Probing the Floor of the Optic Nerve Head in Glaucoma
2011; Elsevier BV; Volume: 119; Issue: 1 Linguagem: Inglês
10.1016/j.ophtha.2011.08.013
ISSN1549-4713
Autores Tópico(s)Retinal Diseases and Treatments
ResumoSpectral domain optical coherence tomography (SD-OCT) applied to the eye is rapidly expanding its scope of usefulness. In this issue of Ophthalmology, the respective authors of 2 different articles1Park H.-Y.L. Jeon S.H. Park C.K. Enhanced depth imaging detects lamina cribrosa thickness differences in normal tension glaucoma and primary open angle glaucoma.Ophthalmology. 2012; 119: 10-20Abstract Full Text Full Text PDF PubMed Scopus (233) Google Scholar, 2Park S.C. De Moraes C.G.V. Teng C. et al.Enhanced depth imaging optical coherence tomography of deep optic nerve complex structures in glaucoma.Ophthalmology. 2012; 119: 3-9Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar describe the use of “enhanced depth imaging” (EDI) to explore the optic disc, and in particular the lamina cribrosa, in the context of glaucoma. The lamina cribrosa is of special interest because the excavation of the optic nerve head, so characteristic of glaucoma in contrast to other optic atrophies, is related to the collapse and posterior bowing of the lamina cribrosa with widening of the scleral opening. The standard SD-OCT of the fundus gives images that are clearest at the innermost retinal layers. The intensity of the illuminating beam is attenuated by scattering and absorption as it passes through tissue, but the retina is practically transparent, so structures are well illuminated unless they are in a shadow–beneath a blood vessel, or deeper than the pigment epithelium. Several methods have been described to improve image information for the dimly illuminated locations,3Spaide R.F. Koizumi H. Pozzoni M.C. Enhanced depth imaging spectral-domain optical coherence tomography.Am J Ophthalmol. 2008; 146: 496-500Abstract Full Text Full Text PDF PubMed Scopus (1789) Google Scholar, 4Chang S. Flueraru C. Mao Y. Sherif S. Attenuation compensation for optical coherence tomography imaging.Optics Commun. 2009; 282: 4503-4507Crossref Scopus (11) Google Scholar, 5Girard M.J.A. Strouthidis N.G. Ethier C.R. Mari J.M. Shadow removal and contrast enhancement in optical coherence tomography images of the human optic nerve head.Invest Ophthalmol Vis Sci. 2011; 52: 7738-7748Crossref PubMed Scopus (228) Google Scholar but EDI3Spaide R.F. Koizumi H. Pozzoni M.C. Enhanced depth imaging spectral-domain optical coherence tomography.Am J Ophthalmol. 2008; 146: 496-500Abstract Full Text Full Text PDF PubMed Scopus (1789) Google Scholar is the first to be commercially available. Enhanced depth imaging makes use of the fact that the SD-OCT image is derived from an interference pattern created by differing path-lengths between the reference beam and the light returning from tissue structures at various depths, the path length being determined by the depth of the tissue structure that reflects the light back. The relevant technical feature for EDI is that there is a plane in the tissue for which the path-length is the same as that of the reference beam. The closer a structure is to this zero reference plane, whether behind or in front of it, the more easily the interference pattern of light reflected from it can be detected, and therefore the more visible the structure is. Thus, if the instrument is positioned to place this reference plane just in front of the retina (standard method), it is the inner retinal features that are most easily delineated. The trick in the EDI method is to place the reference plane just behind the structures of interest instead of just in front of the retina. Although the light intensity is still diminished in the deeper tissue, the interference pattern that is produced is more easily analyzed. The EDI method has been used to evaluate the choroid in glaucoma, with the finding that the choroid becomes thinner with age, but is seemingly not affected by glaucoma.6Mwanza J.-C. Hochberg J.T. Banitt M.R. et al.Lack of association between glaucoma and macular choroidal thickness measured with enhanced depth-imaging optical coherence tomography.Invest Ophthalmol Vis Sci. 2011; 52: 3430-3435Crossref PubMed Scopus (145) Google Scholar The reports in this issue of Ophthalmology turn our attention to the optic disc and deal with the use of EDI to evaluate the lamina cribrosa and other deep structures. In a previous study with standard SD-OCT,7Inoue R. Hangai M. Kotera Y. et al.Three-dimensional high-speed optical coherence tomography imaging of lamina cribrosa in glaucoma.Ophthalmology. 2009; 116 (214–2)Abstract Full Text Full Text PDF Scopus (129) Google Scholar the anterior portion of the lamina cribrosa was visible only in the cup, but not under the rim of neuroretinal tissue. In 42% of the eyes the posterior boundary of the lamina cribrosa could not be identified, even in the region of the cup, so thickness could be measured in only 58% of the eyes and for the most part only at the center. Park et al1Park H.-Y.L. Jeon S.H. Park C.K. Enhanced depth imaging detects lamina cribrosa thickness differences in normal tension glaucoma and primary open angle glaucoma.Ophthalmology. 