Advances in technology have provided SS-OCT devices with several advantages compared to earlier SD-OCT:

The DRI OCT–1 Atlantis (Topcon Corporation, Tokyo, Japan) is a SS-OCT device used for imaging of the posterior segment of the eye. The device uses a 1050 nm wavelength and is capable of scanning 100,000 A-scans per second. The faster scanning speed allows region areas of up to 12 × 9 mm to be imaged with a volume scan. The viewing software is capable of automated segmentation of various layers of the retina and choroid (Fig. 4.3).

The SS-1000 CASIA Anterior Segment OCT (Tomey Corporation, Nagoya, Japan) is a noncontact imaging system which uses a scanning light source centered on a wavelength of 1310 nm (Kawana et al. 2007). The device is capable of 30,000 A-scans per second, with resolution of up to 10 μm axially and 30 transversely (Liu et al. 2011). The SS-1000 CASIA produces cross-sectional images of the cornea and anterior segment and its structures within the angles.

With a wide scan length of 16 mm, the entire cross section of the anterior chamber can be imaged in a single B-scan. The device is able to produce volume scans consisting of 128 horizontal B-scans, each consisting of 512 A-scans. In addition, the SS-1000 CASIA allows 360-degree imaging of the scleral spur, trabecular meshwork, angle recess, and iris root (Kawana et al. 2007).

Age-related macular degeneration (AMD) is the commonest causes of visual loss in developed countries and the third commonest worldwide. Its prevalence is expected to increase as a result of the aging population trends observed in many parts of the world. OCT plays a crucial role in the diagnosis and monitoring of patients with AMD (Fig. 4.4). Quantitative assessments of retinal thickness or qualitative features such as the presence of retinal thickening and cysts and subretinal fluid and the presence of choroidal neovascularization (CNV) are criteria for initiation of treatment and are commonly used in both clinical practice and multicenter randomized controlled trials.

Since the pathology in AMD occurs in the deeper layers of the retina, with the CNV occurring either in the subretinal or sub-RPE space, SS-OCT will contribute greatly to our ability to visualize the pathology in these areas (Fig. 4.5). SS-OCT can be used to assess qualitative features and quantitative features such as choroidal thickness (Tan et al. 2015a, c and, with the use of OCT angiography, visualize the actual CNV lesion.

It has been demonstrated that SS-OCT may be superior to SD-OCT in visualization of the deeper structures, including the sub-RPE space, Bruch’s membrane, and the choroid (Figs. 4.6 and 4.7) (Tan et al. 2015a, c; Yasuno et al. 2009).

In a study by (Yasuno et al. 2009) comparing a prototype SS-OCT device with the Topcon 3D OCT-1000, 9 of 13 (69.2 %) eyes showed marked improvement in the qualitative image contrast of pathologic lesions beneath the RPE in SS-OCT images compared to SD-OCT images. SS-OCT enables better visualization of hyperreflective masses beneath the RPE, visualization of fibrin and structures beneath the CNV lesion, and visualization of Bruch’s membrane as a separate line beneath a PED. In contrast, these structures were not as well seen on SD-OCT scans.

Another role of OCT in the evaluation of patients with AMD is the assessment of choroidal thickness. Various studies have reported that the choroid is thinner among patients with AMD compared to normal controls. Since differences in choroidal thicknesses may be affected by the visualization of the choroid–scleral boundary, the image contrast of the OCT scan is of paramount importance. Several studies have found that the visualization of the choroid–scleral boundary is clearer when SS-OCT is used compared to SD-OCT (Tan et al. 2015c; Yasuno et al. 2009).

While many studies still use point thickness measurements subfoveally or at various points along a horizontal and/or vertical B-scan (Tan et al. 2012b, 2015c), these are limited by the fact that the choroid is a three-dimensional structure with considerable topographic variation (Tan and Cheong 2014; Tan et al. 2014a; Ouyang et al. 2011), and it has been suggested that evaluating the mean choroidal thickness in each sector of the ETDRS grid may give a more accurate representation of the variations in choroidal thickness in different diseases. The faster scanning speed and wider scan area of SS-OCT are an advantage in such studies. A study by Ueda-Arakawa et al. (2014) comparing choroidal thicknesses between patients with reticular drusen only or with neovascular AMD or geographic atrophy and normal controls found that the mean choroidal thickness and volume of each sector were significantly reduced in eyes with reticular drusen compared to normal controls.

