Systemic Review
## 1 Introduction  
Diabetic retinopathy (DR) is a severe eye condition that results in visual loss. Unfortunately, this illness remains silent at the initial stages and is detected through routine eye checkups [1]. DR has become more common as diabetic patients' life expectancy has increased. Untreated and serious cases of DR might result in blindness, so regular retina screening is necessary for DR patients to avoid becoming visually impaired [2]. DR is a crucial symptom of blindness in those under the age of 50 years. According to some experts, 90% of diabetic people who receive an early diagnosis may be saved from the disease [3]. It is predicted that about 600-Millions of people would have diabetes by 2040, and one-third of them will have DR according to WHO [4]. People are becoming more prone to DR daily; the number has been estimated at 191.0 million by 2030. In the early stage, no symptoms are shown; hence, the detection of DR is a difficult task [5]. The back thin layer of the eye is known as the retina; it manages the light-sensing process and converts this light into signals and sends them to the brain [6]. The optic disc (OD) is a disc-like region on the retina created by axons of retinal ganglion cells, which transmit messages from the eye's photoreceptors toward the optic nerve; it provides assistance for vision. All layers of the retina are supplied with blood by tiny blood capillaries, which are vulnerable to damage when blood sugar levels are elevated. When glucose level in blood is increased, the vessels start to disintegrate because the cells do not receive enough oxygen [7]. Blockage in retina vessels might cause serious eye injury. Therefore, the metabolic rate decreases, allowing DR to enter through structural anomalies in vessels [8]. This can lead to blindness at advanced stages. In all over the world, 2.6% amount of DR is the main reason for visual deterioration [9]. DR identification is achieved due to the existence of numerous types of lesions like microaneurysms (MAs), hemorrhages (HMs), hard exudate (HE), soft exudate (SE), and representations of OD and blood arteries in the retina, as shown in Figure 1.  
* MAs are the initial indication of DR, which appear as microscopic red circular marks on retina caused by the breakdown in the walls of vessel. Sharped margins with a size of less than 125 \(\upmu\)m define the dots on retinal fundus images [11]. * Hemorrhages (HMs) show large patches on the retina with irregular edges that are greater than 125 \(\upmu\)m. It appears when the leakage of blood from blocked retinal vessels impairs vision in the eyes. HMs are further classified into two categories, flame (superficial HMs) and blot (deeper HMs) [12]. * Hard exudate (HE) appears as waxy yellow patches on the retina due to plasma leakage. HE is caused by the production of lipoproteins, which flow from MAs and accumulate in the retina. * Soft exudate (SE) appears as white fluffy patches on the retina with distracted edges caused by the swelling of nerve fibers [13]. Dark red lesions on the retina indicate the presence of MAs and HMs, while bright lesions on the retina indicate the presence of HE and SE. The DR process comprises two distinct stages; the first stage is proliferative-DR (PDR) and the second stage is nonproliferative-DR (NPDR), as mentioned in Table 1. PDR arises when pre-existing micro-blood vessels in numerous parts of the retina produce new abnormal blood vessels. NPDR develops when diabetes damages the retina's blood vessels, which causes blood to seep onto the retina's surface. The leakage of blood minimizes the sensitivity of the retina; therefore, the retina becomes swollen and wet [14]. The different lesions of DR, such as MAs, HMs, HE, and SE, occur at this stage [14]. Depending upon the presence of these lesions, NPDR is additionally classified into three phases: NPDR mild (MAs only), NPDR moderate (MAs and HE), and NPDR severe (intra-retinal HMs and intra-retinal microvascular abnormalities) [15].