Ординатура / Офтальмология / Английские материалы / Retinal and Choroidal Angiogenesis_Penn_2008

44.W. Tin, D. W. Milligan, P. Pennefather, E. Hey, Pulse oximetry, severe retinopathy, and outcome at one year in babies of less than 28 weeks gestation, Arch. Dis. Child. Fetal Neo. Ed. 84(2), F106-F110 (2001).

45.W. Tin, U. Wariyar, Giving small babies oxygen: 50 years of uncertainty, Semin. Neonatol. 7(5), 361-367 (2002).

46.C. H. Cole, K. W. Wright, W. Tarnow-Mordi, D. L. Phelps, on behalf of the POST-ROP Study Planning Group, Resolving our uncertainty about oxygen therapy, Pediatrics 112(6), 1415-1418 (2003).

47.Committee of the Institute of Medicine, Division of Health Sciences Policy. Report of a study. Vitamin E and retinopathy of prematurity, (National Academic Press, Washington, DC, 2101 Constitution Ave., NW, Washington, IOM-86-02, 1986).

48.S. C. Shih, M. Ju, N. Liu, L. E. H. Smith, Selective stimulation of VEGFR-1 prevents oxygen-induced retinal vascular degeneration in retinopathy of prematurity [comment], J. Clin. Invest. 112(1), 50-57 (2003).

49.L. C. Shaw, M. B. Grant, Insulin like growth factor-1 and insulin-like growth factor binding proteins: their possible roles in both maintaining normal retinal vascular function and in promoting retinal pathology, Rev. Endocr. Metab. Disord. 5(3), 199-207 (2004).

50.A. Hellstrom, C. Perruzzi, M. Ju, E. Engstrom, A-L. Hard, J-L. Liu, K. AlbertssonWikland, B. Carlsson, A. Niklasson, L. Sjodell, D. LeRoith, D. R. Senger, L. E. H. Smith, Low IGF-I suppresses VEGF-survival signaling in retinal endothelial cells: Direct correlation with clinical retinopathy of prematurity, Proc. Natl. Acad. Sci. USA 98(10), 5804-5808 (2001).

51.J. Stone, T. Chan-Ling, J. Pe’er, A. Itin, H. Gnessin, E. Keshet, Roles of vascular endothelial growth factor and astrocyte degeneration in the genesis of retinopathy of prematurity, Invest. Ophthalmol. Vis. Sci. 37(2), 290-299 (1996).

52.J. D. Reynolds, R. J. Hardy, K. A. Kennedy, R. Spencer, W. A. J. van Heuven, A. R. Fielder, for the Light Reduction in Retinopathy of Prematurity (LIGHT-ROP) Cooperative Group, Lack of efficacy of light reduction in preventing retinopathy of prematurity, N. Engl. J. Med. 38(22), 1572-1576 (1998).

53.D. L. Phelps, J. L. Watts. Early light reduction to prevent retinopathy of prematurity in very low birth weight infants. Neonatal Module of The Cochrane Database of Systematic Reviews, The Cochrane Library. (Disk) Issue 1, Oxford: Update Software (2001).

54.L. Lakatos, Z. Lakatos, I. Hatvani, G. Oroszlan, Controlled trial of use of d-penicillamine to prevent retinopathy of prematurity in very low- birth-weight infants, in Physiologic Foundations of Perinatal Care, edited by L. Stern, W. Oh, B. Friis-Hansen, (Elsevier, 1987) pp9-23.

55.D. L. Phelps, L. Lakatos, J. L. Watts, D-Penicillamine to prevent retinopathy of prematurity. Neonatal Module of The Cochrane Database of Systematic Reviews, The Cochrane Library; (Disk) Issue 1, Oxford: Update Software (2001).

56.M. Hallman, A-L. Jarvenpaa, M. Pohjavuori, Respiratory distress syndrome and inositol supplementation in preterm infants, Arch. Dis. Child. 61, 1076-1083 (1986).

