Ординатура / Офтальмология / Английские материалы / The Retina and its Disorders_Besharse, Bok_2011

as a macrophage chemoattractant and as a promoter of ICAM-1 upregulation.

Placental growth factor

Placental growth factor (PlGF) is a structurally related member of the same superfamily as VEGF, with all four PlGF isoforms acting as ligands for VEGFR1. PlGF has been implicated in a variety of physiological processes, including serving as a chemoattractant for monocytes and endothelial progenitor cells. Although ablation of the PlGF gene did not prevent embryonic angiogenesis, loss of PlGF led to impaired angiogenesis during conditions of ischemia and inflammation. In murine models, PlGF was found to be essential for the full expression of pathological neovascularization in laser-induced choroidal neovascularization (CNV) as well as for angiogenesis in tumors that were resistant to VEGFR inhibitors. Moreover, intravitreal injection of PlGF was shown to induce retinal edema in a rodent model, reflecting a disruptive effect on the retinal pigment epithelium. PlGF accordingly has attracted interest as a possible therapeutic target, and inhibition of its function is believed to contribute to the efficacy of VEGF-Trap, which, as discussed above, has been engineered to include the binding sites of both VEGFR1 and VEGFR2.

Platelet-derived growth factor

The platelet-derived growth factor (PDGF) family includes four members (PDGF-A through PDGF-D), with active forms being dimers and usually occurring in the homodimeric form. They serve as ligands for two receptor kinases, PDGFR-a and PDGFR-b. PDGF-B, acting through PDGFR-b, mediates most of the actions of PDGF in vascular development. Gene knock-out of either PDGF-B or PDGFR-b resulted in perinatal death owing to vascular deformities. In addition to promoting endothelial cell proliferation and capillary tube formation, PDGF-B signaling is especially important for the proliferation and recruitment of mural cells (pericytes and vascular smooth muscle cells) to the embryonic vasculature and for the maintenance of pericyte coverage (Figure 2). These actions are also essential for proper retinal vascular development. Endothelial cell-restricted ablation of PDGF-B produced defective pericyte coverage of retinal vessels and associated proliferative retinopathy; a similar phenotype resulted from the administration of an inhibitor to PDGFR signaling.

Investigations have demonstrated a complex interaction between PDGF and VEGF. During embryonic development, pericyte coverage was found to lag behind growth of retinal capillaries for several days, giving rise

Reduced vSMC proliferation and migration

PDGF-B or PDGFR-β knock-out

to an optimum window in which nascent blood vessels were especially vulnerable to VEGF depletion. Further investigations have revealed that the inhibition of both PDGF and VEGF signaling is quite effective in inhibiting ocular neovascularization. In experiments with three different murine models, VEGF inhibition was more effective for immature vessels while mature vessels were sensitive to PDGF-B. In every case, however, the most effective inhibition was achieved by interference with both signaling pathways (Figure 3).

Notch

The Notch family of cell surface proteins, consisting in mammals of four members, Notch1 through Notch4, is activated by ligands of the Jagged and Delta families and regulates pattern formation of numerous tissues, including those of the kidney, nervous system, and cardiovascular system. In its actions in the vascular system, the

principal ligand for Notch is Dll4. Several recent studies have identified a key role for this signaling pathway as a negative regulator of retinal vessel patterning. Even heterozygous ablation of the dll4 gene led to dramatic increases in branching while other approaches to inhibiting the Notch pathway also caused excessive tip-cell (specialized endothelial cells which form the leading edge of vascular sprouts to initiate vessel branching) formation and endothelial cell proliferation. In a tumor model, blockade of the Notch pathway increased tumor vascularity but the vessels were nonproductive, resulting in decreased tumor growth. These findings are believed to reflect the importance of Dll4/Notch signaling in the negative regulation of VEGF-mediated angiogenic sprouting (Figure 4). Finally, the loss of Notch3 signaling mechanism has been found to interfere with mural cell differentiation, an action that may reflect Notch-mediated upregulation of PDGFR-b in vascular smooth muscle cells.

