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OCT Imaging for the Rest of Us

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OCT Imaging for the Rest of Us TODAY’S PRACTICE BUSINESS PRACTICE OCT Imaging for the Rest of Us OCT has many uses beyond imaging the macula, including following glaucoma patients and evaluating the anterior segment. BY ROBERT BRASS, MD ptical coherence tomography (OCT) is rapidly becoming a must-have technology for all ophthalmology practices, not Oonly those that focus on retinal pathology. OCT has progressed from fuzzy, grainy images of the macula to near histologic quality images of ocular structures. With the development of Fourier-domain OCT, which utilizes a faster scanning rate than time-domain OCT systems, ocular tissue can be examined as never before. In addi- tion to better images of the macula, some OCT technologies can be used to manage glaucoma and to image the cornea. OCT is a noncontact imaging technology that obtains images by measuring the time delay of reflected near-infrared light. Each individual axial reflection is combined over a transverse dimension, yielding the image of the structure. Similarly, information can be obtained about the optic nerve head and ganglion cells for evaluating glaucoma or about the macula for evaluating macular Figure 1. Color-coded thickness maps of the ganglion cell complex (GCC). disease. Some OCT devices also have anterior Significant thinning seen in left eye (OS). segment imaging capabilities, enabling them to image the cornea, the crystalline lens, and other low the health of the optic nerve. Our clinical evaluation anterior structures. of the optic nerve is ultimately limited by what we can This article provides a brief overview of new ways in view with the slit lamp. We determine optic nerve health which OCT can benefit general ophthalmologists and by following trends such as optic nerve cupping, stan- anterior segment subspecialists. dard automated perimetry or visual fields, intraocular pressure (IOP), and corneal thickness. GLAUCOMA EVALUATION The optic nerve head can be monitored for cupping AND MANAGEMENT with optic nerve head drawings, but there can be drastic Perhaps the single most valuable use of OCT in a non- variability from observer to observer.1 Optic nerve head retinal practice is in the area of glaucoma evaluation and photographs are useful but can be limited in their ability management. Glaucoma management requires a combi- to pick up subtle details. Glaucoma patients are typically nation of clinical evaluation and objective testing to fol- older and are not always able to accurately perform the MAY 2011 CATARACT & REFRACTIVE SURGERY TODAY EUROPE 77 TODAY’S PRACTICE BUSINESS PRACTICE them at the slit lamp (Figure 1). Changes in the RNFL and ONHT precede visual field loss. OCT can provide earlier detection of RNFL and OHNT changes, which should ultimately help to decrease visual field loss and provide better glaucoma management. ANTERIOR SEGMENT AND CORNEAL IMAGING Anterior segment and corneal imaging using OCT has obvious applications in evaluating corneal opacities, LASIK flaps, Descemet stripping endothelial keratoplasty but- tons, and in measuring the anterior chamber angle. Other uses include evaluation of corneas for keratoconus and measurement of corneas for true corneal power after LASIK to aid in post-LASIK IOL calculations. OCT devices with anterior-segment imaging capabili- ties can be used to detect abnormal corneal thinning in Figure 2. The RTVue detects abnormal corneal thinning. keratoconus (Figure 2).7 Keratoconus evaluation can be performed by comparing thickness relationships current gold standard visual field test. Nothing can be between the superonasal (SN) octant and the inferotem- more frustrating than trying to make long-term clinical poral (IT) octant, and between the superior (S) and infe- decisions in glaucoma management with visual field data rior (I) octant, and by identifying the thinnest corneal that have large numbers of fixation losses and false posi- thickness and the magnitude of corneal thickness differ- tives and negatives. We then must base the patient’s IOP ences. Using these diagnostic parameters and suggested target on these potentially limited clinical findings of cutoff values (Table 1), the results of a corneal scan can optic nerve health and visual fields. be used to determine the likelihood of keratoconus. The RTVue (Optovue Inc., Fremont, California), an As shown in Table 2, the SN-IT value (124 µm) is greater ultrahigh-speed, high-resolution OCT scanner, enhances than the cutoff (52 µm), which is consistent with kerato- ophthalmologists’ ability to care for the eye. With an conus. The S-I value (210 µm) is much greater than the min- axial resolution of approximately 5 µm and producing imum thickness value (54 µm). The minimum corneal thick- 26,000 A-scans per second, the RTVue offers better reso- ness (336 µm) is less than the cutoff value (464 µm), and the lution than