57.M. Hallman, K. Bry, K. Hoppu, M. Lappi, M. Pohjavuori, Inositol supplementation in premature infants with respiratory distress syndrome, N. Engl. J. Med. 326(19), 1233-1239 (1992).

58.A. Howlett, A. Ohlsson, Inositol for respiratory distress syndrome in preterm infants (Cochrane Review). In: The Cochrane Library, Issue 2, Oxford: Update Software (2002).

59.B. P. Connolly, J. A. McNamara, S. Sharma, C. D. Regillo, W. Tasman, A comparison of laser photocoagulation with trans-scleral cryotherapy in the treatment of threshold retinopathy of prematurity, Ophthalmology 105(9), 1628-1631 (1998).

60.D. G. Hunter, M. X. Repka, Diode laser photocoagulation for threshold retinopathy of prematurity. A randomized study, Ophthalmology 100(2), 238-244 (1993).

61.J. A. McNamara, W. Tasman, J. F. Vander, G. C. Brown, Diode laser photocoagulation for retinopathy of prematurity. Preliminary results, Arch. Ophthalmol. 110(12), 1714-1716 (1992).

62.E. A. Pierce, E. D. Foley, L. E. H Smith, Regulation of vascular endothelial growth factor by oxygen in a model of retinopathy of prematurity, Arch. Ophthalmol. 114(10), 1219-1228 (1996).

63.D. L. Phelps, Reduced severity of oxygen-induced retinopathy in kittens recovered in 28% oxygen, Pediatr. Res. 24(1), 106-109 (1998).

64.STOP-ROP Multicenter Study Group, Supplemental therapeutic oxygen for prethreshold retinopathy of prematurity (STOP-ROP), a Randomized Controlled Trial. I: primary outcomes, Pediatrics 105(2), 295-310 (2000).

Chapter 20

ANGIOGENESIS IN SICKLE CELL

RETINOPATHY

Gerard A. Lutty, PhD, and D. Scott McLeod

Wilmer Ophthalmological Institute, Johns Hopkins Hospital, Baltimore, Maryland

389

J.S. Penn (ed.), Retinal and Choroidal Angiogenesis, 389–405.

© Springer Science+Business Media B.V. 2008

structures. We have examined a transgenic mouse line that produces 72.2% human βs and 44.7% human α-globin. The mice produce RBCs that sickle under hypoxic conditions, and they appear to experience organ damage similar to that observed in human sickle cell disease: splenomegaly and vasoocclusions in lung and kidney. In these mice we have observed retinopathy that is similar to the human: vaso-occlusions occur in the retinal vasculature; adjacent to nonperfused areas, A/V anastomoses form; both intraand extraretinal NV occurs. Choroidal NV and chorioretinal lesions, which resemble the human black sunburst lesion, were observed frequently. In summary, a unique form of angiogenesis called sea fans occurs in sickle cell retinopathy adjacent to peripheral areas of nonperfusion. High levels of VEGF, bFGF, and PEDF are associated with these formations. Animals expressing high levels of human sickle beta globin have a retinopathy similar to human subjects.

1.INTRODUCTION

In each erythrocyte, normal hemoglobin A consists of four polypeptide chains, two alpha and two beta, each with a central ferroprotoporphyrin heme ring. Sickle cell hemoglobinopathy is caused by a point mutation in the beta globin chain of hemoglobin. Two mutations have been observed at residue six on the beta chain. A change from glutamic acid to lysine at this position is called hemoglobin C, and substitution of valine at this position is called hemoglobin S. Abnormal hemoglobin subunits can occur in combination with normal hemoglobin subunits, producing various hemoglobinopathies (Table 1). These include hemoglobin AS (sickle cell trait), hemoglobin SS (sickle cell disease or anemia), hemoglobin SC (sickle cell SC disease), and hemoglobin AC (hemoglobin C trait). If the rate of synthesis of either the alpha or beta polypeptide chain is altered so that there is an excess of one chain, a condition called thalassemia results, which, when combined with the presence of hemoglobin S results in a hemoglobinopathy termed sickle cell thalassemia or SThal disease.