Tumor necrosis factor-a

Tumor necrosis factor-a (TNF-a), a member of a large superfamily of cytokines and their receptors that are involved in regulating numerous physiological processes, is an important mediator of inflammation. Clinical studies have detected TNF-a in the fibrovascular membranes of patients with PDR and CNV. In two small case series, intravenous administration of infliximab, a monoclonal antibody directed against TNF-a, led to the regression of CNV in patients with AMD and to the alleviation of macular edema in patients with DME. In addition, a case report has described a similar regression of uveitisinduced neovascularization with this approach.

The mechanisms underlying the actions of TNF-a in ocular neovascularization have yet to be elucidated. TNF-a has been found to upregulate the synthesis of a number of genes that are important for angiogenesis, including VEGF, VEGFR2, angiopoietins 1 and 2 (Ang1 and Ang2), matrix metalloproteinases (MMPs) 2 and 9, PDGF-B, and the Notch ligand Jagged-1. The increase in Jagged-1 may account for the recent finding that TNF-a can induce the formation of tip cells.

Gene ablation studies in mice have been inconsistent, with some studies finding inhibition of retinal neovascularization in response to interference with TNF-a signaling, while others did not. Inhibition of laser-induced CNV by agents targeting TNF-a has been observed, though, both with intravitreal injection of infliximab and

intraperitoneal administration of etanercept, a fusion protein containing the TNF-a receptor. Also, TNF-a signaling was determined to be important for ischemiamediated neovascularization in the hind limbs of mice. Taken together, these data suggest that TNF-a may serve as a potential molecular target for controlling ocular neovascularization.

Ephrins and Ephs

Ephrins are a family of ligands that bind to the Eph receptor tyrosine kinases. The ephrins fall into two broad classes, the ephrinAs, which are attached to the cell membrane by a glycosylphosphatidyl anchor, and the ephrinBs, which span the cellular membrane and possess a cytoplasmic signaling domain (Figure 5). There is a corresponding division among the Eph kinases, which fall into A and B subclasses, with ephrinAs binding primarily to EphAs and ephrinBs to EphBs. Because of the tethered nature of the ephrins, ephrin–Eph signaling requires cell–cell contact and can proceed in either the forward or reverse direction. In addition to angiogenesis, ephrin–Eph interactions are required for the proper development of the nervous and cardiovascular systems as well as such processes as insulin secretion and trafficking of immune cells.

Evidence has been adduced supporting the roles of both major Eph/ephrin classes in angiogenesis, including retinal neovascularization. A soluble EphA-containing fusion protein significantly inhibited VEGFor ephrinA1-induced

PDZ

P

SH2 P P

P

PDZ

Ligand binding

Kinase

Cysteine-rich

SAM

Fibronectin type III

Figure 5 Ephrins and their Eph receptors. While both ephrins and Eph receptors are membrane-tethered proteins, ephrinBs traverse the membrane and possess a cytoplasmic signaling domain while ephrinAs do not. Ephrin–Eph binding results in receptor clustering followed by autophosphorylation of multiple tyrosine residues and docking of downstream effectors through src-homology domains. The presence of a sterile alpha motif (SAM) and a PDZ domain (shown here for the carboxy terminus of EphA but also present in EphB) promotes ligand-induced receptor clustering. Adapted from Dodelet, V. C. and Pasquale, E. B. (2000). Eph receptors and ephrin ligands: Embryogenesis to tumorigenesis. Oncogene 19: 5614–5619, with permission from Nature Publishing Group.

endothelial cell migration and assembly into capillary tubes in vitro, and when injected intravitreally the fusion protein also suppressed retinal neovascularization in a murine ROP model.

More extensive data are available implicating ephrinB/ Eph signaling. In clinical studies, both EphB2 and EphB3 were found to be expressed in fibroproliferative membranes of patients with ischemic ocular neovascularization. Genetic ablation of either ephrinB2 or EphB4 led to homozygous embryonic lethality owing to aberrant development of the vasculature. Analysis of expression patterns suggested that ephrinB2 is expressed mainly on arteries and EphB4 on veins, and that these molecules act in defining arterial or venous identity.