The faster scanning speed of SS-OCT also makes this a useful modality for OCT angiography. While this is discussed in greater detail in another chapter, several authors have reported that OCT angiography allows noninvasive visualization of CNV lesions among AMD patients that correspond well to the size and location of the CNV seen on fluorescein angiography (FA) (Jia et al. 2014; Moult et al. 2014). In some cases, a feeder vessel can be identified. Other features such as reduced choroidal blood flow beneath and adjacent to regions of CNV have been reported (Jia et al. 2014; Moult et al. 2014).

Polypoidal choroidal vasculopathy (PCV) is a disease characterized by the presence of abnormal vascular channels which supply the polyps. Currently, the gold standard for diagnosis of PCV is indocyanine green angiography (ICGA) (Lim et al. 2010; Tan et al. 2014c, 2015b). This modality, however, is costly and invasive, requiring an intravenous injection of the dye. The use of indocyanine green carries the risk of allergic reactions, especially among patients with known allergies to seafood and shellfish in particular. If OCT can be used to detect PCV, it would provide a fast, cheap, and noninvasive method of monitoring this condition.

SD-OCT devices use a light source with a wavelength of 840 nm, which is scattered at the level of the RPE. Since both the polyps and vascular channels that supply them are believed to lie beneath the RPE, visualization of these lesions on SD-OCT is difficult. In contrast, the longer wavelength of the SS-OCT would allow better visualization of structures located beneath the RPE. If the PCV lesion can consistently be visualized, SS-OCT can be used to detect the presence and level of activity and recurrence of PCV lesions.

Studies using SD-OCT and SS-OCT have reported the presence of hyperreflectivity beneath the RPE, which is presumed to correspond to the polyps (Fig. 4.8) (Yasuno et al. 2009).

Besides the traditional cross-sectional OCT B-scans, we are also able to visualize OCT scans using en face imaging. These allow visualization of lesions throughout the scan area at different planes of the retina, RPE, and choroid. Several papers have used this method on patients with PCV (Alasil et al. 2015; Sayanagi et al. 2015).

Sayanagi et al. (2015) reported the findings of en face SS-OCT on 19 eyes of 18 patients with PCV. Of 43 polypoidal lesions seen on ICGA in 14 eyes, 41 were detected using en face OCT. The polyps appeared as an RPE ring with inner reflectivity on slices obtained from the RPE layer. Within the RPE rings, the inner reflectivity manifested with either a mosaic (72 %) or homogenous (28 %) pattern. In most of the cases, the RPE rings were of the same size as the polypoidal lesions on ICGA, while in a minority, the rings were larger. In addition, en face OCT detected 15 of 17 eyes with a vascular network on ICGA. Using en face OCT, the networks appeared as a hyperreflective mesh-like configuration. Dilated choroidal vessels were seen beneath the RPE ring (the presumed polypoidal lesions) in 22 of 47 (47 %) RPE rings on en face images. These coincided with the dilated choroidal vessels seen on ICGA (Sayanagi et al. 2015).

Another paper by Alasil et al. (2015) demonstrated the relationship between the smaller PEDs that correlated with the polyps and the larger adjoining PEDs. Choroidal vascular abnormalities were detected in all eyes with PCV and included focal dilation, diffuse vascular dilation, and a branching vascular network (Alasil et al. 2015).

Due to the longer wavelength and greater penetration of its light source, it is not surprising that SS-OCT has been reported to be superior to SD-OCT and enhanced depth imaging OCT (EDI-OCT) for lesions deep to the RPE (Francis et al. 2015; Hayashi et al. 2014; Takahashi et al. 2013).

In a comparison of SS-OCT and EDI-OCT from 30 eyes with choroidal nevi (Francis et al. 2015), SS-OCT was found to be superior at visualizing intralesional vessels, intralesional granularity, and abnormal choriocapillaris in melanotic lesions. In contrast, EDI-OCT was found to be equivalent at identifying secondary retinal changes, distended bordering vessels, and visualization of the nevus–scleral interface with hyporeflective gradation beneath the lesion. The authors also found that the morphology of melanotic nevi was better visualized using SS-OCT compared to EDI-OCT (100 % with SS-OCT compared to 53 % with EDI-OCT), whereas both OCT modalities were equivalent in amelanotic lesions.

Hayashi et al. described the SS-OCT findings in two eyes with choroidal osteoma. Calcified regions appeared multilayered and spongelike, while a lamellar reflective pattern was noted in the decalcified regions. Hyperreflective mound-like areas with abnormal outer retinae were observed in both eyes. Of note, the chorioscleral border was visible in both cases.

