Get Familiar With SD-OCT

Spectral domain allows you to detect and monitor several retinal pathologies that you might not see ophthalmoscopically. Here’s how to effectively interpret SD-OCT imagery.

By Samantha Slotnick, O.D., and Jerome Sherman, O.D.

Goal Statement:

Consistent advancements in imaging technology have allowed us to better examine various ocular pathologies. The most recent advance in imaging, spectral-domain optical coherence tomography (SD-OCT), provides a level of detail that parallels—and even enhances—histological observation of retinal integrity. This article provides an extensive tutorial on how to interpret SD-OCT images of outer, middle and inner retinal layer pathologies.

Faculty/Editorial Board:

Samantha Slotnick, O.D., and Jerome Sherman, O.D.

Credit Statement:

This course is COPE approved for 2 hours of CE credit. COPE ID: 25588-PS. Please check your state licensing board to see if this approval counts towards your CE requirement for relicensure.

Joint-Sponsorship Statement:

This continuing education course is joint-sponsored by the Pennsylvania College of Optometry.

Disclosure Statement:

Dr. Slotnick has no relationships to disclose. Dr. Sherman lectures on behalf of Carl Zeiss Meditec.

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During the last 10 years, consistent advancements in imaging technology have allowed us to better examine various ocular pathologies. Several technologies, including optical coherence tomography (OCT), scanning laser ophthalmoscopy (SLO) and scanning laser polarimetry (SLP), have expanded our ability to detect, follow and diagnose retinal complications and glaucomatous presentations. Today, once our suspicions have been raised, we can employ such high-tech equipment to explore any presentation in detail and monitor subtle changes in both structure and function.

The most recent advance in imaging, spectral-domain optical coherence tomography (SD-OCT), provides a level of detail that parallels—and even enhances—histological observation of retinal integrity. Currently, several SD-OCT units are available from different manufacturers, including 3D OCT-1000 (Topcon), Cirrus HD-OCT (Carl Zeiss Meditec), Spectralis OCT (Heidelberg Engineering), RTVue-100 (OptoVue) and 3D SD-OCT (Bioptigen, Inc).

Even in the absence of clinical signs and/or symptoms, SD-OCT allows us to detect retinal pathologies that might not be seen ophthalmoscopically. Additionally, SD-OCT enables us to gain more detailed insight into the etiology of clinical symptoms and signs than we can detect ophthalmoscopically, but cannot accurately diagnose based upon clinical presentation alone. With this deeper understanding of the problem at hand, SDOCT might even present an opportunity for intervention before an emerging pathology becomes clinically significant.

However, to take full advantage of SD-OCT’s enhanced capabilities, we must be able to interpret the data it produces effectively. So, drawing upon five case reports, this article provides an extensive tutorial on how to interpret SD-OCT images of outer, middle and inner retinal layer pathologies.

SD-OCT of a Healthy Eye

Like conventional time-domain optical coherence tomography (TDOCT), SD-OCT shows a cross-section of the retina, which appears as a histological slice perpendicular to the fundus. OCT technology identifies changes in optical density and illustrates them in a color-coded format. When two adjacent structures demonstrate large differences in refractive index, a greater volume of light is reflected at their interface. Large reflections are depicted by warm colors (red through yellow) and mild reflections are depicted by cool colors (green through blue). An absence of reflection appears black. Images in grayscale utilize brighter shading in lieu of warmer colors.

OCT images do not, in fact, depict histological layers, but rather reflective interfaces between structures. So, the layers that appear in a normal SD-OCT image show differences in optical density––some of which occur within a single histo-logical cell layer. Cell bodies, for example, are far denser than the dendritic aspects of the cell, and photoreceptor outer segments are optically denser than photoreceptor inner segments.

• Figure 1 depicts an SD-OCT slice through a healthy fovea. The well-organized structures of the retina form distinct layers. There is a marked change in reflectivity between the vitreous and the internal limiting membrane/retinal nerve fiber layer, depicting the vitreoretinal interface. Proceeding from inner to outer retina, the ganglion cell bodies, the inner plexiform layer and the outer plexiform layer show heightened zones of reflectivity. Continuing outward, a relatively dark area depicts a lack of change in optical density as light passes through the tightly packed outer nuclear layer.