Preclinical studies of the role of ephrinB/EphB in ocular neovascularization have not been consistent, however. In corneal models, neovascularization was promoted by ephrinB2, or fusion proteins containing it, and by an EphB1 fusion construct. In contrast, administration of

soluble forms of EphB4 inhibited laser-induced CNV in rats as well as ischemia-induced retinal neovascularization in mice; the inhibition of retinal neovascularization also was seen when using soluble ephrinB2. Further work clearly is required if the ephrin/Eph systems can be considered as potential therapeutic targets.

Angiopoietins

Ang1 and Ang2 are members of a family of secreted ligands for Tie2, a receptor tyrosine kinase whose function is essential for vascular development and remodeling. Although Ang1 and Ang2 both activate Tie2, they usually act antagonistically. Similar lethal phenotypes involving gross vascular defects resulted from genetic ablation of Tie2 or Ang1 as well as from the overexpression of Ang2. While the effects of both Ang1 and Ang2 are complex, the overall action of Ang1 is to stabilize the quiescent vasculature while Ang2 is a destabilizing agent for the vascular endothelium (Figure 6).

Ang1, which is secreted by vascular smooth muscle cells, acts as a survival factor for endothelial cells in vitro and also can promote endothelial cell sprouting and tissue invasion of new blood vessels. In transgenic mice, coexpression of VEGF and Ang1 led to an additive effect on angiogenesis, but co-expression of Ang1 was found to suppress the leakiness of vessels induced by VEGF alone. Recently, this stabilizing effect has been shown to involve Ang1-mediated inhibition of the VEGF-induced internalization of vascular endothelial cadherin, a key component of adherent junctions. In addition, Ang1 inhibited other VEGF-mediated pro-inflammatory actions, including the upregulation of tissue factor, ICAM-1, and vascular cell adhesion molecule-1. Intravitreal injection of Ang1 also prevented many of the inflammatory changes characteristic of DR. In transgenic models, overexpression of Ang1 inhibited the development of laser-induced CNV and retinal neovascularization.

In contrast to the stabilizing effects of Ang1, Ang2 is primarily a destabilizing agent. In clinical studies, elevated levels of Ang2 and VEGF were detected in the vitreous of patients with DR as well as in choroidal neovascular membranes. Ang2 is synthesized by arterial smooth muscle cells and by endothelial cells where it is stored in Weibel-Palade bodies upon activation in response to stimuli such as thrombin or histamine, and sensitizes the endothelium to pro-inflammatory cytokines such as TNF-a. In addition to being upregulated by hypoxia and VEGF, Ang2 appears to act in concert with VEGF in many contexts, including enhancement of endothelial cell permeability. In several different rodent models, Ang2 was found to co-operate with VEGF in promoting neovascularization, but if VEGF levels were low, overexpression of Ang2 led to its regression.

Taken together, these findings suggest that Ang1 could have therapeutic potential as an antiangiogenic agent on

Quiescent/resting vasculature

Ang-1/Ang-2

Activation/WPB release

Ang-1/Ang-2

Activated/responsive vasculature

Figure 6 Regulation of vascular responsiveness by Ang1 and Ang2. Angiopoietin 1 (Ang1) (multimeric, white) is secreted constitutively at a low level by mural (periendothelial) cells, and acts on the quiescent endothelium to sustain a low-level activation of Tie2, thereby helping to maintain the luminal cell surface in an antithrombotic and antiadhesive state. Ang2 (dimeric, gray) is stored in Weibel-Palade bodies (WPB) in the endothelium and during endothelial cell activation is released from them, along with other stored factors, leading to the Ang1/ Ang2 ratio being altered more in favor of Ang2. As a result, the endothelial cell layer becomes destabilized and more responsive to pro-inflammatory stimuli. Reproduced with permission from Pfaff, D., Fiedler, U., and Augustin, H. G. (2006). Emerging roles of the Angiopoietin–Tie and the ephrin–Eph systems as regulators of cell trafficking. Journal of Leukocyte Biology 80: 719–726.

its own. For Ang2, the situation is more complex, although it may have promise if administered together with a VEGF-suppressive agent.

