Ординатура / Офтальмология / Английские материалы / Ocular Disease Mechanisms and Management_Levin, Albert_2010

Box 65.1  Retinopathy in type 1 and type 2 diabetes

•Diabetic retinopathy seems not to be different in type 1 and type 2 diabetes

•The severity of lesions seems directly related to the severity of hyperglycemia, but the type of lesions that develop are not different between type 1 and type 2 diabetes

capillary nonperfusion, vascular leakage, and hemorrhage) to date (Table 65.1), although attention has also been paid to function of the neural retina.15–24 There have been fewer attempts to inhibit the retinopathy in diabetic patients than there have been in diabetic animals, undoubtedly due to the long durations and significant costs required to demonstrate any effect, and those attempts have been far less successful compared to the animal studies (described below). Pharmacologic studies of histopathology lasting about 3 years or less have not been successful (whether this is due to a faulty hypothesis or insufficient study duration is not clear) to date. The Diabetes Control and Complications Trial (DCCT) did demonstrate efficacy of insulin therapy on development or progression of histopathology after about 5 years,25 suggesting that 5 or more years may be required for an objective test of drug therapies in clinical studies for these parameters. Effects on retinal edema (secondary to vascular leakage) and retinal function have required lesser durations of study.

Advanced retinopathy can currently be treated by laser photocoagulation or intravitreal steroids. Panretinal (scatter) photocoagulation, which is performed using a relatively high-energy laser, ablates relatively large areas of retina, presumably to reduce the hypoxia of the remaining retina. It results in a decrease in the formation of proliferative vessels, intravitreal hemorrhage, and retinal detachment, and can significantly reduce the risk for severe vision loss.26 Focal/ grid laser photocoagulation to the retina can reduce the risk of loss of vision by nearly 50% in patients with clinically significant diabetic macular edema.27–29 Intravitreal steroids likewise are having dramatic effects on visual impairments

Table 65.1  Therapies and their effects on diabetic retinopathy in patients

due to macular edema.30,31 Nevertheless, vision loss continues in some patients, and these approaches do not address the underlying etiology of the retinopathy.

Intensive insulin therapy has been shown to inhibit development of vascular lesions of diabetic retinopathy in patients,25 dogs,32,33 and rats transplanted with exogenous islets.34 DCCT25 showed that intensive control of blood glucose inhibited the progression of existing retinopathy by 54% in patients with type 1 diabetes. Likewise, both the UK

Prospective Diabetes Study (UKPDS)35 and the Kumomoto study36 demonstrated a protective effect of glycemic control on the development of retinopathy in type 2 diabetes. Nevertheless, the DCCT and the follow-up Epidemiology of Diabetes Interventions and Complications (EDIC) studies have shown that instituting tight glycemic control in diabetic patients does not immediately inhibit the progression of retinopathy (Box 65.2). Adverse effects of prior poor glycemic control continue to progress even if hyperglycemia is reduced or eliminated in diabetic patients,25 in diabetic dogs33 and rats,37 and the benefits of good control persist even if the good glycemic control is not maintained: benefits of a few years of modestly improved glycemic control continued to be apparent for the decade after the glycemic control was relaxed.38 This phenomenon, commonly referred to as “metabolic memory,” has also been observed in diabetic dogs and rats.33,39,40 The molecular basis of this memory is not yet known, but it is initiated early in the course of diabetes. Apparently, the level of glycemia results in a longterm imprinting on the cell.

Studies have also demonstrated that blood pressure medications, notably ß-blockers and inhibitors of angiotensinconverting enzyme, slow the development of capillary degeneration in diabetic animals,41 and the progression of advanced stages of diabetic retinopathy in diabetic

Box 65.2  “Inertia” of diabetic retinopathy

•Diabetic retinopathy develops slowly, and resists arrest after the process has begun

•“Metabolic memory” describes a poorly understood process by which cells are changed as a function of their previous exposure to hyperglycemia. In the Diabetes Control and Complications Trial/Epidemiology of Diabetes Interventions and Complications trial, beneficial effects of previous good glycemic control are maintained for many years (even if glycemia is no longer as good), but, likewise, adverse effects of previous poor glycemic control continue even after re-establishing relatively normal glycemia

patients.42,43 In the UKPDS43,44 type 2 diabetics allocated to tight control of blood pressure had a 34% reduction in risk in the proportion of patients with deterioration of diabetic retinopathy by two steps, and a 47% reduced risk of deterioration in visual acuity. Likewise, lipid levels have been shown to influence the development or progression of the retinopathy in diabetic animals.45,46 Lipid-lowering therapy using fenofibrate reduced the need for laser treatment for diabetic retinopathy,47 although the mechanism of this effect was not regarded as secondary to plasma concentrations of lipids.