A fine, but distinct, boundary appears next. This boundary demarcates the adjacent inner and outer segments of the photoreceptors. The photoreceptor layer is extremely well organized, demonstrating both horizontal alignment between adjacent photoreceptor cells and vertical alignment with an orientation perpendicular to the fundus. This nonhistological boundary is referred to as the photoreceptor integrity line (PIL), and it serves as the junction between the inner and outer segments of the photoreceptors. On SD-OCT, the PIL is present in virtually all healthy eyes.

The next distinct boundary occurs at the interface between the outer segments of the photoreceptors and the retinal pigment epithelium (RPE).

Finally, the choroid is the most external layer, which also demonstrates areas of dense reflectivity.

Figure 1 depicts the foveal pit while simultaneously demonstrating intact and continuous seeing structures of the fovea. Here, the PIL provides evidence of an intact photoreceptor layer. The dark zone beneath the foveal pit (between the PIL and RPE boundaries) is normal––not a cyst. This area represents the extent of the photoreceptors’ outer segments, which are elongated at the fovea.

• Figure 2 shows another healthy patient with intact cell layers. In this grayscale scan, a band of highly reflective intensity is observed at the continuous PIL, above the RPE. The fundus image shows the location of the scan (superior to the macula), which transects a large blood vessel. The dim area observed on the line scan at the cursor’s location is not a retinal defect, but a result of the blood vessel attenuating the amount of light reaching the underlying anatomy. It is important to recognize that this attenuation occurs evenly to all layers below the level of the blood vessel. The PIL is faint but present, as is the RPE.

A closer look reveals several small areas of apparent signal dropout along the length of the line scan. The accompanying fundus photo reveals that this slice was made through several arterioles much smaller than the branch retinal artery, and their presence is also depicted as vertical linear shadows with equal signal attenuation below.

As we proceed through the following cases, keep in mind the ways that the SD-OCT images have been formed in addition to retinal anatomy. It is critical to understand how the technology works to effectively interpret the imagery.

Case 1: Outer Retinal Pathology

• History and diagnostic data. A 78-year-old male presented with no ocular complaints; he came in only because he had broken his glasses. His best-corrected visual acuity measured 20/30 O.U. On fundus examination, we identified some pigment mottling O.D. and two subtle areas of geographic atrophy of the RPE, both superior and inferior to the fovea O.S.

How does a patient with geographic atrophy maintain 20/30 visual acuity? What is the effect of geographic atrophy on retinal integrity? Where is the disruption to the visual pathway? Is this disruption reversible, now or in the future? Close inspection of figure 3 provides answers to these particular questions.

• Discussion. The area indicated centrally (at the red arrow), beneath the foveal pit, shows an intact PIL. This patient was fortunate because the central fovea was not impacted, preserving visual acuity in the left eye.

The PIL and RPE are interrupted both superior and inferior to the fovea, coincident with the zones of geographic atrophy seen in the fundus photo. Both the PIL and RPE re-emerge as a unit in the inferior aspect of the scan.

SD-OCT clarifies the physiological effect of dry macular degeneration, which often progresses to geographic atrophy.¹ This process causes deterioration of the RPE, which results in apoptosis of the overlying photoreceptors.

Take a moment to recall the effect of blood vessels in the normal eye (figure 2)–– additional light is absorbed by the blood vessel, resulting in the attenuation of light transmitted to (and reflected from) the areas below the vessel. In figure 3, however, note that the areas below the geographic atrophy show a much more detailed appreciation of the choroid, including visualization of blood vessel cross-sections in this underlying space. Why are the blood vessels apparent here, but not underneath the areas where the PIL and RPE are still intact? Where these two highly reflective boundaries (the PIL and RPE) are missing, more light is transmitted to the structures below. Accordingly, both Bruch’s membrane and the choroid are more clearly visualized.

What can we appreciate about the integrity of the retina’s inner layers? SD-OCT shows that the overlying portions maintain their structure and organization. If the unaffected components of the visual pathway are intact (without ischemia, there is no present cause for cell death), then the potential to transfer visual information to the brain remains.

The potential for sight exists if technological advancements provide a method for light detection and signal transfer to the visual pathway with retinal implantation above the areas of geographic atrophy. Currently, this sort of cutting-edge technology is under development, and it is important to understand in which cases the potential for sight remains.