Erythropoietin

While primarily identified as a promoter of erythropoiesis, erythropoietin also acts as a neuroprotective agent and as a promoter of angiogenesis. It plays an essential role in establishing the vascular network following vasculogenesis in that homozygous deletion of erythropoietin or its receptor leads to embryonic lethality with extensive vascular anomalies. In ischemic conditions, erythropoietin (which is itself upregulated by hypoxia) upregulates VEGF and VEGFR2 and promotes the recruitment of bone marrow-derived endothelial progenitor cells.

Correlative clinical findings implicate erythropoietin in ocular neovascular disease. Elevated ocular levels of

erythropoietin were detected in patients with DME and PDR; in addition, treatment of premature infants with erythropoietin has been correlated with an increased risk of ROP. Preclinical studies are limited to one report in which intravitreal administration of a soluble erythropoietin receptor inhibited ischemia-induced neovascularization in a murine model. While not extensive, the available data suggest that further studies are warranted.

Integrins

Integrins comprise a large family of transmembrane cell surface receptors that transduce signals from ligands in the extracellular matrix to the cytoplasm. Each integrin is composed of one a and one b subunit; each subunit class contains many representatives, and more than 24 a–b combinations have been observed. The principal focus of integrin research has been their relevance to cancer, and in this context they play important roles in angiogenesis.

Studies with small molecule antagonists have identified a role for the a5b1 integrin in murine models of ocular neovascularization. These inhibitors include JSM5562, which inhibited corneal neovascularization, and a related molecule, JSM6427, which inhibited laser-induced CNV and ischemia-induced retinal neovascularization. Studies indicated that a5b1 contributed to an angiogenic pathway that was distinct from VEGF-mediated angiogenesis, suggesting that combinatorial approaches may offer the promise of even greater efficacy.

Matrix metalloproteinases

The MMPs comprise a large family of enzymes that degrade the extracellular matrix, facilitating the tissue penetration of nascent blood vessels. While the primary focus of their physiological impact has been in the context of cancer, they also have been shown to be involved in ocular neovascularization. MMP action has been implicated in the exposure of a cryptic epitope of collagen IV that promoted laser-induced CNV in a murine model. MMPs also generated soluble fragments of VEGF from matrix-bound forms; as MMP expression by cultured retinal pigment epithelium cells is upregulated by VEGF, this may provide for a positive feedback amplification of the angiogenic response.

Components of the complement cascade

In addition to the genetic studies implicating specific mutations in the complement system as risk factors for AMD (see above), both clinical and preclinical studies have directly implicated factors C3a and C5a in promoting ocular neovascularization. Both of these factors have been found in the drusen of patients with AMD as well as in the eyes of mice with laser-induced CNV; genetic ablation of their corresponding receptors reduced VEGF expression, leukocyte recruitment, and induction of CNV. Moreover, these

factors induced upregulation of VEGF expression when injected intravitreally in mice. Similar effects on CNV and VEGF expression were found in mice in which factor C3 was ablated. Finally, depletion of complement by administration of cobra venom factor has been shown to inhibit the development of the inflammation, which is often accompanied by neovascularization, in an animal model of uveitis. Taken together, these findings suggest that targeting certain components of the complement cascade could provide a therapeutic option for treating ocular neovascularization.

Inhibitors of Angiogenesis

Pigment epithelium-derived factor

Pigment epithelium-derived factor (PEDF) is a 50-kDa glycoprotein, originally found to be expressed in the retinal pigment epithelium that exhibits properties directly opposing VEGF: inhibition of VEGF signaling through VEGFR1, downregulation of VEGF expression in the context of ischemia, and inhibition of VEGF-induced increases in endothelial cell permeability. PEDF induces macrophage apoptosis, suggesting that it serves also as a modulator of inflammation. Systemic or intravitreal administration of PEDF, as well as its expression from a transgene, was found to inhibit ischemia-induced retinal neovascularization in mice. PEDF also induced apoptosis of cultured endothelial cells and suppression of VEGF-induced endothelial cell proliferation and migration. However, one study reported that the effects of PEDF were dose dependent, with both laser-induced CNVand endothelial cell differentiation in vitro being inhibited at low doses and promoted at high doses. Clinical data have been similarly inconsistent. While the expression of PEDF in Bruch’s membrane was reduced in patients with AMD relative to controls, vitreous levels of PEDF were elevated in patients with PDR. The potential of PEDF as an antiangiogenic agent for ocular neovascularization thus remains to be established.