2012; 119: 10-20Abstract Full Text Full Text PDF PubMed Scopus (233) Google Scholar of The Catholic University of Korea, made measurements at 3 locations along the vertical midline of the disc (in the cup), to avoid shadows caused by blood vessels and other overlying tissue. They report that among 137 eyes with glaucoma, the front surface of the lamina cribrosa could be seen in all, even with the standard mode. The posterior surface was adequately seen in only 66% with the standard mode, but in 93% with EDI. They also found a greater repeatability when measuring the lamina cribrosa thickness with EDI than in the standard mode (within observer, between observers, within a visit, and between visits). With regard to glaucoma, they found that the lamina cribrosa was thickest in healthy eyes, less thick in eyes with high-pressure glaucoma, and thinner yet in eyes with normal-tension glaucoma, particularly in those in which disc hemorrhages were seen. Of particular note is that the lamina cribrosa seemed to become thin while glaucoma was not yet evident by visual field testing, at least as judged by the mean deviation (MD) index, which had been noted before.7Inoue R. Hangai M. Kotera Y. et al.Three-dimensional high-speed optical coherence tomography imaging of lamina cribrosa in glaucoma.Ophthalmology. 2009; 116 (214–2)Abstract Full Text Full Text PDF Scopus (129) Google Scholar Park et al2Park S.C. De Moraes C.G.V. Teng C. et al.Enhanced depth imaging optical coherence tomography of deep optic nerve complex structures in glaucoma.Ophthalmology. 2012; 119: 3-9Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar of the New York Eye and Ear Infirmary, eliminated 26% of 188 EDI-mode images for reasons of quality. Of the remaining EDI images, the anterior lamina surface could be identified near the center in all, but at the circumference the details were often obscured in the shadow of sclera and nerve tissue, pigment of the retina or choroid, or blood vessels, with the edge visible at some location in only 65%. In 76% of the eyes, pores of the lamina cribrosa could be seen in regions of the disc, mainly centrally or temporally. They made note of other structures as well. The central retinal vessels could be seen in all eyes, and in 86% at least one short posterior ciliary artery was seen. In a minority, other details were observed, including the anterior termination of the subarachnoid space, a patch of absent lamina cribrosa, and an instance of a nonvascular cavity within the choroid. The authors thus illustrate new, but perhaps very infrequent, features that accompany glaucomatous disease. Of interest is not only the early thinning of the lamina cribrosa, but also the very early posterior migration of its insertion onto the sclera, sometimes taking up position as far back as the pia mater. This connective tissue remodeling was first noticed by histology in monkeys with experimental glaucoma,8Yang H. Williams G. Downs J.C. et al.Posterior (outward) migration of the lamina cribrosa and early cupping in monkey experimental glaucoma.Invest Ophthalmol Vis Sci. 2011; 52: 7109-7121Crossref PubMed Scopus (131) Google Scholar but has recently been documented by SD-OCT in humans as well with the EDI technique.9Park S.C. Kiumehr S. Dorairaj S. et al.In-vivo, 3-dimensional imaging of the lamina cribrosa horizontal central ridge in normals and lamina cribrosa deformation in glaucoma.Invest Ophthalmol Vis Sci. 2011; 52 ([ARVO Meeting E-Abstracts]): 3063Google Scholar Thus, EDI has not only enabled study of the choroid (and possibly sclera), but is beginning to open new windows to the depths of the optic nerve head. Already details are emerging about the collapse and thinning of the lamina cribrosa, as well as the posterior migration of its insertion into the sclera, sometimes as far back as the pia mater. These events seem to occur in the early stages of glaucomatous cupping. Although histological verification that structures are correctly identified would be valuable, and it is too soon to determine the temporal sequence with regard to axon loss, since the MD index is not the most sensitive indicator of early damage to axons. In addition, while EDI is a major step forward, the image of deeper structures is still imperfect. Additional enhancements such as the use of a swept source and longer wavelengths,10Srinivasan V.J. Adler D.C. Chen Y. et al.Ultrahigh-speed optical coherence tomography for three-dimensional and en face imaging of the retina and optic nerve head.Invest Ophthalmol Vis Sci. 2008; 49: 5103-5110Crossref PubMed Scopus (247) Google Scholar combined with image processing methods,4Chang S. Flueraru C. Mao Y. Sherif S. Attenuation compensation for optical coherence tomography imaging.Optics Commun. 2009; 282: 4503-4507Crossref Scopus (11) Google Scholar, 5Girard M.J.A. Strouthidis N.G. Ethier C.R. Mari J.M. Shadow removal and contrast enhancement in optical coherence tomography images of the human optic nerve head.Invest Ophthalmol Vis Sci. 2011; 52: 7738-7748Crossref PubMed Scopus (228) Google Scholar may need to be applied to refine the information further. The ultimate hope is that we not only come to understand the pathogenic process, but can use the information in making clinical evaluation and decisions. Enhanced Depth Imaging Detects Lamina Cribrosa Thickness Differences in Normal Tension Glaucoma and Primary Open-Angle GlaucomaOphthalmologyVol. 119Issue 1PreviewTo confirm the advantages of the enhanced depth imaging (EDI) mode over the standard mode of the Heidelberg Spectralis spectral domain optical coherence tomography (SD-OCT) for imaging of the lamina cribrosa, and to compare laminar thicknesses of various glaucoma types with or without disc hemorrhage in a similar state of visual field loss. Full-Text PDF Enhanced Depth Imaging Optical Coherence Tomography of Deep Optic Nerve Complex Structures in GlaucomaOphthalmologyVol. 119Issue 1PreviewTo assess the usefulness of enhanced depth imaging (EDI) optical coherence tomography (OCT) for evaluating deep structures of the optic nerve complex (ONC; optic nerve head and peripapillary structures) in glaucoma. Full-Text PDF
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