Pathology

Histologically, vascular lesions in the early stages of diabetic retinopathy in humans and animals are characterized by the presence of saccular capillary microaneurysms, pericytedeficient capillaries, and obliterated and degenerate capillaries. These degenerate capillaries are not perfused,2,3 and so increases in their frequency represent reductions in retinal perfusion. Capillary occlusion and degeneration initially occur in single, isolated capillaries, and have no clinical importance when only few capillaries have become non­ perfused. As more and more capillaries become occluded, however, retinal perfusion likely decreases, at least locally (Figure 65.2). No one of these lesions is totally specific for diabetic retinopathy, but in combination, they are quite unique.

The clinically demonstrable changes to the retinal vasculature in diabetes have led to the general assumption that the retinopathy is solely a microvascular disease. Nevertheless, diabetes can also damage nonvascular cells of the retina, resulting in alterations in function,23,24,46,48–50 loss of ganglion cells, horizontal cells, amacrine cells, and photore- ceptors,20,51–62 and activation52,54,56–58,63–68 or death of Müller glial cells in some,52,59 but not all, studies.16,66,69,70 Findings in diabetic mice have not necessarily been in agreement with findings in rats.57,60,68,71,72 These important topics exceed the breadth of the present review, and are covered elsewhere.

Diabetes

Blood hexose

Metabolic abnormalities

Figure 65.3  Schematic summarizing the postulated pathogenesis of diabetic retinopathy.

Pathophysiology

Diabetic retinopathy is an important cause of visual impairment in diabetes, but the pathogenesis of the condition remains unclear. A current working model of diabetic retinopathy is that the clinically significant (proliferative) phase of the retinopathy is a direct consequence of earlier changes, especially increased leakage and degeneration of retinal capillaries (Figure 65.3). Efforts to inhibit the development of the early stages of diabetic retinopathy have focused to a great extent on histologic endpoints (degeneration of retinal capillaries and neurons), and these studies have provided considerable insight into the pathogenesis of the retinopathy. Table 65.2 summarizes a number of therapies reported to inhibit retinal vascular histopathology in diabetes, grouped by their presumed mode of action. Most of the research focus to date has been on the role of hyperglycemia and its sequelae in the pathogenesis of the retinopathy. Also, most of these studies have been conducted in rodents, and accordingly, the histologic parameters of diabetic retinopathy that develop in those species (degeneration of retinal capillaries and pericyte loss) are the endpoints for most of these studies.

Other biochemical or metabolic abnormalities, including impaired insulin signaling,55,73,74 have been postulated to contribute to the retinopathy, but effects of correcting these abnormalities have not been demonstrated histologically to date.

Hyperglycemia is strongly associated with development of diabetic retinopathy, and this has been strongly supported by clinical and animal studies showing that reduction in the severity of hyperglycemia significantly inhibited development of the retinopathy.25,32,35 Nevertheless, this evidence does not prove that hyperglycemia per se is the critical abnormality, because intensive insulin therapy can normalize defects also related to lipids and proteins in diabetes. The strongest evidence that hyperglycemia is sufficient to initiate the vascular lesions of diabetic retinopathy is the evidence that lesions that are morphologically identical to those of diabetic retinopathy also develop in nondiabetic animals made experimentally hyperglycemic by feeding a galactoserich diet.75–80

Although one would assume that all vascular cells should be exposed to the same concentration of blood glucose in

Pathophysiology

Box 65.3  Cell types involved in diabetic retinopathy

•Which retinal cell types are involved in the etiology of diabetic retinopathy is not fully understood

•Retinal endothelial cells and pericytes have long been understood to be altered in diabetes, but other cells, including retinal ganglion cells, Müller cells, photoreceptors, and even white blood cells are being found to contribute to the retinopathy in unexpected ways

a given individual, there is unexplained regional variability in susceptibility to diabetes-induced microvascular disease even within the same retina. Microaneurysms and acellular capillaries have been found to develop in a nonuniform distribution even within the same retina in diabetic patients81–84 and in experimentally diabetic or galactosemic dogs.85 In both species, lesions were most common in the superior and temporal portions of the retina. Likewise, neovascularization in diabetic patients has been noted to be more common in the superior and temporal portions of the retina than in other regions.86

Diabetic retinopathy is not made up of a single lesion (Box 65.3). It is a spectrum of abnormalities, none of which is totally unique to diabetic retinopathy, but which in combination offer a clinical picture that is relatively unique to diabetes. Whether or not these individual lesions share a common pathogenesis or differ in aspects remains to be determined. Thus, the individual lesions of early diabetic vascular disease in the retina are discussed individually below.