VEGFxxxb isoforms

VEGFxxxb denotes a parallel family of alternately spliced VEGF isoforms that are identical in length to the canonical VEGF family but differing in the last six amino acids. VEGFxxxb isoforms can bind VEGFR2 but have little or no ability to initiate downstream pathways. They therefore act as competitive inhibitors of VEGF. In clinical studies, VEGFxxxb was found to predominate in the vitreous of control eyes but to constitute only 12% in patients with PDR; similar downregulation of VEGFxxxb has been observed in several cancers. In preclinical studies, VEGFxxxb inhibited both corneal and retinal neovascularization. These findings suggest that downregulation of VEGFxxxb levels may contribute to pathological neovascularization and that members of this family may have potential as antiangiogenic therapies.

Soluble VEGF receptor 1

Soluble VEGFR1, also generated by alternative splicing, is a natural inhibitor that can bind VEGF but lacks the exons required for signaling. It is essential for maintaining the avascularity of the cornea. As an integral component of the engineered therapeutic agent VEGF-Trap (see above), the ligand-binding site currently is being evaluated in clinical trials as a VEGF-targeting agent.

Complementary regulatory protein CD59

In addition to containing components that promote pathological neovascularization, the complement cascade also includes regulatory components that are potential antiangiogenic agents. One such candidate protein is CD59, whose ablation facilitated the development of CNV in mice; in the converse experiment, laser-induced CNV was inhibited by intravitreal or intraperitoneal administration of a soluble fusion protein containing CD59.

Tryptophanyl-tRNA synthase fragment

Tryptophanyl-tRNA synthase fragment (T2-TrpRS), another naturally occurring molecule, was found to inhibit physiological retinal angiogenesis in mice as well as ische- mia-induced preretinal neovascularization. Recently, it was shown that intravitreal injection of T2-TrpRS, together with an anti-VEGF aptamer, led to potent inhibition of ischemia-induced retinal neovascularization, suggesting yet another avenue for combinatorial therapy.

Slit/Roundabout4

Slit/Roundabout4 (Robo4) is a member of a family of transmembrane receptors for the Slit family of secreted ligands. While Slit/Robo signaling was initially shown to be important in neuronal patterning, it also has been implicated in angiogenesis. Recently, intravitreal injection of Slit2 was found to inhibit both ischemia-induced neovascularization and VEGF-retinal permeability; none of these effects were seen if Robo4 was genetically ablated. Slit2-mediated inhibition of VEGF-induced endothelial cell migration and tube formation also was dependent on the presence of Robo4. Taken together, these data suggest that Slit/Robo4 axis acts as a negative regulator of VEGFinduced proangiogenic signaling.

Other inhibitors

In addition to the inhibitors discussed herein, numerous other molecules have been identified that also possess antiangiogenic activity but whose involvement in ocular neovascularization is not as well characterized (Table 1).

New Directions in Antiangiogenic Therapy

While most research into pathological angiogenesis has been directed toward ablating the unwanted vasculature, recent studies into the control of malignant tumors suggest

that antiangiogenic therapy may act to normalize, rather than to destroy, the tumor vasculature. This normalization facilitated tumor penetration of chemotherapeutic agents and increased their efficiency. Similar concepts have emerged from studies of Dll4/Notch signaling where inhibition led to more profuse tumor vasculature but slower tumor growth. The molecular mechanisms underlying vascular normalization are yet to be determined. Inhibition of the regulator of G protein signaling 5 (RGS5), a protein found in pericytes, has been shown to promote vascular normalization in a mouse tumor model; this allowed the influx of immune cells into the tumor parenchyma and led to increased survival. Upregulation of RGS5 resulted in a significant reduction in the susceptibility of nascent vessels to regression by VEGF inhibition, suggesting that RGS5 contributes to peri- cyte-endothelial cell interactions and vascular maturation.