Takahashi et al. described the OCT findings in a 60-year-old man with presumed retinal pigment epithelium hamartoma. Whereas standard SD-OCT demonstrated an elevation of the retina with complete shadowing of optical transmission, SD-OCT using the EDI setting and SS-OCT were able to image deeper structures within the pigmented lesion.

In a study by Sato et al., 26 eyes of 15 patients with optic disc drusen were evaluated using EDI-OCT and SS-OCT (Sato et al. 2013). EDI-OCT and SS-OCT both showed multiple optic disc drusen at different levels; most were located immediately anterior to the lamina cribrosa. The drusen appeared as ovoid regions of lower reflectivity that were bordered by hyperreflective material. In 12 eyes (46.2 %), there were internal hyperreflective foci. The mean diameter of the optic disc drusen was 686.8 μm (standard deviation ± 395.2 μm), and there was a significant negative correlation between the diameter of the optic disc drusen and the global retinal nerve fiber layer thickness (r = −0.61, p = 0.001).

SS-OCT has been reported to be useful for imaging of patients with pathologic myopia (Fig. 4.9). In a comparative observational case series of five patients with pathologic myopia (mean spherical equivalent, −16.00 ± 4.70 D) (Lim et al. 2014), it was found that SS-OCT better visualized retino-choroidal structures. The choroid (SS-OCT 8/8 vs. SD-OCT 1/8), inner segment/outer segment line (SS-OCT 6/8 vs. SD-OCT 1/8), and external limiting membrane (ELM) (SS-OCT 7/8 vs. SD-OCT 2/8) were clearly seen in a higher proportion of SS-OCT compared to SD-OCT scans (p < 0.01 for all). In contrast, visualization of the sclera and retinal pigment epithelium (RPE) was similar using both OCT devices. The wider SS-OCT scan revealed additional pathology along the walls of the staphylomata, which were not visible using SD-OCT in 4/8 images. These included incomplete posterior vitreous detachment in one eye and peripheral retinoschisis in 3/8 eyes. Vitreoschisis was visible in 3/8 SS-OCT images but not in the SD-OCT images.

Using SS-OCT, Ellabban et al. described the tomographic features of dome-shaped macula (DSM) in 51 highly myopic eyes (35 patients). DSM was first described by Gaucher and associates as an inward convexity of the macula that occurred within the concavity of a posterior staphyloma in highly myopic eyes. In all reconstructed 3D images of the RPE, two outward concavities were seen within the posterior staphyloma, and a horizontal ridge was formed between these two concavities. In 42 of these eyes, this horizontal ridge was band shaped. The vertical OCT section through the fovea showed a convex configuration of RPE, but the horizontal section showed an RPE line that was almost flat. In nine eyes, 3D images showed a typical dome-shaped convexity within the staphyloma. OCT scans showed no outward protrusions in the external scleral surface, but marked scleral thinning was seen in the region of the two outward concavities of the RPE. The sclera of the fovea (518.6 ± 97.6 μm) was significantly thicker than that in all four quadrants of the parafoveal area (range, 277.2–360.3 μm; p < 0.001). It has been suggested that dome-shaped macula is not related to the type of staphyloma, but rather is related to an anatomic or structural change within the sclera and can occur in eyes with any type of staphyloma (Ellabban et al. 2013a).

Pedinielli et al. described focal scleral ectasia in areas of macular/perimacular patchy chorioretinal atrophy secondary to pathologic myopia in 26 eyes of 13 patients on SS-OCT and 78 eyes of 39 patients on EDI-OCT (Pedinielli et al. 2013). Focal scleral ectasia is characterized by an abrupt posterior bowing of the sclera with different degrees of scleral schisis on its borders. It is noteworthy that the outer retina, the RPE, and the choroid were absent in areas of macular/perimacular patchy atrophy, as well as in focal scleral ectasia. The absence of these structures, together with the deep penetration OCT signal (both with EDI-OCT and SS-OCT), allowed visualization of retrobulbar vessels perforating the sclera at the borders or bottom of the focal scleral ectasia and running through the superficial scleral thickness for the whole extension of the atrophic area. These features were found in 12 out of 68 eyes (11 out of 39 consecutive patients, mean age 65.7 ± 11.9 years) with macular/perimacular patchy chorioretinal atrophy and were always observed inferior or temporal to the macula. The authors postulated that focal scleral ectasia in areas of macular/perimacular patchy atrophy may have developed from a scleral stretch-associated schisis, which is more pronounced inferiorly (possibly due to oculomotor, gravitational, and staphyloma-associated stretching forces), in areas of openings of anterior emissary for the short posterior ciliary arteries in the sclera. They are different from the recently described macular intrachoroidal cavitation. In focal scleral ectasia, the choroid, the RPE, and the outer retina are absent; there are various degrees of thinning/schisis of the inner retina, and there is an exaggerated focal scleral protrusion with an aspect of schisis on its borders.

