Chapter 11 |
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Assessing Diabetic Macular Edema |
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with Optical Coherence Tomography |
Sebastian Wolf
Core Messages
■Diabetic retinopathy (DR) and diabetic macular edema (DME) are common microvascular complications in patients with diabetes.
■DR and DME are the leading causes of blindness in the working population.
■Diagnosis of DME is based on slit-lamp biomicroscopy and stereo fundus photography.
■Optical coherence tomography (OCT) has been used primarily to analyze macular thickness in DME.
■Structural changes in DME can be assessed by OCT. These include retinal swelling, cystoid macular edema, and subretinal fluid.
■The spectral-domain high-resolution OCT images not only allow to analyze structural changes in DME, but also to analyze the integrity of single retinal layers.
■The presence and integrity of the external limiting membrane (ELM), the photoreceptor inner segment (IS), the outer segment (OS), and the retinal pigment epithelium (RPE) appears to be a good prognostic feature for visual improvement after treatment for DME.
11.1 Diabetic Macular Edema
Diabetes mellitus (DM) is the most common endocrine disease in developed countries. The prevalence ranges between 2 and 5% of the world’s population. Diabetic retinopathy (DR) and diabetic macular edema (DME) are common microvascular complications in patients with diabetes. They are the leading causes of blindness in the population aged 20–74 years and responsible for 12% of new cases of blindness each year [1]. As the prevalence of diabetes will double over the next 20 years, DR and DME will continue to cause substantial vision loss unless adequately treated [2].
11.2 Examinations in Diabetic Macular Edema
Diagnosis of DME is traditionally based on slit-lamp biomicroscopy and stereo fundus photography [3, 4]. Additionally, fluorescein angiography is used to evaluate patients with DME to evaluate the extent and origin of fluid leakage as well as the extent of capillary ischemia in
the macula [5]. However, these methods are relatively insensitive to determine changes in retinal thickness. More recently, optical coherence tomography (OCT) has been used to analyze DME [6–9]. OCT was introduced into the clinical routine during the past decade as a noninvasive means to assess the posterior pole [10–13]. In the past, OCT has been used primarily to analyze macular thickness in DME [8, 9]: Additionally, OCT in patients with DME has revealed several structural changes in the retina. These include retinal swelling, cystoid macular edema, and subretinal fluid [14, 15]. OCT is an evolving technology. Current advances, e.g., the introduction of Fourier analysis (spectral OCT), made both high resolution and fast scanning speed possible [16–18]. High resolution allows for differentiation of as much as 11 structural characteristics within the retina [19–21].
11.3 Treatment of Diabetic Macular Edema
Currently, laser photocoagulation is the only proven treatment of DME by large-scale studies. The Early Treatment
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11 Assessing Diabetic Macular Edema with Optical Coherence Tomography |
in Diabetic Retinopathy Study (ETDRS) showed that laser photocoagulation reduces the risk of moderate visual acuity loss by about 50% in DME [5]. However, visual acuity improved only in a small percentage of patients
11 and a significant number of eyes did not respond to laser photocoagulation. Other treatment modalities for DME include pars plana vitrectomy, intravitreal triamcinolone acetonide injection, and intravitreal injection of antiVEGF drugs [22–24]. Especially, intravitreal anti-VEGF treatment appears to be very promising, showing significant visual improvements in several small studies [25].
The response to various treatment modalities for DME is variable. Some patients show excellent visual improvement whereas others show only minimal response. Thus, there is a significant unmet need to develop diagnostic methods that allow identifying prognostic features for the visual outcome after treatment for DME.
11.4High-Resolution Optical Coherence Tomography in Diabetic Macular Edema
The Spectralis™ HRA + OCT combines high-resolution spectral-domain OCT with an SLO. The system allows for simultaneous OCT scans with high-resolution scanning laser retinal imaging. The instrument uses broadband 870 nm SLD for the OCT channel. The retina is scanned at 40,000 A-scans per second, creating highly detailed images of the structure of the retina. The OCT optical depth resolution is 7 µm, the digital depth resolution is 3.5 µm. The combination of highresolution scanning laser retinal images and spectraldomain OCT allows for real-time tracking of the eye movements and real-time averaging of scanning laser images and OCT scans, reducing speckle noise of the OCT images [26].
Fig. 11.1 Normal retina as imaged by Spectralis™ HRA + OCT. The ganglion cell layer GCL; the inner plexiform layer IPL; the inner nuclear layer INL; the outer plexiform layer OPL; outer nuclear layer ONL; external limiting membrane ELM; the photoreceptor inner segments IS; the outer segments OS; and the retinal pigment epithelium RPE are marked with arrows
Fig. 11.2 DME as imaged by Spectralis™ HRA + OCT. Note the epiretinal membrane ERM; subretinal fluid accumulation SRF; and intraretinal fluid accumulation IRF
11.4 High-Resolution Optical Coherence Tomography in Diabetic Macular Edema |
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The spectral OCT images allow analyzing the integrity of the retinal layers. In a normal retina imaged by Spectralis™ HRA + OCT, the ganglion cell layer (GCL), the inner plexiform layer (IPL), the inner nuclear layer (INL), the outer plexiform layer (OPL), outer nuclear layer (ONL), the external limiting membrane (ELM), the photoreceptor
inner segments (IS), the outer segments (OS), and the retinal pigment epithelium (RPE) can be seen (Fig. 11.1). In patients with DME, high-resolution OCT revealed various pathologic findings. These include epiretinal membranes, subretinal fluid, intraretinal fluid accumulation, and cystoid macular edema (Figs. 11.2 and 11.3).
Fig. 11.3 Cystoid DME as imaged by Spectralis™ HRA + OCT. Note the integrity of the outer retinal layers, external limiting membrane, the photoreceptor inner segments, the outer segments, and the retinal pigment epithelium
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Fig. 11.4 Patients with cystoid macular edema before (a) and 4 weeks after (b) treatment with intravitreal antiVFGF therapy (ranibizumab). Visual acuity improved from 20/60 to 20/30. Note the normal outer retinal layers at baseline. At the inner surface of the retina, an epiretinal membrane can be seen
Fig. 11.5 Patient with DME and severe cystoid changes. Note that the outer retinal layers such as the external limiting membrane and photoreceptor layers are severely disturbed. Visual acuity was unchanged in this patient after anti-VEGF therapy