Normalization of the vasculature also has been proposed as a contributor to the utility of antiangiogenic therapy in pathological ocular neovascular diseases. Retinal neovascularization may reflect an adaptive response to hypoxia that has gone awry. It remains to be seen whether inducing regression or normalization of aberrant retinal vasculature will be the most effective in providing favorable visual outcomes.

Conclusions

The systematic study of the mechanisms underlying physiological and pathological angiogenesis has yielded a wealth of knowledge as to underlying molecular and cellular mechanisms involved in retinal neovascular diseases, including the actions of proangiogenic and antiangiogenic factors that modulate these processes. Drugs targeting VEGF have already proved their clinical utility. It is clear, moreover, that a variety of other agents offer promise, either alone or as adjunctive therapy with agents targeting VEGF, and that there is reason for optimism that the range of treatments for retinal neovascular diseases soon will be expanded.

See also: Breakdown of the RPE Blood–Retinal Barrier; Development of the Retinal Vasculature; Retinal Vasculopathies: Diabetic Retinopathy; Retinopathy of Prematurity.

Further Reading

Adamis, A. P. (2002). Is diabetic retinopathy an inflammatory disease?

British Journal of Ophthalmology 86: 363–365.

Andrae, J., Gallini, R., and Betsholtz, C. (2008). Role of platelet-derived growth factors in physiology and medicine. Genes and Development 22: 1276–1312.

Arcasoy, M. O. (2008). The non-haematopoietic biological effects of erythropoietin. British Journal of Haematology 141: 14–31.

Avraamides, C. J., Garmy-Susini, B., and Varner, J. A. (2008). Integrins in angiogenesis and lymphangiogenesis. Nature Reviews Cancer 8: 604–617.

Bora, N. S., Jha, P., and Bora, P. S. (2008). The role of complement in ocular pathology. Seminars in Immunopathology 30: 85–95.

Dodelet, V. C. and Pasquale, E. B. (2000). Eph receptors and ephrin ligands: Embryogenesis to tumorigenesis. Oncogene 19: 5614–5619.

Ferrara, N., Damico, L., Shams, N., Lowman, H., and Kim, R. (2006). Development of ranibizumab, an anti-vascular endothelial growth factor antigen binding fragment, as therapy for neovascular age-related macular degeneration. Retina 26: 859–870.

Fiedler, U. and Augustin, H. G. (2006). Angiopoietins: A link between angiogenesis and inflammation. Trends in Immunology 27: 552–558.

Gragoudas, E. S., Adamis, A. P., Cunningham, E. T., Jr., et al. (2004). Pegaptanib for neovascular age-related macular degeneration.

New England Journal of Medicine 351: 2805–2816.

Hellstrom, M., Kalen, M., Lindahl, P., Abramsson, A., and Betsholtz, C. (1999). Role of PDGF-B and PDGFR-beta in recruitment of vascular smooth muscle cells and pericytes during embryonic blood vessel formation in the mouse. Development 126: 3047–3055.

Ishida, S., Usui, T., Yamashiro, K., et al. (2003). VEGF164-mediated inflammation is required for pathological, but not physiological, ischemia-induced retinal neovascularization. Journal of Experimental Medicine 198: 483–489.

Jo, N., Mailhos, C., Ju, M., et al. (2006). Inhibition of platelet-derived growth factor B signaling enhances the efficacy of anti-vascular endothelial growth factor therapy in multiple models of ocular neovascularization. American Journal of Pathology 168: 2036–2053.

Pfaff, D., Fiedler, U., and Augustin, H. G. (2006). Emerging roles of the Angiopoietin-Tie and the ephrin-Eph systems as regulators of cell trafficking. Journal of Leukocyte Biology 80: 719–726.

Ribatti, D. (2008). The discovery of the placental growth factor and its role in angiogenesis: A historical review. Angiogenesis 11: 215–221.

Rosenfeld, P. J., Brown, D. M., Heier, J. S., et al. (2006). Ranibizumab for neovascular age-related macular degeneration.

New England Journal of Medicine 355: 1419–1431.