Case 2: Outer Retinal Pathology

• History and diagnostic data. A 16-year-old white female presented after she failed a high school eye screening.² She reported blurred vision, but claimed that her acuity had been stable during the last several years. Clinical evaluation revealed a small refractive error with unreliable responses on subjective testing. Her best-corrected visual acuity was 20/60 O.D. and 20/70 O.S. The patient correctly identified 2/8 Ishihara color plates. No nystagmus was observed. Fundus examination revealed a very subtle macular lesion O.U. Carefully evaluate figure 4 before reading on.

• Discussion. SD-OCT reveals a distinct gap beneath the foveal pit. The PIL is absent in an area that corresponds to the central 5° of the visual field. A lack of a PIL explains the reduced visual acuity; but, what etiology explains the lack of a PIL?

Recall the patient’s clinical profile––she is a 16-year-old female who performed poorly on color vision testing. Her best visual acuity measures about 20/60. The gap in the SD-OCT is limited to the central 5°. Recall that the cones are concentrated at the fovea and that the rods predominate outside of the central 5°. The most likely diagnosis is incomplete rod monochromatism. There is some evidence of cone preservation, which explains why her vision is relatively good.

Additional evidence on SD-OCT confirms a lack of cones. Note that there is an area of increased reflectance above the gap in the PIL, and that this interface is not observed in the adjacent areas where the PIL is present. Where the cones are absent, there is a distinct lack of cells beyond the outer limiting membrane (likely composed of Mueller cells). The increase in reflectivity represents the transition between the presence and absence of cells beyond the outer limiting membrane. The next area of reflectivity occurs at the interface between this gap in the photoreceptors and the RPE.

Careful inspection of the SDOCT image reveals the presence of some cones in this gap area, which is consistent with limited color detection and a moderately reduced visual acuity level.

Case 3: Middle Retinal Pathology

• History and diagnostic data. A 75-year-old Hispanic male presented with blurred vision O.D. that had persisted for several months. His general health was unremarkable. Clinical evaluation revealed 4.00D of hyperopia O.U., with a best-corrected visual acuity of 20/40 O.D. and 20/25- O.S.

On fundus examination, we noted a slight elevation of the macular area O.D. We performed SDOCT imaging to investigate the nature of the probable macular elevation O.D. Figure 5 shows both the fundus and the SD-OCT cross-section taken through the macula and the optic nerve head. Evaluate this image before reading on.

• Discussion. There are several common differential diagnoses for retinal elevation near the macula, including intraretinal cystoid macular edema, central serous chorioretinopathy, retinoschisis and retinal detachment.^3,4 This SD-OCT image identifies several locations of intraretinal insult. We can see multiple retinal splits that extend from the optic disc to the macula O.D. with cystoid spaces occurring between different retinal layers.

Now, assess the area below the foveal depression. What is the potential for vision here? Note that the PIL remains intact, although less organized in its present location. The thin layer visualized above the PIL is the outer limiting membrane, and this too remains continuous temporal to the macula and at the fovea. At the macula, the retinal separation occurs between the RPE and the PIL. The retinal elevation may induce some distortion in addition to a hyperopic refractive shift, but the acuity is only mildly reduced to 20/40 O.D., which is consistent with the observation of a relatively intact PIL. A separation that occurs between the RPE and the photoreceptors is a neurosensory retinal detachment.

Next, observe the cystoid areas between the macula and the optic nerve head within the layers of the middle retina. The presentation seen here is a retinoschisis.⁵ Retinoschisis may occur when multiple cystoid spaces within the retinal layers coalesce and divide the retina into an inner layer and an outer layer. Alternatively, the cystoid spaces may be secondary to longstanding retinal splits.

A well-known etiology of retinoschisis that occurs adjacent to the optic nerve head is the presence of an optic pit. Optic pits (a form of coloboma) create a communication for fluid between the optic cup and retinal stroma. However, a review of all 128 sections scanned around this patient’s optic disc failed to reveal an optic pit. Subsequently, we performed a fluorescein angiogram, which did not reveal any leakage between the optic disc and the area of the retinoschisis, ruling out the presence of an optic pit. Therefore, we considered this presentation to be an idiopathic retinoschisis. Whether intravitreal anti-VEGF (vascular endothelial growth factor) treatment or intravitreal gas injections to flatten the macula would be of any value in this case is unclear.

Case 4: Inner Retinal Pathology

• History and diagnostic data. A 73-year-old white male presented for a routine eye exam with no complaints. He was pseudophakic, and his best-corrected visual acuity was 20/25 O.U. Fundus examination revealed a probable epiretinal membrane at the macula (EMM)

O.D. We performed SD-OCT imaging to better visualize the EMM. Before reading on, examine figure 6, which demonstrates both the fundus appearance O.D. and the location of the SD-OCT section.

