Ординатура / Офтальмология / Английские материалы / Diabetes and Ocular Disease Past, Present, and Future Therapies 2nd edition_Scott, Flynn, Smiddy_2009

24Diabetes and Ocular Disease

7.Wilkinson CP, Ferris FL III, Klein RE, et al. Proposed international clinical diabetic retinopathy and diabetic macular edema disease severity scales. Ophthalmology. 2003;110:1677–1682.

8.Fukuda M. Clinical arrangement of classification of diabetic retinopathy. Tohoku J Exp Med. 1983;141(Suppl):331–335.

9.Verdaguer TJ.Screening para retinopatia en latin America. Rev Soc Brasil Retina

Vitreo. 2001;4:14–15.

10.National Health and Medical Research Council. Clinical Practice Guidelines: Management of Diabetic Retinopathy. Canberra: NHMRC; 1997.

11.Klein RE, Klein BE, Moss SE, et al. The Wisconsin Epidemiologic Study of Diabetic Retinopathy. IX. Four-year incidence and progression of diabetic retinopathy when age at diagnosis is less than 30 years. Arch Ophthalmol. 1989;107:237–243.

12.Klein RE, Klein BE, Moss SE, et al. The Wisconsin Epidemiologic Study of Diabetic Retinopathy. X. Four-year incidence and progression of diabetic retinopathy when age at diagnosis is 30 years or more. Arch Ophthalmol. 1989;107:244–249.

13.Shekelle PG, Kahan JP, Bernstein SJ, et al. The reproducibility of a method to identify the overuse and underuse of medical procedures. N Engl J Med. 1998;338:1888–1895.

26 Diabetes and Ocular Disease

macular edema. Proliferative diabetic retinopathy (PDR) describes both intraretinal pathology as well as neovascular changes that extend beyond the internal limiting membrane of the retina and may extend along the surface of the disc and retina or may be elevated by partial posterior vitreous detachment. Proliferative disease may lead to retinal detachment, preretinal hemorrhage, and neovascular glaucoma. The cornea, ciliary body, crystalline lens, and retinal glia may also be affected in patients with diabetes mellitus. In this chapter, the ocular histopathological changes of diabetes mellitus will be reviewed.

MECHANISM OF DIABETIC RETINOPATHY

The mechanisms that lead to the histopathologic changes in diabetes mellitus are complex and are likely secondary to dysregulation of a number of metabolic pathways. These include the polyol pathway, the formation of advanced glycosylation end products (AGEs), the pathological activation of protein kinase C (PKC), and increased oxidative stress by free radicals [1].

The polyol pathway, which becomes activated with high glucose levels, may lead to early changes in the retinal vasculature including loss of vascular pericytes and thickening of the basement membrane [2]. High intracellular levels of glucose may saturate the normal pathway and shunt the remaining glucose into the aldose reductase pathway. Aldose reductase reduces glucose to sorbitol and uses nicotinamide adenine dinucleotide phosphate (NADPH) as a cofactor. Sorbitol is then oxidized to fructose via sorbitol dehydrogenase with NAD(+) used as a cofactor. The overproduction of NADPH and increase in the NADH/NAD ratio is thought to alter enzyme activities and contribute to the formation of reactive metabolites that may lead to cellular dysfunction and damage [3]. Subsequent cellular dysfunction may cause pericyte loss [4–8] and basement membrane thickening [7,9]. While the use of an aldose reductase inhibitor has been proposed to reduce the formation of sorbitol and slow the progression of, or even prevent, diabetic retinopathy, studies have demonstrated mixed results. Aldose reductase inhibitors have been shown to prevent thickening of basement membrane in the retinal vessels in galactosemic and diabetic rats [10], while no benefit was shown in preventing or slowing retinopathy in a randomized clinical trial of sorbinil (aldose reductase inhibitor) in type 1 diabetic patients [7].