Table 20-1. Mutations in hemoglobin and their corresponding diseases

When exposed to hypoxia, acidosis, or hyperosmolarity, deoxygenated hemoglobin S polymerizes within the erythrocyte and causes it to take on a sickle shape.1 These changes lead to decreased cell pliability, increased hemolysis and blood viscosity, and vaso-occlusion. The occlusions occur in all organ systems of the body. During sickle cell crisis, the occlusions in marrow cause the characteristic debilitating pain that sickle cell patients endure.

Sickle cell hemoglobin is most prevalent in both central and west Africa, where it provided a survival advantage against malaria infection. In North America, about 10% of African descendents have abnormal hemoglobin. 8.5% of these have sickle cell trait (AS), 0.4% have sickle cell disease (SS), 0.2% have sickle cell SC disease (SC), and 0.03% have sickle cell thalassemia (SThal).2

The systemic effects of sickle cell hemoglobinopathy are the most severe in patients with SS disease and are due in part to the presence of more than 90% hemoglobin S within their erythrocytes. In patients with SS disease, intravascular sickling may occur in the microvascular circulation, leading to red blood cell sludging, hemolysis, decreased erythrocyte survival, and subsequent anemia, despite increased erythrocyte production. In bone marrow, infarcts may cause sclerosis and even aseptic necrosis of the femoral head. Sickling may also result in painful joints, abdominal pain, pulmonary infarcts, and cerebrovascular accidents. Systemic manifestations are less likely in SC, SThal, or AS heterozygotes. SC or SThal heterozygotes are only mildly anemic and usually have an uneventful systemic course with very few crises per year. Despite minimal systemic findings, however, these patients have the most severe ocular symptoms of all the hemoglobinopathies. AS heterozygotes, on the other hand, rarely experience systemic or ocular morbidity, except under severe hypoxic conditions, since they have only 50% abnormal hemoglobin S.3

From the studies of Goldberg, proliferative sickle cell retinopathy occurs in about 33% of patients with SC disease, in 14% of patients with SThal hemoglobinopathies, and in only about 3% of patients with SS disease.4-6 Recent data from Clarkson suggest a greater incidence of ocular complications in all genotypes.7 Vaso-occlusion in the peripheral retinal vasculature begins the cascade of events in proliferative sickle cell retinopathy that may culminate in traction and/or rhegmatogenous retinal detachment.

2.CLINICAL FEATURES

Erythrocyte sickling can occur in any microvascular network of the eye. Depending on the anatomical location of the vaso-occlusion(s), visual function may or may not be affected. One site of vaso-occlusion that may be a diagnostic tool is the conjunctival vasculature, where vaso-occlusions result in characteristic comma-shaped vessels full of trapped sickled erythrocytes.8,9

SC and SThal patients are more likely to exhibit ocular manifestations than patients with other hemoglobinopathies; however, the reasons for this are not well understood. It may be related in part to the rate of sickling, blood viscosity, and hematocrit.3 The increased rigidity of sickled cells hampers effective circulation through the microvasculature and increases blood viscosity. Blood viscosity is determined by the most abundant cell type, the erythrocyte, especially in the small-caliber vessels.10 When erythrocytes containing sickle hemoglobin traverse a deoxygenated capillary bed, sickling may occur. Sickling in patients with higher hematocrits leads to an even higher viscosity.11

The marked increase in viscosity contributes to vaso-occlusive events. For example, the hematocrit in SC and SThal patients is significantly higher than in SS disease. Thus, SC or SThal heterozygotes have greater blood viscosity during a sickling episode and, therefore, may experience more vaso-occlusive events in the retinal microvasculature where red blood cell characteristics are a critical factor. Despite the large number of sickled red cells in SS subjects, their lower hematocrit and thus lower viscosity may protect the vessels in the retina from vaso-occlusions. Another factor that may give SC subjects the highest incidence of retinopathy is that occlusions are almost always in peripheral retina, where the hematocrit and the viscosity are normally the highest.12 Thus, abnormally high hematocrits in SC subjects would have a profound effect on RBC sludging in peripheral retina. The relationship between sickling, vessel wall adhesion, and the presence of various serum factors in predisposing toward and/or promoting vaso-occlusion in various hemoglobinopathies is currently being investigated.