Studies have described the use of SS-OCT to detect intrachoroidal cavitations in pathologic myopia (myopic spherical equivalent > −8 diopters). Ohno-Matsui et al. described patchy atrophy identified by funduscopy in macular intrachoroidal cavitation in 31 of 56 eyes (55.4 %) in the macular area and compared them to 68 consecutive patients with pathologic myopia without patchy atrophy (Ohno-Matsui et al 2013). The SS-OCT images showed that the sclera was bowed posteriorly in and around the patchy atrophy compared to neighboring sclera, whereas none of the 68 patients without patchy atrophy showed this finding. Macular intrachoroidal cavitation had OCT features similar to peripapillary intrachoroidal cavitation: the choroid in the macular intrachoroidal cavitation area appeared thickened, and the retina was caved into the cavitation. A direct communication between the vitreous cavity and intrachoroidal cavitation has been observed in some eyes. Retinoschisis was observed significantly more frequently in or around the patchy atrophy in eyes with macular intrachoroidal cavitation than in those without cavitation. The authors surmised that these findings suggest that patchy atrophy affects the scleral contour within posterior staphyloma beyond the funduscopically identified patchy atrophy by macular intrachoroidal cavitation. Such deformation of sclera may facilitate the development of retinoschisis in and around the patchy atrophy. Ohno-Matsui et al. also described the incidence of peripapillary intrachoroidal cavitation located temporal to the optic disc (defined as intrachoroidal cavitation located temporal to the optic disc seen in horizontal OCT section through the optic disc center) in 125 patients with pathologic myopia. In that series, 17 eyes of 16 patients had temporal intrachoroidal cavitation. All of the eyes had temporal or temporally wider annular conus. The intrachoroidal cavitation was observed temporal to the optic disc in 15 of 17 eyes, and two of the remaining eyes also had inferior intrachoroidal cavitation. Even in the two eyes with both temporal and inferior intrachoroidal cavitation, the temporal intrachoroidal cavitation was much wider than the inferior intrachoroidal cavitation. The average width of the temporal intrachoroidal cavitation in the horizontal OCT scan across the center of the optic disc was 1467.8 ± 1328.1 μm with a range of 442–6200 μm. In two eyes, the temporal intrachoroidal cavitation extended beyond the central fovea, and in one of these, the subfoveal choroidal thickness in the vertical OCT scan through the fovea appeared thickened because of a separation at the suprachoroidal space. Inner retinal defects at the border of the conus and temporal intrachoroidal cavitation were detected in two eyes. Although the authors postulated that the characteristics of temporal intrachoroidal cavitation did not provide clues on why they tend to be large, one possibility could be that the area temporal to the optic disc between the conus and central fovea might be the area where the mechanical tension is the highest in eyes with pathologic myopia. On the other hand, in eyes with an inferior intrachoroidal cavitation, the inferior edge of the staphyloma was generally close to the inferior border of the optic disc, and the inferior intrachoroidal cavitation did not appear to spread beyond the inferior edge of the staphyloma. The reason why the intrachoroidal cavitation develops temporal to the optic disc is not clear. All of the eyes with temporal intrachoroidal cavitation had a temporal or temporally wider annular conus. They suggested that the deformation of the posterior wall of the eye is more pronounced in the area of the conus because overlying layers are thinner than in the more normal regions temporal to the conus.