AGEs form as a result of the nonenzymatic glycation of intracellular and extracellular proteins and lipids. AGE formation is directly related to the amount and duration of hyperglycemia. Mild increases in glucose concentration have been shown to produce large increases in AGE accumulation [11]. AGEs alter the function of basement membrane matrix components including type IV collagen and laminin. AGEs interact with type IV collagen and inhibit the lateral association of these molecules into a network-like structure. Effects on laminin include decreased binding to type IV collagen and decreased self-assembly [12]. Alterations to the matrix components are thought to account for the thickening in the basement membranes of tissues seen in diabetic patients. AGEs alter the cellular function by binding to the receptors for advanced glycosylation end products (RAGEs) [13,14].

Ligand binding of AGE-specific receptors on endothelial cells increases coagulation factors (factors IX and X) [14], decreases anticoagulation factors (thrombomodulin) [14], and induces vasoconstrictive factors (endothelin-1) [15], which may lead to vasoconstriction and thrombosis in the setting of AGEs. AGEs may also alter DNA and the nuclear proteins of cells by nonenzymatic modification with resultant altered gene expression [16]. AGEs increase the extraluminal accumulation of plasma proteins including low density lipoproteins by chemically binding to reactive AGE precursors of matrix proteins [17,18]. These alterations caused by AGEs have been proposed to be responsible for the pathological changes seen in diabetic retinopathy [19].

Aminoguanidine, an AGE formation inhibitor, has been tested in animal models to evaluate the role of AGEs in the formation of diabetic retinopathy. Treatment with aminoguanidine for 26 weeks in diabetic rats prevented endothelial cell proliferation and reduced pericyte dropout when compared with controls. After 75 weeks, treated rats had an 80% reduction in the number of acellular capillaries and had no microaneurysm formation [20]. In addition, blockage of RAGEs has been shown to inhibit the AGE-induced impairment of endothelial barrier function and reverse the early vascular hyperpermeability seen in diabetic rats [21].

Free radical production is increased in states of hyperglycemia through oxidative phosphorylation and glucose autoxidation [22]. Reactive oxygen species are thought to contribute to some of the manifestations of diabetic retinopathy. [23–26].

PKC and diacylglycerol (DAG) are intracellular signaling molecules responsible for vascular functions including permeability, vasodilator release, endothelial activation, and growth factor signaling. PKC and DAG activation is increased in animal models with diabetes [27] and activated PKC may lead to vascular damage, increased growth factor expression and signaling, which has been proposed to account for the pathological changes seen in diabetic retinopathy [28–30].

The proposed pathways described above lead to alterations in gene expression and protein function, which may then manifest as cellular dysfunction with the resultant vascular changes seen in diabetes. While the relative roles of the different pathways is not clear, it is likely that the combined or interactive effects of all of these pathways may be responsible for the changes seen in diabetic retinopathy.

Nonproliferative Diabetic Retinopathy. NPDR describes intraretinal microvascular changes, which include basement membrane thickening, pericyte loss, microaneurysm formation, venous caliber abnormalities, and intraretinal microvascular abnormalities (IRMAs). The vascular changes may lead to macular edema, hard exudate formation, cotton-wool spots (soft exudates), and intraretinal hemorrhages. In the Early Treatment Diabetic Retinopathy Study (ETDRS), NPDR has been categorized as mild, moderate, severe, and very severe. Mild NPDR is defined as the presence of one microaneurysm, but hemorrhages and microaneurysms are less than ETDRS standard photograph 2A in all four retinal quadrants. There is no evidence of moderate, severe, or very severe disease. Moderate NPDR is defined as the presence of hemorrhages and/or microaneurysms greater than those pictured in ETDRS standard photograph 2A in at least one field but less than four

of normal growth and repair of the endothelial cells in the retinal vascular system [37]. Selective loss of intramural pericytes with a decreased ratio of intramural pericytes to endothelial cells occurs in the capillaries of the retina in patients with diabetic retinopathy [32,38–42]. Loss of pericytes leaves empty dropout spaces in the capillary wall that are referred to as pericyte “ghosts.” Pericyte loss is only detectable by histological examination, and cannot be seen clinically.