Capillary nonperfusion and degeneration

In its simplest form, mechanisms postulated to contribute to the nonperfusion and degeneration of retinal capillaries in diabetes include abnormalities that have developed within the vascular cells themselves (intrinsic abnormality) and those which have developed outside the vascular cells, but then stimulate a change in the cells of the retinal vasculature. These mechanisms, some of which are listed below, are not mutually exclusive.

Capillary cell death caused by metabolic abnormalities within capillary cells

Metabolic sequelae of hyperglycemia that may damage the vasculature have been the most extensively studied mechanism for development of diabetic retinopathy. Numerous of these postulated metabolic abnormalities are summarized below. Initiator and effector caspases have been found to become activated in the vasculature of the retina of diabetic animals or in retinal vascular cells incubated in elevated glucose concentration.87 A variety of therapies have reduced the number of terminal uridine deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive capillary cells or degenerate capillaries compared to control,24,41,57,88–98 suggesting that related metabolic abnormalities also contribute to the capillary cell death.

The rate of capillary cell showing evidence of apoptosis (TUNEL-positive at any given moment) is very small com-

pared to the total number of capillary cells. Apoptosis may not be the only form of cell death occurring in diabetic retinopathy. Joussen and coworkers90,99,100 have reported that only 9 days of diabetes caused an increase in intracellular accumulation of propidium iodide (commonly used as a marker of necrosis) in the retinal vasculature. Thus, focusing solely on apoptotic cell death may underestimate the total number of cells dying at any time. On the other hand, propidium iodide may overestimate necrosis, since it can enter even viable cells having impaired membrane integrity.101,102

510

Capillary cell death due to extrinsic abnormalities

Vaso-occlusion by white blood cells or platelets

Attraction and adhesion of leukocytes to the vascular wall are significantly increased in retinas of diabetic animals,90,93,100,103–112 and may contribute to the capillary nonperfusion or death of retinal endothelial cells in diabetic retinopathy.90 Leukocyte stiffness is increased in diabetes (decreased filterability) and contributes to the occlusion of retinal vessels.104,113 Each instance of occlusion by a blood cell is likely to be short-lived, but cumulative effects of such repeated ischemia/reperfusion injuries over a prolonged

interval are not known. Abnormal leukocyte adherence to retinal vessels in diabetes occurs via adhesion molecules, including intercellular adhesion molecule 1 (ICAM-1). Diabetes increases expression of ICAM-1 in retinas of animals and humans,95,100,106,114,115 and interaction of this adhesion molecule on retinal endothelia with the CD18 adhesion molecule on monocytes and neutrophils contributes to the diabetes-induced increase in adherence of white blood cells to the vascular wall in retinal vessels.106 Using in situ perfusion methods, changes consistent with capillary occlusion secondary to leukostasis have been observed in occasional retinal vessels. Diabetic mice lacking ICAM-1 and CD18 are protected from development of diabetes-induced increase in leukostasis, vascular permeability, and degeneration of retinal capillaries.93 Whether the development of the retinal disease in diabetes results from ICAM-1-mediated capillary occlusion or some other mechanism, however, has not been explored.

Increased platelet aggregation has also been postulated to contribute to capillary nonperfusion in diabetes. Platelet microthrombi are present in the retinas of diabetic rats and humans, and have been spatially associated with apoptotic endothelial cells.116,117 Nevertheless, the selective antiplatelet drug clopidogrel did not prevent neuronal apoptosis, glial reactivity, capillary cell apoptosis, or degeneration of retinal capillaries in diabetic rats,96 suggesting that platelet aggregation does not play a critical role in the development of the vascular lesions of early diabetic retinopathy.