• Discussion. Upon evaluation of the SD-OCT image, a few details appear noteworthy. First, as this section shows, the foveal pit was not observed. (None of the sections through the macula revealed a foveal pit.) The foveal pit appears to be compromised because of traction created by the EMM.

The epiretinal membrane itself is depicted on the SD-OCT scan as a layer of increased reflectance that overlies the center of the macula. Centrally (beneath the EMM), the macula appears distinctly thicker than it does in the adjacent areas, temporal and nasal to the membrane. In fact, it is possible to visualize an upsweep in the trajectory of the middle retinal layers, which point toward the membrane above.

Below the location of the EMM, the PIL appears to be intact, explaining the nearly normal acuity. The minor visual acuity reduction is likely attributed to the epiretinal membrane.⁶ This patient should be monitored for changes in the EMM. An Amsler grid may be used to track progression at home.

Case 5: Inner Retinal Pathology

• History and diagnostic data. An 80-year-old black male presented for a routine eye exam. He has had a history of type II diabetes for about seven years, but has not demonstrated diabetic retinopathy to date. The patient’s best-corrected visual acuity measured 20/30 O.U.

Fundus examination revealed a subtle, round lesion at the macula O.D. We performed an SD-OCT to better assess the nature of the lesion detected ophthalmoscopically. Before continuing, examine figure 7.

• Discussion. Evaluation of the SD-OCT images reveals a surface of reflectance above the level of the retina. This additional surface adheres to the retina at the fovea. The retinal nerve fiber layer is clearly visualized below this additional layer, confirming that the additional layer is internal to the retina.

What creates this additional reflection? This is the posterior surface of the vitreous body, which has begun to separate from the retina, yet remains connected to the retina via its macular adhesion. The distortion of the foveal pit at the location of this adhesion suggests that this presentation is the result of vitreomacular traction. The subtle, round lesion at the macula was created by a concentration of the vitreous at its central macular adhesion. This is better seen on the 3-D SDOCT image. It is rather challenging to appreciate vitreomacular traction ophthalmoscopically.

The 2-D image reveals that the PIL is mostly intact through the scan. A small interruption in the PIL can be seen nasal to the fovea.

The 3-D image reveals the extent of posterior hyaloid and the remaining traction of the macula. In most cases, vitreomacular traction subsides spontaneously as the posterior vitreal detachment becomes complete. More infrequently, the traction results in a macular hole.

As these five cases show, SD-OCT is able to illustrate retinal anatomy in better detail than clinical observation alone. Whether exploring unsubstantiated clinical symptoms or investigating a subtle lesion detected ophthalmoscopically, SD-OCT provides a new dimension to our current clinical capacity. And ultimately, the more familiar we become with SD-OCT, the better we can care for our patients.

Dr. Slotnick is a behavioral optometrist who practices in Dobbs Ferry and Mahopac, N.Y. Dr. Sherman is a distinguished teaching professor at State University of New York College of Optometry and the Schnurmacher Institute of Vision Research. He also practices at The Eye Institute and Laser Center, New York City, and is the current president of the Optometric Retina Society. They wish to thank Yuliya Bababekova for her help in organizing this article.

References

1. Lujan BJ, Rosenfeld PJ, Gregori G, et al. Spectral domain optical coherence tomographic imaging of geographic atrophy. Ophthalmic Surg Lasers Imaging 2009 Mar-Apr;40(2):96-101.
2. Clinical case and imagery courtesy of Kerry Head, O.D. Photographs reprinted with permission of the author.
3. Ozdemir H, Karacorlu M, Karacorlu SA. Serous detachment of macula in cystoid macular edema associated with latanoprost. Eur J Ophthalmol 2008 Nov-Dec;18(6):1014-6.
4. Pollack AL. Peripheral retinoschisis and exudative retinal detachment in pars planitis. Retina 2002 Dec;22(6):719-24.
5. Hassenstein A, Richard G. Optical coherence tomography in optic pit and associated maculopathy. Ophthalmologe 2004 Feb;101(2): 170-6.
6. Mitamura Y, Hirano K, Baba T, Yamamoto S. Correlation of visual recovery with presence of photoreceptor inner/outer segment junction in optical coherence images after epiretinal membrane surgery. Br J Ophthalmol 2009 Feb;93(2):171-5.