other OCT instruments. minimum-median value (-137 µm) is less than the cutoff OCT with the RTVue adds a new set of data to help diag- value (-33 µm). The location on the y axis of the thinnest nose and manage glaucoma. With this instrument, we can point of the cornea is 1.47 mm, which is less than the -1.1 quickly and accurately measure optic nerve head thickness mm cutoff value. This cornea is abnormal in five of five (ONHT) at all points circumferentially and the peripapillary parameters, a condition highly suggestive of keratoconus. retinal nerve fiber layer (RNFL). Because the sensitivity and A growing problem facing ophthalmologists is calculat- specificity of these measurements are independent of the ing IOL powers for cataract surgery in post-LASIK patients. patient’s ability to respond to standard automated perime- Many formulas are used to estimate corneal power in try testing, we have a more standardized way to document these patients, but using anterior-segment OCT to meas- the health of the optic nerve and aid in choosing a desired ure corneal pachymetry and curvature, true corneal power IOP. This modality is also less affected by cataract. can be calculated. Using a vergence formula, the corneal Reduced macular thickness in glaucoma has been power as measured by OCT can be used to accurately pre- reported by Ziemer et al.2 Measuring the inner RNFL, TAKE-HOME MESSAGE the ganglion cell layer, and the inner plexiform layer, referred to collectively as the ganglion cell complex • In glaucoma management and evaluation, the RTVue can (GCC), has been shown to increase diagnostic accura- measure optic nerve head thickness at all points cy for glaucoma.3,4 RNFL measurements combined circumferentially and the peripapillary retinal nerve fiber layer. with changes in the ONHT provide more data to fol- 5,6 • OCT can be used to evaluate corneal opacities, LASIK flaps, low glaucoma progression. OCT can provide infor- Descemet stripping endothelial keratoplasty buttons, and to mation about changes in the health of the optic nerve measure the anterior chamber angle. and the GCC earlier than we as clinicians can detect 78 CATARACT & REFRACTIVE SURGERY TODAY EUROPE MAY 2011 TODAY’S PRACTICE BUSINESS PRACTICE TABLE 1. SUGGESTED KERATOCONUS CUTOFF VALUES Parameter Keratoconus Cutoff (1 percentile) SN-IT >52 µm S-I >54 µm Min <464 µm Min-Med <-33 µm Y Min Y<-1.1 mm TABLE 2. KERATOCONUS ANALYSIS FOR EYE IN FIGURE 2 2–5 mm 5–6 mm Zonal/Hemispheric Analysis SN-IT 124 µm 116 µm S-I 210 µm 118 µm ST-IN 141 µm 67 µm Superior-Inferior 129 µm 81 µm Min=336 µm, Min-Median=-137 µm, Min-Max=-0.252 µm within 5-mm diameter circle Minimum thickness: 336 µm; location at (0.083 mm, -1.473 mm) Corneal power: 61.56 D, standard deviation (SD)=0.19 Anterior power: 72.48 D, SD=0.13, curvature radius=5.19 mm Posterior power: -11.17 D, SD=0.09, curvature radius=3.58 mm dict the IOL power that should be used in post-LASIK The ability to image the cornea with OCT provides one eyes. This is a promising technique that is currently being more technique to diagnose and manage corneal disease. studied and has shown great accuracy.8 Measuring true corneal power will better allow surgeons to predict accurate IOL powers in post-LASIK patients CONCLUSION before cataract surgery. ■ OCT has always been a great way to image the mac- ula, and it continues to be a useful technology to Robert Brass, MD, is the lead physician and quickly and easily evaluate patients for retinal thicken- surgeon at the Brass Eye Center, Latham, New ing related to an epiretinal membrane, central serous York, and an Associate Clinical Professor at retinopathy, cystoid macular edema, or diabetes. But Albany Medical College, Albany, New York. Dr. now OCT is a technology that can also significantly Brass states that he has no financial interest in benefit anterior segment surgeons and general oph- the products or companies mentioned. He may be reached thalmologists. at e-mail: [email protected]. For patients requiring glaucoma evaluation and management, I typically perform one OCT per year 1.Cox ML,et al.Inter-observer variability in a computer-assisted optic nerve head assessment system.Ophthalmic Physiol Opt.1992;12(1):69-71. and one visual field per year, each separated by 6 2.Zeimer R,Asrani S,Zou S,Quigley H,Jampel H.Quantitative detection of glaucomatous damage at the posterior months. Once patients experience the passive OCT pole by retinal thickness mapping:a pilot study.Ophthalmology.1998;105(2):224-231. scan, many complain about the rigorous nature of 3.Ishikawa H,et al.Macular segmentation with optical coherence tomography.Invest Ophthalmol Vis Sci. 2005;46(6):2012-2017. visual field testing. With straightforward training of 4.Tan O,Li G,Lu AT,Varma R,Huang D.Mapping of macular substructures with optical coherence tomography for technicians, the OCT scans can be obtained quickly glaucoma diagnosis.Ophthalmology.2008;115(6):949-956.
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