Abnormalities initiated by binding to extracellular receptors

In addition to physical obstruction of capillaries by non­ vascular cells, binding of external stimuli such as advanced glycation endproducts (AGEs) or cytokines (such as interleukin (IL)-1ß) to extracellular receptors on vascular cells may damage or kill those cells. Interfering with signaling from the receptor for advanced glycation endproducts (RAGE) or the IL-1ß receptor has been found to inhibit diabetes-induced degeneration of retinal capillaries in diabetic mice,46,118 demonstrating their role in the pathogenesis of the capillary degeneration.

Contribution of outer retina to degeneration of retinal capillaries in diabetes

The outer retina has also been implicated as a source of an unidentified extrinsic abnormality that contributes to dia- betes-induced degeneration of retinal capillaries. Since photoreceptors are a major consumer of oxygen in the retina, it was postulated that photoreceptors may deprive a marginally oxygenated retina of needed oxygen or other products. This hypothesis was tested in mice having photoreceptor degeneration (rhodopsin knockout mice) that were made experimentally diabetic.119 Hypoxia-specific immunostain increased in the retina of wild-type diabetic animals compared to that in nondiabetic controls, but this diabetes-induced change was absent in the diabetic animals having retinal degeneration. As expected, vascular density was subnormal in diabetic wild-type controls, but, remarkably, this capillary degeneration did not develop in the diabetics lacking photoreceptors. Thus, loss of the outer retina seems to reduce the severity of diabetes-induced degeneration of retinal capillaries, possibly via less consumption of oxygen.

Pathophysiology

Postulated effect of retinal neurodegeneration on capillary degeneration in diabetes

Retinal neuroglial cells have been observed to begin to degenerate in diabetic rats after only about 1 month of diabetes.53 This is considerably before capillary degeneration has been detected, and consequently it was postulated that the neurodegeneration may play some role in the later development of capillary degeneration. Data to support this postulate are still only speculative, and evidence that topical nepafenac inhibited the diabetes-induced degeneration of retinal capillaries without any beneficial effect on neurodegeneration24 does not support this hypothesis.

Invasion of retinal vessels by processes from glial cells

Cellular processes from retinal glial cells have been found inside occasional degenerate capillaries (identified from the basement membrane tube that surrounds vessels)120–122 (Figure 65.4). It is not clear whether this glial invasion precedes and causes the capillary to degenerate, or is a result of the capillary cells dying (glial cells filling empty spaces in tissues).

Several gaps exist in our knowledge of capillary degeneration and its significance. Specifically, does capillary degeneration cause subretinal oxygenation or ischemia of retina in diabetes? Does neurodegeneration develop especially in areas having a compromised vasculature? How does the outer retina contribute to capillary degeneration in diabetes? Does neurodegeneration contribute to capillary degeneration in diabetic retinopathy?

Capillary permeability

Breakdown of the blood–retinal barrier in diabetes, resulting in increased vascular permeability, has been attributed to increases in leukostasis, cytokines, and growth factors, to

Figure 65.4  Degenerate retinal capillary is filled with processes of glial cells. Endothelial cells and pericytes are no longer present, but the basement membrane that remains clearly indicates that this was previously a functional capillary. This obliterated capillary (from a diabetic dog) is from the outer retina, indicating that these glial processes are from Müller cells. × 29 100.

name a few causes.90,123–126 Molecular alterations, such as in proteins of the tight junction complex, have also been demonstrated to play a significant role in the diabetes-induced increase in capillary permeability.127 Controversy remains as to how fast the permeability defect develops in retinas of diabetic rodents, with reports ranging from 8 days to more than 6 months after onset of diabetes.100,125–132 Permeability was also increased in dogs diabetic for several years.133 A number of therapies have been found to inhibit the diabetes-induced increase in vascular permeability within the retina, including aldose reductase inhibitors, protein kinase C inhibitors, tyrosine kinase inhibitors, aspirin, a cyclooxygenase-2 inhibitor, steroids, vascular endothelial growth factor (VEGF) antagonist, tumor necrosis factor-α receptor antagonists, and peroxisome proliferator-activated receptors (PPAR) gamma ligands.100,112,129,130,134–144

types have been studied in both humans and animals, the animal data have both agreed32,33,41,151 and disagreed98,152,153 with results obtained in human trials. It is possible that the pathogenesis of the lesions is different in animals than in humans, but evidence that such differences play a critical role in the development of retinopathy is lacking. Other differences stand out, however. In hindsight, many studies of diabetic retinopathy in humans have been conducted for too short a duration, started too late (when appreciable retinopathy is already present), and using undesirably low levels of drug. In addition, the endpoints of human and animals studies are not usually the same: color fundus photography (the main method used for quantitation of retinopathy in human clinical trials) is far less able to demonstrate capillary degeneration than the high-resolution, microscope-based techniques to analyze the isolated retinal vasculature routinely used in animal studies.