Sainson, R. C. and Harris, A. L. (2008). Regulation of angiogenesis by homotypic and heterotypic notch signalling in endothelial cells and pericytes: From basic research to potential therapies.

Angiogenesis 11: 41–51.

Perimetry

D B Henson, University of Manchester, Manchester, UK

ã 2010 Elsevier Ltd. All rights reserved.

Glossary

Change probability – Change probability compares each test location with that of a baseline measure and establishes whether or not there has been any significant change. Results are presented in the form of a visual field plot where each location is classified according to a series of cut-off probability levels, for example, p < 0.05.

Kinetic perimetry – With the kinetic examination strategies, the perimetrist selects a stimulus of a given size and intensity and moves it from outside the visual field toward its center noting the position at which it first becomes visible.

Perimetry – Perimetry, or campimetry, is the technique used to measure the extent of the visual field or to assess the sensitivity of the visual system to stimuli presented within the visual field (IPS standards).

Standard achromatic perimetry (SAP) – SAP is the most widely used form of perimetry where a white stimulus is projected onto a white background, often called white-on-white perimetry.

Static threshold perimetry – In static threshold perimetry, an estimate of the patient’s sensitivity is derived at a series of predetermined test locations. Visual field – Tate and Lynn defined the visual field as ‘‘all the space that one eye can see at any given instant,’’ it normally extends from the fixation axis: 60 up, 75 down, 100 temporally, and 60 nasally. The superior and nasal fields are limited by facial contours.

Perimetric Techniques

Kinetic Perimetry

With the kinetic examination strategies, the perimetrist selects a stimulus of a given size and intensity and moves it from outside the visual field toward its center noting the position at which it first becomes visible. This is repeated along a series of different meridians and the points at which the stimulus first became visible are then joined together by a line which is called an isopter. Scotomas within the area of an isopter are detected by continuing to move the stimulus toward the center of the visual field

after it has first been detected. The patient is asked to report, if at any time, it disappears.

The perimetrist can repeat the whole process with stimuli of different sizes and/or intensities, in order to build up a map of the patient’s visual field, such as that given in Figure 1. Kinetic examination strategies were very popular in the early days of perimetry. They have, however, been largely replaced by static techniques although they are still retained for specific cases such as small residual islands of vision and where patients have difficulty in performing automated perimetry.

Static Threshold Perimetry

In static threshold perimetry, an estimate of the patient’s sensitivity is derived at a series of predetermined test locations. There are many different algorithms that can be used to establish the threshold and a great deal of effort has gone into deriving ones that minimize test time and error (difference between the true and the measured threshold).

The first widely used threshold algorithm (Full Threshold) was developed by Spahr and Bebie and their colleagues. It is a two-reversal staircase strategy in which the step size reduced from 4 to 2 dB after the first reversal.

This algorithm suffers from four major drawbacks:

1.long test time,

2.demanding and exhausting for the patient,

3.poor repeatability, and

4.a significant learning effect.

In an attempt to shorten test times, the fast threshold staircase algorithm was introduced, in which there was a single step size of 3 dB with only one reversal. While being faster, this algorithm is more variable than the Full Threshold algorithm and thus has not been widely adopted in clinical practice.

Tendency-orientated perimetry was similarly developed in order to reduce test times. The algorithm combines data from several neighboring locations and reduces test times by effectively reducing spatial resolution.

The Swedish interactive threshold algorithm (SITA) is currently one of the most widely used algorithms. It is considerably faster than the full threshold algorithm (approximately 5 min per eye for a normal field) with similar repeatability. This has been achieved by:

1.using maximum-likelihood procedures in combination with a 4–2 dB staircase algorithm;

2.removing the need for false-positive catch trials; and

3.speeding up the rate of stimulus presentation in patients who respond quickly. The SITA algorithm monitors the patient’s response rate and adjusts the presentation rate accordingly.

The SITA algorithm is currently available in two forms, standard and fast. SITA Fast has more liberal terminating criteria to further reduce overall test times.

Static Suprathreshold Perimetry

In a suprathreshold examination, the stimuli are initially presented at an intensity that is calculated to be above the patient’s threshold. If the stimuli are seen then it is assumed that no significant defect exists. This strategy has largely been developed as a screening procedure for conditions such as glaucoma.