Ohno-Matsui et al. described the incidence and characteristics of pitlike structures around the optic disc and myopic conus in eyes with high myopia that were barely visible ophthalmoscopically but can be demonstrated by using SS-OCT (Ohno-Matsui et al. 2013). The authors evaluated 198 eyes of 119 patients with pathologic myopia and compared them to 32 eyes of 32 emmetropic subjects. Pitlike clefts were found at the outer border of the optic nerve or within the adjacent scleral crescent in 32 of 198 highly myopic eyes (16.2 %), but none were observed in the emmetropic eyes. The eyes with these pits were more myopic, had significantly longer axial lengths, and had significantly larger optic discs than the highly myopic eyes without pits. The pits were located in the optic disc area (optic disc pits) in 11 of 32 eyes and in the area of the conus outside the optic disc (conus pits) in 22 of 32 eyes. One eye had both optic disc pits and conus pits. The optic disc pits existed in the superior or inferior border of the optic disc. All but one eye with conus pits had a type IX staphyloma, and the location of the conus pits was present nasal to the scleral ridge or outside the ridge temporal to the nerve. The optic disc pits were associated with discontinuities of the lamina cribrosa, whereas the conus pits appeared to develop from a scleral stretch-associated schisis or to emissary openings for the short posterior ciliary arteries in the sclera. The nerve fiber tissue overlying the pits was discontinuous at the site of the pits. The authors hypothesized that the optic disc is first enlarged by a mechanical expansion of the papillary region in highly myopic eyes and becomes megalodisc-like. This mechanical expansion of the optic disc area results in stretching and expanding of the lamina cribrosa. Because of the mechanical tension on the lamina and surrounding peripapillary sclera, coupled with the myopic thinning of these structures, the lamina might dehisce from the peripapillary sclera, particularly at the superior and inferior poles, which are sites of decreased connective tissue support. This stage is observed by SS-OCT as a hyporeflective gap at the junction of the lamina cribrosa and the peripapillary sclera. Further increases of the gap seemed to be associated with disruption of the overlying nerve fibers or herniation of these fibers into the defect. The clinical importance of the pits lies in the disruption of the nerve fiber overlying the pits. In addition to the discontinuity of nerve fiber tissue overlying the pits, the lamina was dehisced from the peripapillary sclera at the site of optic disc pits. The dehisced lamina cribrosa due to optic disc pits and discontinuous peripapillary sclera due to conus pits could cause a condition similar to that of the material of a trampoline with its springs broken.

Shinohara et al. described 38 eyes of 29 patients who were highly myopic, and 16 eyes of 15 patients were not highly myopic with tilted disc syndrome (TDS) (Shinohara et al. 2013). This has been thought to be a congenital anomaly caused by an incomplete closure of the fetal fissure of the eye, and it occurs in 1–2 % of the populations. The ophthalmic features of TDS include an inferonasal tilting of the optic disc, the presence of an inferior or inferonasal crescent, myopia, astigmatism, and an ectasia of the lower fundus or inferior staphyloma. The representative OCT findings of the optic disc were a sloping of the lamina cribrosa posteriorly from the upper part to the lower part, a protrusion of the upper edge of Bruch’s membrane, and choroid. The distance and the depth of the most protruded point from the fovea (the most protruded point occurred along the vertical section across the fovea in 11 eyes and occurred 15° inferotemporal to the fovea in the remaining eyes) were significantly greater in the eyes with non-highly myopic TDS than those with highly myopic TDS (P = 0.002 for distance, P = 0.03 for depth). The average distance between the most protruded point and the fovea was 3497.4 ± 763.3 μm with a range of 2237–4473 μm in nine non-highly myopic eyes and 2451.2 ± 691.5 μm with a range of 1195–4290 μm in 15 highly myopic eyes. Similarly, the average depth of the most protruded point from the foveal plane was 935.8 ± 396.8 μm with a range of 537–1730 μm in the nine non-highly myopic eyes and 611.5 ± 284.4 μm with a range of 207–1100 μm in the 15 highly myopic eyes. The authors inferred from the similarities of the deformities in eyes with TDS and eyes with high myopia without TDS; the essential pathology of TDS might be a posterior bulge involving a wide area of inferior globe. The appearance of the optic disc might be influenced only by the relationship between the attachment site of the optic nerve and the upper edge of posterior bulge.

SS-OCT allows noninvasive, volumetric, and measurable in vivo visualization of the anatomic microstructural features of the posterior vitreous and vitreoretinal interface (Fig. 4.10). It is superior to SD-OCT for vitreous imaging, because it maintains high sensitivity over a much longer imaging range. In addition, the long wavelengths reduce artifacts caused by ocular opacities and induce less light scattering by the vitreous fibers. The high speed of SS-OCT combined with registration motion correction and merging enables volumetric imaging over a wider scan length and compensates for the movement of the vitreous fibers (Liu et al. 2015).