Microaneurysms

Microaneurysms, or dot hemorrhages, are detectable ophthalmoscopically, and the presence and number of micro­ aneurysms have strong predictive value with respect to progression of the retinopathy.145,146

The capillary outpouchings have been identified as being predominantly around areas of occluded capillaries. Minute red-color lesions typically measuring 10–100 µm in diameter are located primarily in the posterior pole in early disease but may become widespread with disease duration and severity. Focal damage results in alveolar bulging of the capillary wall and loss of endothelial cell tight junction integrity. Areas of capillary leakage frequently have a microaneurysm at the center, suggesting that they leak more than other adjacent areas. Despite being one of the strongest predictors of progression of diabetic retinopathy, we know very little about the pathogenesis of microaneurysm in diabetes, the extent to which they contribute visual complications of diabetes, and the basis for their prognostic power.

As suggested by Ashton147 and consistent with findings of Aguilar et al,148 microaneurysms may be aborted attempts at neovascularization due to focal retinal ischemia. Since pericytes have been demonstrated to inhibit endothelial pro­ liferation,149 and microaneurysms have been regarded as relatively free of pericytes, it was postulated that microaneurysm formation may result from loss of endothelial growth suppression following pericyte loss.150

Why numerous therapies seem to inhibit the early stages of diabetic retinopathy in animals, but seem less effective in diabetic humans is an important question. Many of the pharmacologic approaches studied in animal models have not been studied also in patients, but when similar drug

Proliferative diabetic retinopathy

Neovascularization is a major contributor to visual dysfunction in diabetes, and accordingly has been a major therapeutic target in recent years. Success is being achieved using laser photocoagulation, and more recently using anti-VEGF therapies. These approaches are described in Chapter 66.

Conclusions and summary

We have learned much about the pathogenesis and how to treat diabetic retinopathy. Vascular abnormalities are still believed by many to account for most of the clinically meaningful visual consequences of diabetes, but the possible contribution of changes in the neural retina continues to be explored. Attention rightfully has focused on inhibiting clinical meaningful causes of visual impairment, such as neovascularization and retinal edema, but focusing on the later stages of the disease means that retinal vascular damage will have already occurred. Success is now being made also in the earlier stages of the retinopathy, with hopes that inhibiting the early damage (such as capillary degeneration) will inhibit development of the more advanced stages of the retinopathy. Unexplained observations about the retinopathy, however, demonstrate that there is still a considerable amount to learn about the retinal disease. Studies to date have offered statistical insight on efficacy of a given therapy towards a given population of patients or animals, but these studies offer little insight as to how an individual patient will respond to a given therapy.

11.Diabetes Control and Complications Trial Research Group. Clustering of long-term complications in families with diabetes in the diabetes control

and complications trial. The Diabetes Control and Complications Trial Research Group. Diabetes 1997;46: 1829–1839.

25.Diabetes Control and Complications Trial Research Group. The effect of intensive treatment of diabetes on the development of long-term

complications in insulin-dependent diabetes mellitus. N Engl J Med 1993;329:977–986.

53.Barber AJ, Lieth E, Khin SA, et al. Neural apoptosis in the retina during experimental and human diabetes. Early onset and effect of insulin. J Clin Invest 1998;102:783–791.

70.Bresnick GM, Palta M. Predicting progression of severe proliferative diabetic retinopathy. Arch Ophthalmol 1987;105:810–814.

75.Engerman RL, Kern TS. Experimental galactosemia produces diabetic-like retinopathy. Diabetes 1984;33:97–100.

83.Niki T, Muraoka K, Shimizu K. Distribution of capillary nonperfusion in early-stage diabetic retinopathy. Ophthalmology 1984;91:1431–1439.

88.Mizutani M, Kern TS, Lorenzi M. Accelerated death of retinal microvascular cells in human and experimental diabetic retinopathy. J Clin Invest 1996;97:2883–2890.

93.Joussen AM, Poulaki V, Le ML, et al. A central role for inflammation in the pathogenesis of diabetic retinopathy. Faseb J 2004;18:1450–1452.

