Ординатура / Офтальмология / Английские материалы / Handbook of Optical Coherence Tomography_Bouma, Tearney_2002

17.2OPTICAL COHERENCE TOMOGRAPHY OF THE NORMAL POSTERIOR SEGMENT

One of the most attractive features of optical coherence tomography is the relative ease and rapidity with which the scans are performed and interpreted. The recognition of pathology on OCT images requires familiarity with the OCT representation of the normal posterior segment. Figure 1a (see color plate) is an OCT image of a normal retina through the optic nerve and fovea. The posterior boundary of the neurosensory retina is well delineated by a highly reflective red layer corresponding to the retinal pigment epithelium and the choriocapillaris. The outer segments of the photoreceptors are represented by a dark layer of minimal reflectivity just anterior to the highly reflective band of the RPE and choriocapillaris. The inner margin of the retina, the nerve fiber layer, is represented by a red band due to bright backscatter that is easily seen in contrast to the nonreflective vitreous. Intervening structures between the highly reflective red bands of the RPE and nerve fiber layer are represented as alternating layers of moderate and low reflectivity. This is due to the stratified structure of the retina with moderate backscatter from the fibrous inner and outer plexiform layers that are oriented perpendicular to the incident beam [1,2]. The nuclear layers have cell bodies that are oriented parallel to the incident beam and therefore have minimal backscatter and are represented by dark bands [1,2]. The retinal blood vessels are identified by their shadowing of deeper structures.

Longitudinal surveillance of patients with macular disease is accomplished by obtaining six radial scans centered at the fovea [3]. Retinal thickness is then computed for 600 macular locations and displayed as a false color topographic map (Fig. 1b). Retinal thickness is also reported as a numerical average in nine regions. Evaluation of the entire macular region is possible using the retinal thickness map. Tomograms of the optic nerve are obtained by scanning the nerve head around two radii of curvature (2.25 and 3.37 mm). The nerve fiber layer is then plotted schematically (Fig. 1c). The nerve fiber layer is thickest in the superior and inferior quadrants. Normal nerve fiber layer thickness is a mean of 148:6 m superiorly, 143:5 m inferiorly, 66:9 m temporally, and 117:2 m nasally [4].

17.3MACULAR DISEASE

Optical coherence tomography (OCT) has become a valuable tool for a number of macular diseases, particularly those involving the vitreoretinal interface. OCT has led us to a better understanding of the anatomical relationships and pathogenesis of macular holes and epiretinal membranes and has also proven useful in the management of patients with age-related macular degeneration. The high resolution imaging of OCT also allows accurate longitudinal monitoring of patients with diabetic macular edema and central serous chorioretinopathy.

17.3.1 Macular Holes

Traditionally, the diagnosis and staging of macular holes have been accomplished by using contact lens slitlamp biomicroscopy. There are a number of lesions such as partial thickness or lamellar holes, pseudoholes, and macular cysts that may be difficult to distinguish from full thickness macular holes. OCT facilitates the identification of macular holes and aids in the staging according to the Gass classification

of idiopathic macular holes: stage I, foveal detachment; Stage II, small full thickness hole; Stage III, fully developed, full thickness hole; and stage IV, fully developed hole with posterior vitreous detachment [5].

On OCT, a stage I macular hole appears as a decreased foveal depression with an abnormal, minimally reflective space beneath the neurosensory retina in the fovea (Fig. 2a) (see color plate). The foveolar detachment is consistent with Gass’ stage I hole. Vitreous fibrils may be demonstrated inserting obliquely onto the fovea. An OCT image of a stage II hole demonstrates a flask-shaped full thickness defect (Fig. 2b). The retinal defect in stage II holes is small, and eccentric stage II holes may have a flap of retina attached to their surface. Stage III holes demonstrate a larger full thickness defect without a retinal flap (Fig. 2c). OCT images of stage IV macular holes demonstrates a full thickness defect with complete separation of the vitreous (Fig. 2d). This is in contrast to stages I–III, where the vitreous is usually seen inserting onto the retinal surface. Partial thickness holes, pseudoholes, and retinal cysts are readily distinguished from macular holes by using OCT. All of these disorders lack the full thickness defect and fluid cuff with its classic flask-shaped appearance.