3.NONPROLIFERATIVE RETINOPATHY

As just mentioned, occlusions occur predominantly in far peripheral retina. In the posterior pole, the major retinal vessels usually appear to be normal. Occasionally, increased vascular tortuosity may be present; this has been attributed to arteriovenous shunting in the retinal periphery.13 In the

periphery, the major retinal vessels may end abruptly, often in hairpin loops.14 Peripheral microvascular occlusions have been observed in SS subjects as young as 20 months of age.14 The nonperfused areas observed in middle-aged subjects may represent a continuum of vaso-occlusive events; only capillaries and small arterioles are occluded in children, but major arteries and veins are involved in adult sickle cell subjects.

The cause of the vaso-occlusions seems intuitive: sickled erythrocytes. Erythrocytes may certainly be the initiating cell type, as observed even in large blood vessels at A/V crossing.14 However, although these sites are characterized by packed sickled erythrocytes downstream, leukocytes, particularly polymorphonuclear leukocytes (PMNs), are often observed upstream. PMNs were observed to be significantly elevated in sickle cell retina compared to a non-sickle cell control subject.15 In addition, we found that the leukocyte adhesion molecules P-selectin, VCAM-1, and ICAM-1 were significantly elevated in retinas of some sickle cell subjects.15

Although the majority of vaso-occlusions and vascular changes are in peripheral retina, vascular changes may occur in the macula in older subjects. Macular arteriolar occlusion and subsequent nonperfusion is

common in SS homozygotes and may precede the development of retinal thinning and atrophy.16,17 It may also be associated with a mild decrease in

visual acuity. Roy and associates found that macular blood flow velocity in sickle cell subjects was reduced and that the reduction in leukocyte velocity was related to the density of sickled erythrocytes.18

The histopathological characteristics of nonproliferative (no neovascularization) or background sickle cell retinopathy also include the salmon patch hemorrhage, iridescent spots, and black sunburst lesions. These changes may be intricately associated with sites of occlusion.

A salmon patch hemorrhage is an oval or round, preretinal or superficial intraretinal collection of blood that has a flattened or dome-shaped appearance with well-defined borders and a size of up to one disc in diameter. These hemorrhages occur most commonly in the midperiphery adjacent to an intermediate-sized retinal arteriole.19 They may form due to vessel rupture at the site of a sudden arteriolar occlusion by sickled erythrocytes. The blood changes to a red-orange or salmon color with time. The patch may be localized beneath the internal limiting membrane or may diffuse into the vitreous or subretinal space.

The site of resorption of a salmon patch hemorrhage may develop into a small retinoschisis cavity containing yellowish granules that represent multiple hemosiderin-laden macrophages.20 This cavity is termed an iridescent spot. Histopathological examination demonstrates a small retinoschisis space following resorption of the intraretinal blood20 containing

intraand extracellular iron. The cavity is lined by the internal limiting membrane anteriorly and by the sensory retina posteriorly.

The black sunburst lesion consists of hyperplastic retinal pigment epithelial (RPE) cells that have migrated into the sensory retina.13 Romayananda hypothesized that RPE migration into the retina was stimulated by blood that had dissected into the subretinal space,20 but the etiology of the black sunburst may be multifactorial. The black sunburst may evolve directly from a salmon patch hemorrhage depending on the plane of dissection of the resulting hemorrhage21 as has been clearly demonstrated in a 17-year-old man with SC disease during a 6 year follow-up period.22 However, it is interesting that hemorrhages occur often in diabetic retinopathy, yet no black sunburst-like lesion occurs in diabetic subjects.