133.Wallow IH, Engerman RL. Permeability and patency of retinal blood vessels

Key references

in experimental diabetes. Invest Ophthalmol 1977;16:447–461.

165.Sorbinil Retinopathy Trial Research Group. A randomized trial of sorbinil, an aldose reductase inhibitor, in diabetic retinopathy. Arch Ophthalmol 1990;108:1234–1244.

168.Effect of ruboxistaurin in patients with diabetic macular edema: thirty-month results of the randomized PKC-DMES clinical trial. Arch Ophthalmol 2007; 125:318–324.

Clinical background

Introduction

Diabetes mellitus is a metabolic disorder caused by defects in insulin secretion (type 1), insulin action (type 2), or both. It is characterized by chronic hyperglycemia which ultimately may result in dysfunction and damage to various organ systems, including the brain, kidneys, eyes, and peripheral nerves. Diabetic retinopathy may be broadly classified in terms of the presence or absence of retinal neovascularization. The term nonproliferative diabetic retinopathy (NPDR) is used to describe intraretinal microvascular changes that occur in the early stages of diabetic retinopathy. The etiology and pathogenesis of NPDR are discussed in Chapters 65 and 67. Proliferative diabetic retinopathy (PDR) is used to indicate the presence of newly formed vessels, fibrosis, or both, arising from the retina or optic disc and extending along the inner retinal surface and/or into the vitreous cavity. PDR may be characterized by neovascularization of the iris (NVI) as well. The focus of this chapter will be on the proliferative changes seen in these advanced stages of diabetic retinopathy.

Epidemiology

Diabetic retinopathy is the leading cause of new cases of blindness in people aged 20–74 years in the USA.1 The incidence of diabetic retinopathy increases with the duration of diabetes mellitus, and it is found in the vast majority of patients who have had diabetes for 20 years or more.2 After 20 years of diabetes, PDR affects about 50% of patients with type 1 diabetes, 5–10% of patients with noninsulindependent type 2 diabetes, and 30% of patients with insulindependent type 2 diabetes3 (Box 66.1). In the USA, African Americans and Hispanics have a higher prevalence of diabetes, approximately 25%, compared with 6.2% in the remainder of the population.4 The major risk factors for progression of diabetic retinopathy are the duration of diabetes mellitus, poor glucose control, high blood pressure, and elevated cholesterol.2 The Diabetes Control and Complications Trial (DCCT) and the UK Prospective Diabetes Study (UKPDS) have demonstrated the efficacy of intensive glucose control in reducing the incidence and progression of diabetic retin-

Neovascularization in

diabetic retinopathy

Corey B Westerfeld and Joan W Miller

opathy.5,6 However, results of these studies have also confirmed the difficulty of achieving and maintaining appropriate glycemic control over a long period. As such, recent attention has been given towards further elucidating the pathogenesis of diabetic retinopathy in an attempt to develop better, more targeted therapies for this prevalent and visually disabling condition.

Clinical features

Diabetic retinopathy is characterized by the appearance of microaneurysms, increased vascular permeability, occlusion of capillaries, and formation of new, abnormal vessels.7 There are two primary pathological features in diabetic retinopathy responsible for vision loss: diabetic macular edema (DME) and retinal neovascularization. DME is the most common cause of vision loss in diabetes and is generally associated with other nonproliferative changes.8 NPDR is characterized by microaneurysms, small “dot and blot” hemorrhages, “flame” hemorrhages, intraretinal microvascular abnormalities, and “cottonwool” spots. Later stages of diabetic retinopathy are characterized by the formation of new vessels on the optic nerve or in the retina which may extend along the surface of the retina and/or into the vitreous cavity. These proliferative changes occur as a programmed response to ischemia in the inner retina in an effort to improve tissue oxygenation. However, the new vessels are weak and may break, resulting in vitreous hemorrhage. Furthermore, the combination of neovascularization, fibrous tissue proliferation, and recurrent vitreous hemorrhage may lead to tractional retinal detachment (Box 66.2).

Etiology

The development of diabetic retinopathy is primarily related to the duration of diabetes, severity of hyperglycemia, and the existence of contributing factors such as hypertension and hyperlipidemia.2 Hyperglycemia is the primary pathogenic factor in the development of diabetic retinopathy.2,5 However, diabetic retinopathy may occur at higher rates in some patient groups in spite of relatively good glucose control and vice versa, suggesting that there are other contributing factors.