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Figure 2 (a) OCT image of a stage I macular hole with loss of the foveal depression but no full thickness defect. (b) OCT image of a stage II macular hole showing a small full thickness macular hole with an eccentric flap of retinal tissue. (c) OCT image of a stage III macular hole with a larger full thickness flask-shaped retinal defect. (d) OCT image of stage IV macular hole with posterior vitreous detachment. (See color plate.)

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Figure 2 (continued)

Optical coherence tomography is also valuable for the longitudinal documentation of the progression of macular holes and can aid in the timing of surgical intervention. Approximately 50% of stage I macular holes will spontaneously improve, whereas the majority of stage II holes will progress to stage III [6]. This progression or resolution is easily demonstrated on OCT images. Additionally, patients with an idiopathic macular hole in one eye may be at increased risk for developing a macular hole in the fellow eye, and OCT may be used to evaluate the vitreoretinal interface and identify impending macular holes in fellow eyes [7].

17.3.2 Central Serous Chorioretinopathy

Central serous chorioretinopathy (CSCR) is a common retinal disorder characterized by idiopathic detachments of the neurosensory retina in the macular region [8]. OCT has proven useful for multiple purposes in CSCR. Small or shallow neurosensory detachments may be difficult to see clinically. Because of micrometer-scale resolution, OCT is capable of detecting neurosensory detachments that are difficult to detect by slitlamp biomicroscopy (Fig. 3) (see color plate). The contrast in optical

Figure 3 OCT image of a patient with CSCR revealing a small collection of fluid seen as an optically empty area beneath the highly reflective red RPE band. (See color plate.)

reflectivity between the nonreflective serous fluid and the more highly reflective posterior boundary of the neurosensory retina allows detection of small amounts of subneurosensory fluid. OCT is also capable of imaging the same retinal area over time, making it a noninvasive method of monitoring the clinical course of patients longitudinally [9]. A frequent problem arises when CSCR is present in older patients or patients with drusen or pigmentary changes in the macula. OCT images may document the absence of subretinal neovascular complexes or detect abnormalities in the choriocapillaris and RPE and aid in distinguishing between subretinal neovascularization and CSCR.

17.3.3 Epiretinal Membrane

Epiretinal membrane is a preretinal proliferation of fibrocellular material that may occur in healthy eyes or eyes with a pathological condition such as inflammation or retinal breaks [10]. OCT is well suited for imaging epiretinal membranes regardless of their etiology and has been used to characterize epiretinal membranes secondary to trauma, inflammatory disease, proliferative disease, intraocular surgery, and idiopathic causes [11]. The appearance of epiretinal membranes on OCT images is variable, depending on how tightly adherent the membrane is to the retinal surface. Membranes that are tightly adherent to the retinal surface may appear as a contrast in reflectivity between the membrane and the surface of the retina. Some adherent membranes may show only a deepening of the foveal pit or the formation of a pseudohole (Fig. 4a) (see color plate). A tuft or edge of membrane may be seen in some cases. Occasionally, epiretinal membranes are clearly visible on OCT as reflective tissue anterior to the retinal surface (Fig. 4b). Membranes of this type are distinguished from the posterior hyaloid by the difference in thickness of the anterior reflective band, with epiretinal membranes having a thickener reflective band.

Measurements of retinal thickness related to epiretinal membranes have shown correlation with visual acuity [11]. Furthermore, OCT images may be useful in predicting membranes that may be difficult to treat surgically and therefore aid in the timing of surgical intervention.

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Figure 4 (a) OCT image of a pseudomacular hole with deepening of the foveal pit. Note that retinal tissue at the trough of the pseudohole. (b) OCT image of an epiretinal membrane showing skip areas of retinal adherence. (See color plate.)

17.3.4 Age-Related Macular Degeneration

Optical coherence tomographic images are useful in several aspects of age-related macular degeneration (ARMD). OCT has been used in nonexudative ARMD to characterize soft drusen and retinal pigment epithelial atrophy [12]. OCT has also been used to image detachments of the retinal pigment epithelium, detachments of the neurosensory retina, and subretinal neovascularization [12,13]. Soft drusen appear as focal elevations of the retinal pigment epithelial layer (Fig. 5a) (see color plate). Retinal pigment epithelial atrophy appears as enhanced backscatter from the choroid (Fig. 5b). In exudative ARMD, common pathologies such as serous pigment epithelial detachment (PED), hemorrhagic PED, fibrovascular PED, and neurosensory detachment have characteristic appearances. Serous PED presents as a focal elevation of the reflective RPE band over an optically empty clear space (Fig. 6a) (see color plate). The angle of elevation of the detachment is typically acute, possibly secondary to the tight adherence of the RPE cells to Bruch’s membrane at the edge of the detachment. Hemorrhagic PED can be distinguished by the presence of a reflective band beneath the RPE reflective layer corresponding to the sub-RPE blood (Fig. 6b). OCT images of fibrovascular PED show a moderately reflective layer throughout the sub-RPE space beneath the detachment (Fig. 6c).