Basement Membrane Thickening. Basement membrane is composed primarily of type IV collagen. Thickening of the vascular basement membrane is seen early in the course of patients with diabetes mellitus. Experimental studies in rats [43] and dogs [44] have shown that a high galactose diet can induce basement membrane thickening with striated collagen deposition. Clinical evidence has shown that basement membrane thickening is directly related to hyperglycemia and can be reversed with good diabetic control [45]. Increased synthesis of basement membrane with decreased turnover appears to be the cause of the thickening [46]. Decreased proteoglycan content is present in association with the thickening, which reduces the electrical charge barrier function and increases the membrane permeability [46,47]. The increase in membrane permeability may lead to the increased vascular permeability and the extravasation of intravascular fluid seen in diabetic retinopathy.

Microaneurysms. An early clinical manifestation of diabetic retinopathy is microaneurysm formation. Microaneurysms are dilations of the capillaries, terminal arterioles, or small venules caused by proliferation and outpouching of the capillary endothelium in areas of intramural pericyte loss [38,48]. These microaneurysms are located most often on the venous side, range in size from 25 to 100 microns in diameter, and are found in the posterior fundus, especially temporal to the macula [49]. Clinically, they appear as tiny red dots in the retina. The color is initially red because the wall of the microaneurysm is transparent and the red blood cells give the aneurysm a red hue. Over time, the wall of the microaneurysm thickens, becomes less transparent and may appear orange to yellow-white in color [50]. They may increase and decrease in number over time [51] secondary to the development of new aneurysms and the obliteration of some of the aneurysms by endothelial proliferation.

Microaneurysms are often difficult to identify through the ophthalmoscope and may be visualized best using fluorescein angiography. There are two types of aneurysms: saccular and fusiform (Fig. 3.2A and B). Saccular aneurysms involve the dilation of all sides of the vessel wall and fusiform aneurysms involve dilation of only one side of the vessel wall. An increase in the number of these microaneurysms in the retina is associated with progression of retinopathy [52–54]. When the number of microaneurysms in an eye exceeds 10, fluorescein angiography usually shows capillary abnormalities including dilation, nonperfusion, and leakage from capillaries or microaneurysms [55]. The microaneurysms occur adjacent to acellular capillaries and a proposed shunt theory suggests that the loss of pericytes leads to dilation of the capillaries and preemption of blood flow with secondary atrophy and obliteration of adjacent capillaries [56]. Other theories suggest that

30 Diabetes and Ocular Disease

A

B

Figure 3.2. (A) A capillary microaneurysm is present that is dilated with a thinned wall (arrow) (Source: Courtesy W. Richard Green, MD.) (B) Capillary microaneurysms are present as saccular dilatations in the trypsin digest preparation. (Source: Courtesy W. Richard Green, MD.)

microaneurysm formation is a result of the net effects of pericyte loss and vascular endothelial growth factor (VEGF)-induced endothelial proliferation. Obstruction and occlusion of retinal vessels may occur secondary to the proliferation of endothelium into the lumen [57,58] with resultant ischemia of the adjacent retina.

Intraretinal microvascular abnormalities. IRMAs (Fig. 3.3) refer to shunt vessels and neovascularization within the neural retina located in areas of dilated capillaries and retinal nonperfusion [59,60] that may be associated with leakage, hard exudates, and hemorrhage [61]. IRMAs is a nonspecific term that was given to avoid the controversy of whether new tortuous, hypercellular retinal vessels in areas of occluded capillaries and nonperfused retina represent either retinal neovascularization, aberrant forms of aneurysms, or preexisting vessels that became dilated “shunts” in areas of nonperfusion [61,62]. Histologically, IRMAs have been described as thin-walled dilated vessels in the inner retina composed of endothelium with a thickened basement membrane and a decreased number of surrounding pericytes [63].

Figure 3.3. Intraretinal microvascular abnormalities: An area of intraretinal neovascularization with proliferation of blood vessels (arrows). (Source: Courtesy W. Richard Green, MD.)