Genetics

Siblings of individuals with diabetic retinopathy have a higher risk of developing diabetic retinopathy themselves. This risk is in addition to the baseline risk of diabetes and is greater than the expected rate of diabetic retinopathy, indicating that there is a genetic component.9 A variety of candidate genes have been investigated in diabetic patients and animal models, but large studies have not yet proven any direct correlation with diabetic retinopathy. The development of diabetic retinopathy is multifactorial, and as such, relevant genetic factors are probably modulated by many environmental factors as well. Recent studies have shown that polymorphisms in genes coding for intracellular adhesion molecule-1 (ICAM-1) and transforming growth factor-β are risk factors for diabetic retinopathy.10,11 The authors propose that mutations result in leukocyte activation and adhesion to the retinal vascular endothelium, leading to the development of vascular leakage and capillary closure. Larger studies are needed to evaluate further these and other possible genetic components of diabetic retinopathy.

Box 66.1  Epidemiology

•Diabetic retinopathy is the leading cause of new cases of blindness in persons aged 20–74 in the USA

•Incidence of diabetic retinopathy increases with duration of disease

•After 20 years of diabetes, proliferative diabetic retinopathy affects:

50% of type 1 diabetes

5–10% of noninsulin-dependent type 2 diabetes

30% of insulin-dependent type 2 diabetes

Box 66.2  Clinical features

Nonproliferative diabetic retinopathy

•Microaneurysms

•Dot-blot hemorrhages, flame hemorrhages, intraretinal microvascular abnormalities

•Cottonwool spots

Proliferative diabetic retinopathy

•Neovascularization on the optic nerve, retina, iris

•Vitreous hemorrhage

•Traction retinal detachment

Pathophysiology

Pathology

The earliest histologic change in diabetic retinopathy is the loss of pericytes. Pericytes line the retinal vascular endothelium and provide structural support to the retinal vasculature. Loss of pericytes leads to progressive dilation of capillaries and the formation of microaneurysms. A complete discussion of the classic histologic features of NPDR can be found in Chapter 65. PDR is characterized by the formation of new vessels which are evident histologically as thin-walled vessels devoid of pericytes.

Pathophysiology

Pathologic mechanisms in diabetic retinopathy

Several biochemical mechanisms may be responsible for the progression of diabetic retinopathy (Figure 66.1). Hyperglycemia causes the formation of reactive oxygen intermediates (ROIs) and advanced glycation endproducts (AGEs). ROIs and AGEs may cause direct damage to pericytes and vascular endothelial cells and also stimulate the release of vasoactive factors.12 Chronic hyperglycemia also causes activation of the polyol pathway leading to increased glycosylation of cell membranes and extracellular matrix as well as the accumulation of sorbitol by increased aldose reductase expression.13 Glycosylation and sorbitol accumulation cause further vascular endothelial damage and dysfunction of endothelial enzymes.14 It is likely that activation of these pathways in association with vascular damage produces inflammation which further exacerbates the condition. Hyperglycemia may also impair autoregulation of retinal blood flow causing perfusion-related damage to endothelial cells.15 The ultimate effects of glucose toxicity on pericytes and endothelial cells cause impaired circulation, hypoxia, inflammation, and further activation of angiogenic stimuli. Finally, hyperglycemia also causes activation of the protein kinase C (PKC) intracellular signaling pathway.16 PKC influences progression of diabetic retinopathy in two ways. First, it directly promotes the activation of VEGF and other growth factors.7 Second, binding of VEGF to its target receptors requires the presence of the PKC signaling protein.17 Clinical trials investigating PKC inhibitors have demonstrated efficacy in the treatment of NPDR and further studies are ongoing.18

Production of vasoactive factors

The combined effects of these pathways lead to the production of vasoactive factors such as vascular endothelial growth factor (VEGF), nitric oxide, prostacyclin, insulin-like growth

factor (IGF)-1, and endothelin (ET). These vasoactive factors act in concert with hyperglycemia to produce dysfunction of pericytes and vascular endothelial cells. One mechanism by which this occurs is via VEGF stimulation of ICAM-1 expression in the retinal vasculature. ICAM-1 promotes leukocyte binding to the vascular endothelium which triggers a Fas/Fas ligand-mediated endothelial cell death and breakdown of the blood–retinal barrier.19 The culmination of these events leads to thrombosis and closure of retinal capillaries. Occlusion of capillaries gives rise to focal retinal ischemia and hypoxia. Local hypoxia induces further overexpression of angiogenic stimuli. In response, new vessels begin to form. However, the new vessels have reduced structural integrity including a fragile basement membrane, deficient tight junctions between endothelial cells, and lack of pericytes.20 The walls of the vessels are porous, allowing leakage of plasma proteins and even hemorrhage into the retina or vitreous.