There are many different types of suprathreshold tests and Table 1 highlights some of the major differences.

Selection of the test intensity is an important part of a suprathreshold test. If the intensity is set too high then the test will become insensitive to shallow defects. If the intensity is set too low then the test looses specificity. Most suprathreshold tests set the intensity at 5–6 dB above the threshold estimate.

Multiple stimulus tests are faster (approximately twice as fast in a person without a defect). The need to verbally respond to each presentation helps maintain patient attention, improves threshold estimates and reduces variability.

Test Targets

Most modern perimeters use stimuli whose sizes were defined by Goldmann (see Table 2).

Standard Achromatic Perimetry

Standard achromatic perimetry (SAP) is the most widely used form of perimetry where a white stimulus is projected onto a white background, often called white-on-white perimetry. It normally uses a size III or V Goldmann stimulus on a background luminance of 31.5 apostilbs (10 cd m–2). The intensity of the stimuli is given in decibels of attenuation from a fixed value of 10 000 apostilbs (this can vary from instrument to instrument). Thus, 0 dB corresponds to 10 000 abs, 10 dB to 1000 abs, and 20 dB to 100 abs.

Short-Wavelength Automated Perimetry

Patients with glaucoma often have an associated color vision defect in which their sensitivity to blue light is reduced. This observation led researchers to question whether or not the blue sensitive mechanism was more susceptible to glaucomatous damage and whether or not a perimeter that specifically targeted the blue mechanism might not be more sensitive than those that use the conventional SAP stimuli.

A problem encountered when trying to test the blue mechanism is the relatively low sensitivity of blue cones. Even at short wavelengths (440 nm) the blue sensitive cones are only marginally more sensitive than the red and green cones. Selective damage to the blue cones would have relatively little effect upon sensitivity as the red and green cones would simply step in once the blue cone sensitivity dropped below that of the other receptors. To overcome this problem, the red and green cones are desensitized by adapting the eye to a yellow light. Figure 2 shows the sensitivity of the three receptors after adaptation. The blue cones are exposed at 440 nm wavelength. The extent of exposure is important. If we consider that any damage to the blue cones mechanism is likely to lower its sensitivity we can see from Figure 2 just how much loss can be tolerated before the red and green receptors again become the most sensitive (approximately 1.5 log units).

There have been a number of studies that have demonstrated that blue-yellow defects precede those for SAP, however, the test has not been widely adopted because:

1.blue-yellow perimetry is particularly sensitive to lens opacities;

Figure 2 The log threshold sensitivity of the blue, green, and red cone systems after adaptation with a yellow light that desensitizes the red and green cones. Under these conditions the blue cones can be tested using 440 nm wavelength test light. Note that even after adaption, blue cones are approximately 1.5 log units less sensitive than green and red cones.

2.short-wavelength automated perimetry is more variable than SAP;

3.patients find the test difficult; and

4.most patients seen in a glaucoma clinic either have no visual field loss or have defects that can be detected with SAP.

High-Pass Resolution Perimetry (Ring

Perimetry)

High-pass resolution perimetry uses ring-shaped targets of varying size (see Figure 3). The luminance inside each ring target is the same as the background while the core of the ring is brighter and its inner and outer edges darker. The overall intensity profile of the ring is such that when it cannot be resolved it cannot be detected. Patients press a response key when they see the stimulus (presentation time 165 ms) and a repetitive bracketing strategy is used to establish the minimum resolvable ring size.

High-pass resolution perimetry has good sensitivity and specificity when compared to SAP. It has also been shown to have threshold variability that is independent of sensitivity and can detect progressive loss earlier than conventional perimetry. High-pass resolution perimetry is also fairly fast taking on average only 5.5 min to test 50 locations.

On the negative side, the technique is sensitive to blur, either refractive or due to media changes, unable to measure defect depth within small circumscribed lesions, and is unable to detect scotomata whose size is less than the local liminal test target. With the current monitor technology, there is also a relatively small dynamic range of stimuli which limits the test ability to monitor loss in patients with significant loss.