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Figure 5 (a) Optical coherence tomographic image of soft drusen showing focal elevations of the RPE layer. (b) OCT image of RPE atrophy beneath the fovea. Note the increased reflectivity of the choroidal band in bright red. (See color plate.)

Optical coherence tomography may also be useful in identifying subretinal neovascular complexes. For this purpose, OCT is used in conjunction with fluorescein and indocyanine green angiography. Subretinal neovascularization may be represented as a fibrovascular pigment epithelial detachment or as a well-defined or poorly defined membrane. Membranes that are well defined on fluorescein angiography typically appear as fusiform or discoid thickening of the reflective band of the RPE/choriocapillaris that extends anteriorly, elevating the RPE (Fig. 7) (see color plate). Membranes that are poorly defined on fluorescein appear as diffuse areas of reflectivity beneath the RPE without discernible borders. A potential use for OCT is to localize choroidal neovascular membranes to either the sub-RPE space or the subretinal space. In such cases, OCT may be useful in identifying appropriate surgical candidates.

17.3.5 Macular Edema

Macular edema is a common complication of a number of retinal disorders such as diabetic retinopathy and retinal vascular disease as well as uveitis and intraocular surgery. Fluorescein angiography is the current gold standard for qualitative detection of leakage from retinal vessels. It does have several limitations: It is minimally

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Figure 6 (a) Optical coherence tomographic image of a serous pigment epithelium detachment (PED) showing the optically empty space beneath the RPE band. (b) OCT image of a hemorrhagic PED showing a reflective band beneath the retinal pigment epithelium. (c) OCT image of a fibrous PED showing moderate backscatter beneath the RPE, which is elevated. (See color plate.)

invasive with known morbidity, results are difficult to reproduce, quantitative comparison between subsequent angiograms is difficult, and the extent of leakage does not correlate with visual function [14]. OCT has the advantage of being a noninvasive, objective, reproducible, and quantitative method of measuring retinal thickness [15]. Furthermore, retinal thickness measurements through the central fovea have

Figure 7 Optical coherence tomographic image of a well-defined subretinal neovascular membrane protruding forward beneath the RPE. (See color plate.)

been shown to correlate with visual acuity [15]. This is clinically useful for a number of reasons. First, OCT images can display early retinal thickening before it is clinically evident to the average observer. Second, OCT images can be used to direct laser treatment of intraretinal thickening. Finally, OCT can be used to follow patients longitudinally either for worsening of edema or resolution of thickening after laser treatment.

Macular edema is characterized on OCT by retinal thickening. Intraretinal areas of decreased reflectivity secondary to fluid accumulation may be present (Fig. 8a) (see color plate). Cystoid macular edema is classified by round, optically clear regions within the neurosensory retina (Fig. 8b).

17.3.6 Vitreomacular Traction

Vitreomacular traction is characterized by persistent vitreous attachment in the center of the macular causing traction on the retina. This may cause a cystoid configuration and result in decreased vision. OCT images of vitreomacular traction show incomplete detachment of the vitreous with persistent attachment to the inner retina in the macula (Fig. 9) (see color plate). Varying degrees of retinal involvement can be present, ranging from loss of the normal foveal pit to intraretinal thickening with a cystoid appearance. Longitudinal examinations may provide information as to the appropriate timing of surgical intervention.

17.4PERIPHERAL RETINAL DISORDERS

The main clinical utility of optical coherence tomography in peripheral retinal disorders is the differentiation between retinal detachment and retinoschisis. These two entities may appear similar clinically. Optical coherence tomography is an objective and reproducible test capable of differentiating between these two disorders.

Retinoschisis is a splitting of the layers of the retina in the outer plexiform layer that has an appearance similar to retinal detachment. In most cases, these two entities are distinguishable clinically. In cases where the diagnosis is not clear on the basis of clinical appearance, ancillary tests such as perimetry, laser photocoagulation, or B- scan ultrasound may be helpful. OCT is a reliable and objective method of distinguish-