Ординатура / Офтальмология / Английские материалы / Diabetes and Ocular Disease Past, Present, and Future Therapies 2nd edition_Scott, Flynn, Smiddy_2009

134 Diabetes and Ocular Disease

Figure 7.12. Ultrasonography reveals a broad vitreoretinal interface causing a “table-top” tractional retinal detachment.

on kinetic testing. In contrast, table-top detachments exhibit a broader area of vitreoretinal adherence. Ultrasonographically, the detachment is seen as a highly reflective membrane with an adherent posterior hyaloid (Fig. 7.12). The decision to intervene surgically is often based on the location and progression of the detachment as seen on ultrasound.

At times, it may be difficult to distinguish between vitreous hemorrhage, PVD, fibrovascular membrane, and retinal detachment on ultrasound. Furthermore, subretinal blood may simulate a shallow retinal detachment. Several modalities may be used to differentiate these entities. The A-scan spike is higher with a retinal detachment, often a 100% signal, as compared to vitreous detachment. Similarly, on B-Scan, the signal from a retinal detachment generally persists with lower gain settings. Furthermore, the reflectivity of a PVD will decrease in the periphery, whereas a retinal detachment will retain its high reflectivity in the periphery. A retinal detachment always remains attached at the optic nerve (Fig. 7.13). Therefore, a signal that does not insert on the nerve represents a PVD. Finally, kinetic B-scan can be helpful in discriminating between retina, vitreous, and blood.

CONCLUSIONS

Fundus photography has an important role in monitoring progression of diabetic retinopathy. It is also becoming a popular method of screening for diabetic retinopathy in the community setting. Fluorescein angiography is useful in identifying areas of nonperfusion, increased vascular permeability, and neovascularization. It has an essential role in guiding laser treatment of CSME. Ultrasound is useful when cataract or vitreous hemorrhage obscures visualization of the posterior segment. It is important for identifying fibrovascular proliferation, vitreous hemorrhage,

Figure 7.13. Ultrasound demonstrates a total retinal detachment inserting on the optic nerve.

and retinal detachments. Timing of surgical intervention frequently depends on findings from ultrasound examination.

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Optical coherence tomography (OCT) has emerged as an important diagnostic adjunct and management tool for ophthalmologists managing patients with diabetic retinopathy [1–4]. In addition to medical history, laboratory

review, and clinical examination (stereoscopic slit-lamp biomicroscopy of the anterior and posterior segment), ancillary tests such as fundus photography, fluorescein angiography, and OCT has provided the ophthalmologist with a new appreciation for the dynamics of vision loss in eyes with a spectrum of diabetic retinopathy (Table 8.1) [4,5]. Although OCT was not available for the Diabetic Retinopathy Study (DRS) and the Early Treatment Diabetic Retinopathy Study (ETDRS) [6–10], virtually every new and ongoing clinical trial (e.g., Diabetic Retinopathy Clinical Research network, unpublished data) includes OCT as an important secondary outcome variable [11–22]. This chapter focuses on clinical examples that demonstrate the utility of OCT in the evaluation and management of patients with diabetic retinopathy and diabetic macular edema (DME) [23–30]. The following examples (Figs. 8.1–8.15) illustrate how both the qualitative evaluation and quantitative assessment provided by OCT aids in the diagnosis and management of various vitreoretinal abnormalities in eyes with diabetic retinopathy.

BACKGROUND

Although OCT has been commercially available since 1996, its popularity among ophthalmologists and retina specialists flourished with the introduction of the most recent version of the technology known as Stratus OCT, colloquially referred to as OCT-3 (Carl Zeiss Meditec, Dublin, CA) [11–23,31–33]. A detailed explanation of OCT technology, the various OCT image acquisition sequences, and an extended discussion of OCT image creation are beyond the scope of this chapter. A more comprehensive evaluation of the technology is reviewed in several excellent sources [34–37].

Briefly, the OCT unit utilizes a noncontact transpupillary infrared laser to illuminate the retina with multiple axial scans in a rapid fashion and a detector to capture the reflected light [34,37]. The basis for OCT is the Michelson interferometer that measures the phase difference between the reflected light from the retinal tissue as compared to an internal reference beam [34,37]. This difference is plotted as a false-color image of individual axial scans of the retina, with pixels of varying colors corresponding to areas of differential light reflectivity within the retina.

Table 8.1. Application of OCT in the Management

of Diabetic Retinopathy

The OCT unit has image interpolation software that allows it to identify the inner and outer retinal boundaries. The inner retinal boundary is identified by the OCT unit as the interface between the typically low reflectance of the vitreous cavity with the high reflectance associated with the internal limiting membrane and nerve fiber layer. The outer retinal boundary is identified as the so-called highly-reflective external band, consisting of the retinal pigment epithelium (RPE), Bruch’s membrane, and the choriocapillaris. The highly reflective external band is often incorrectly labeled the RPE, but this discrimination of the RPE from Bruch’s membrane and the choriocapillaris is beyond the resolution capabilities of the current commercially available OCT technology. Once the inner and outer retinal boundaries are identified, a simple arithmetical calculation allows determination of the retinal thickness along the scan length. Several of these individual axial scans from various vantages can be combined to create an interpolated en face image of the macula, with an appearance much like a topographical map (Fig. 8.1A) with color coding or number labels indicating relative and absolute average retinal thicknesses for each region of the macula, respectively. Macular volume can be calculated in a similar manner [34,37].

RADIAL LINES SCAN

The most common scan type for the Stratus OCT is the radial lines scan (Fig. 8.1A). With the patient fixating, six consecutive 6 mm scans are obtained from various directions. All six scans have their center on the fovea. The scans are: inferior to superior (one vertical scan at 90°), inferotemporal to superonasal (two separate oblique scans, one at 60° and one at 30°), temporal to nasal (one horizontal scan at 0°), and superotemporal to inferonasal (two oblique scans, one at 330° and one at 300°) [37].

By the machine operator moving the fixation target, it is possible to evaluate extrafoveal regions with the radial lines scan. This is helpful, for example, when trying to characterize a traction retinal detachment along the temporal vascular arcades or when evaluating peripapillary traction. In the latter case, a radial line can be obtained by sweeping the scan horizontally from the temporal peripapillary retina, over the optic disc, and then to the nasal peripapillary retina. This type of scan helps determine the status of the posterior hyaloid in relation to the optic nerve and helps determine if abnormally firm vitreopapillary traction is present in

pattern with hard exudates and numerous dot and blot hemorrhages throughout the posterior pole. The macular thickness map shows a mirror image of the right eye, with a sector of massively thickened retina involving the foveal center. Central foveal thickness measured 502 microns. The vertically oriented radial OCT scan (top row, right) shows a faint surface membrane, microcystic retinal thickening, scattered highly reflective intraretinal foci consistent with hard exudates, and a shallow foveal detachment. This eye also received PRP and multiple intravitreal triamcinolone acetonide injections with interval and sustained benefit over a 2.5-year period. After the 6th intravitreal triamcinolone acetonide injection, the color fundus photograph shows marked reduction in macular edema (bottom row, left), the contour map has flattened and reveals a central foveal thickness of 213 microns (bottom row, center), and nearly normal appearing foveal contour is shown on the vertically oriented radial OCT scan (bottom row, right). As a result, the VA improved to 20/40.