Others have demonstrated choroidal neovascularization that has occurred within the black sunburst lesion23,24 and may develop following a localized choroidal occlusion.25,26

4.PROLIFERATIVE RETINOPATHY

The initiating pathogenic event in proliferative sickle cell retinopathy is peripheral retinal arteriolar occlusion resulting in angiogenesis and sea fan NV formation, the hallmark of proliferative retinopathy. Goldberg4 proposed a five-stage grading system for sickle cell retinopathy in which stage 1 is peripheral arteriolar occlusion. By fluorescein angiography, occlusions appear to occur mostly in the precapillary arterioles,27 but histologically, capillary nonperfusion is the first change in peripheral retina, and it is even observed in young children.14 The sickled erythrocytes act as microemboli and impede local blood flow or cause intravascular thromboses. Arteriolar flow at this site is terminated, and the affected retina becomes nonperfused. Occlusion within a portion of a vein may exert substantial back pressure within the proximal vessel and result in focal vascular extrusion.14 This vessel may further enlarge as the elevated intralumenal pressure persists. Stretching of the vascular structures may lead to endothelial cell proliferation28 and NV.29

At the border of the perfused and nonperfused retina, peripheral A/V anastomoses form due to vascular remodeling, which Goldberg termed stage 2. These abnormal vessels shunt blood from the occluded arterioles to nearby medium-sized venules anterior to the equator. These anastomoses retain intralumenal fluorescein during angiography, unlike true neovascular tissue that usually leaks dye; thus, these vessels appear to represent the creation of preferential vascular channels from preexisting retinal vasculature by enlargement of pre-existing capillaries30 rather than a neovascular

formation. The formation of these abnormal arteriolar-venular connections may be a hydrostatic response to the occlusion of the distal vessels.

The first form of NV is the hairpin loop, the site of major vessel occlusion. These loops represent an abrupt end of an artery or vein and continuation of blood flow in a new channel that formed by a recanalization of the original vascular wall (Figure 1). The new channel ends at the first branch point encountered by the new vascular segment. These structures are detected frequently at the border of perfused and nonperfused peripheral retina.

Figure 20-1. Two retinal vascular hairpin loop formations in a 54-year-old SC sickle cell subject. In A and B, the ADPase activity in the loops is viewed with dark field illumination showing that the artery (A) and the vein (B) abruptly terminate at the border (arrowheads) of perfused and nonperfused peripheral retina. Subsequent sectioning peripheral to those structures (bottom of A and B) demonstrated collagenous tubes where the original vessels were. When sectioned, it is apparent that a new channel had formed beside the original artery

(C) in the original arterial wall. In sections of the venous loop (D), however, the new channel is posterior to the original channel in the wall, which appears sclerotic. From McLeod, D. S. et al., Arch. Ophthalmol. 111:1234-1245, 1993.

Also at this border, buds of NV form. There may be multiple buds breaching the internal limiting membrane in close proximity (Figure 2). The new capillaries initially grow into a fan shape, forming fine red channels that may be overlooked on indirect ophthalmoscopy because of their small size and coloration. The peripheral superotemporal quadrant is most frequently involved, followed by the inferotemporal, superonasal, and inferonasal quadrants.

Figure 20-2. Multiple buds of angiogenesis at the border of perfused and nonperfused retina in a 20-year-old woman with sickle cell anemia (SS). ADPase activity is always elevated in NV, so the buds of NV (arrowheads) and an IRMA (intraretinal microvascular abnormality) formation (arrow E) have more activity in the flat perspective (A and, with higher magnification, B) and, therefore, are easily found in flat perspective analysis. Most buds are present in the venular part of the vasculature, and both arteriole (a) and venule (v) terminate in hairpin loops (curved arrow). When sectioned (C-E), the buds can be seen near the internal limiting membrane (ILM) (C) or breaching it (arrow in C). Even the IRMA-like formation (E) is at the ILM. From McLeod, D. S. et al., Am. J. Ophthalmol. 124:455-472, 1997.