Macular Edema. Macular edema develops secondary to microaneurysm formation, breakdown of the blood-retinal barrier, increased vascular permeability and leakage of fluid and exudate. It is the principal mechanism of vision loss in patients with NPDR. In the ETDRS, macular edema is defined as retinal thickening from accumulation of fluid within one disc diameter of the macula [64,65]. Macular edema is defined as clinically significant macular edema (CSME) if any of the following three features are present: (1) thickening of the retina at or within 500 microns of the center of the macula; (2) hard exudates at or within 500 microns of the center of the macula, if associated with thickening of the adjacent retina; or

(3) a zone or zones of retinal thickening 1 disc area or larger, any part of which is within 1 disc diameter of the center of the macula [66] (Fig. 3.4A).

The incidence of macular edema over a 10-year period has been estimated at 20.1% of patients with type 1 diabetes, 25.4% of patients with type 2 diabetes who require insulin, and 13.9% of patients with type 2 diabetes who do not require insulin [67]. The fluid is composed of water, protein, and lipid material and often collects in the outer plexiform layer of the parafoveal region because more distension can occur in this area of the retina due to the anatomical configuration (Fig. 3.4B) [49]. The water and protein component of the exudates is absorbed by blood vessels and the retinal pigment epithelium, which leads to deposition of lipid-rich material in the outer plexiform layer [49] seen clinically as hard exudates. Hard exudates appear as well-defined yellowish-white intraretinal deposits at the border of edematous and nonedematous areas of the retina [68]. They typically form in clusters and may form a circinate pattern adjacent to groups of microaneurysms. A macular star pattern develops when these hard exudates form in a circinate pattern around the fovea. The exudates are composed of extracellular lipid-rich deposits consisting primarily of polyunsaturated fats [49]. In the ETDRS, it was determined that elevated serum lipid levels were associated with an increased risk of retinal hard exudate in persons with diabetic retinopathy [69]. Chronic macular edema may progress to macular retinoschisis and partial or complete macular hole formation [70].

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A

B

Figure 3.4. (A) Yellow material (hard exudates) is present within the posterior pole (arrows).

(B) Proteinaceous material is present in the outer plexiform layer corresponding to that seen grossly (asterisk).

Intraretinal Hemorrhages. Intraretinal hemorrhages are an early sign of NPDR and are the result of ruptured microaneurysms, leaking capillaries, and IRMAs. The two types of hemorrhages that may be seen are dot-blot hemorrhages and flame-shaped hemorrhages (Fig. 3.5A–C). Dot-blot hemorrhages occur in the inner plexiform, inner nuclear, and outer plexiform layer and appear round because the cellular architecture in these areas runs perpendicular to the retinal surface. Flame-shaped hemorrhages occur in the nerve fiber layer and appear in this configuration because the nerve fiber layer runs parallel to the surface of the retina. The red blood cells may break through the internal limiting membrane and form preretinal or intravitreal hemorrhages [71,72].

Cotton-wool Spots (Soft Exudates). Cotton-wool spots are microinfarctions of the nerve fiber layer that appear clinically as gray and semiopaque lesions with poorly circumscribed, feathery edges (Fig. 3.6A). They frequently have striations running parallel to the nerve fiber layer and occur around blood vessels. The lesions were

A

B C

Figure 3.5. (A) Retinal hemorrhage. Fundus photograph demonstrates a flame-shaped hemorrhage (white arrow) and a dot-blot hemorrhage (black arrow). (B) The retinal hemorrhage is present in the nerve fiber layer which appears clinically as a flame-shaped hemorrhage.

(C) Retinal hemorrhage is present in the outer plexiform and surrounding nuclear layers corresponding to a dot-blot hemorrhage seen clinically.

first observed microscopically as cellular appearing bodies with a “psuedonucleus” in the nerve fiber layer and given the name cytoid bodies [73–77]. They represent swollen nerve endings in the areas of ischemia. The swollen nerve endings are caused by the accumulation of cytoplasmic debris due to the interruption of axoplasmic flow [78] (Fig. 3.6B). Cotton-wool spots are not specific to diabetic retinopathy and can occur in a variety of disease processes, including systemic hypertension, retinal vein occlusions, and acquired immunodeficiency syndrome (AIDS) [79,80].

Venous Caliber Abnormalities. Venous caliber abnormalities in patients with diabetic retinopathy include venous dilation, venous beading, and venous loop formation. Venous dilation is a functional change in response to hyperglycemia and can