Figure 66.2  Angiogenic factors in proliferative diabetic retinopathy. VEGF, vascular endothelial growth factor; bFGF, basic fibroblast growth factor, IGF-1, insulin-like growth factor-1; ET, endothelin; PEDF, pigment epitheliumderived factor.

Advanced PDR

In advanced PDR, the new vessels are accompanied by fibrous tissue and grow from the retinal surface into the vitreous cavity to form fibrovascular membranes. Several studies have demonstrated that the vitreous plays a role in the pathogenesis of PDR.21 Hyperglycemia causes changes in type 2 collagen in the vitreous, leading to liquefaction and vitreous syneresis.22 Additionally, hypoxia and resultant abundance of growth factors lead to a thickening of the posterior vitreous cortex.23 The resulting vitreous instability due to loss of the gel state without dehiscence at the vitreoretinal interface may induce retinal traction. Such traction may not only lead to retinal tears but may also contribute to the neovascular process. In support of this theory, the development of PDR is rare if the vitreous has detached completely, presumably since the scaffold for proliferating cells is removed.24 Liberated serum proteins such as fibronectin accumulate at the junction of attached retina and vitreous25 and mediate the migration and adhesion of proliferating endothelial cells.26 In later stages of PDR, contraction of the posterior hyaloid causes rupture of proliferating vessels, vitreous hemorrhage, and traction and/or rhegmatogenous retinal detachment.

Iris neovascularization

NVI may also occur in PDR. The stimulus for the formation of NVI is the release of vasoactive factors by ischemic retina. The most common causes of NVI are central retinal vein occlusion, diabetes, and ocular ischemic syndrome. Angiogenic factors such as VEGF diffuse anteriorly into the aqueous and stimulate growth of new vessels.27 The new vessels begin as capillary buds at the inner circle of the iris and then extend radially forming a fine vascular network.28 The vessels may cross the trabecular meshwork of the angle and block aqueous outflow causing neovascular glaucoma.

Angiogenic factors in proliferative diabetic retinopathy

Angiogenesis is regulated by the counterbalancing of angiogenic stimulators and angiogenic inhibitors (Figure 66.2).

Box 66.3  Vascular endothelial growth factor

•Induced by hypoxia

•Stimulates vascular proliferation

•Increases vascular permeability

•Elevated in diabetic retinopathy (proliferative diabetic retinopathy > nonproliferative diabetic retinopathy)

In the normal adult retina, angiogenic inhibitors predominate, maintaining relative quiescence of the retinal vasculature. In pathologic conditions such as diabetic retinopathy, the balance swings in favor of proangiogenic stimuli leading to the development of retinal neovascularization. Proangiogenic factors include VEGF, basic fibroblast growth factor (bFGF), IGF-1, and ET, all of which have been implicated in diabetic retinopathy.29 Alternatively, decreased levels of angiogenic inhibitors, such as pigment epithelium-derived factor (PEDF), angiostatin, and endostatin, likely contribute to the formation of new vessels in diabetic retinopathy. Angiogenic factors and angiogenic inhibitors are discussed further in Chapter 70.

Vascular endothelial growth factor

VEGF is possibly the most important biochemical agent in the development of PDR. VEGF is produced by numerous retinal cells and is induced by hypoxia (Box 66.3). VEGF acts to produce both vascular proliferation and increased vascular permeability.30 Studies have confirmed that VEGF levels are increased in the retina and vitreous in patients with diabetic retinopathy.31 Furthermore, as expected, VEGF levels are higher in patients with PDR than with NPDR.32 Laser photocoagulation has been associated with a 75% decrease in VEGF levels in patients with PDR, suggesting that the formation and regression of new vessels are correlated with VEGF levels.31 Elevated VEGF levels have been confirmed in animal models of diabetic retinopathy as well. In the oxygeninduced retinopathy model, retinal VEGF levels are elevated and correlate with progression of retinal neovascularization.33 In a primate model, induction of retinal ischemia via