The choroid is a vascular meshwork between the retina and sclera and plays a vital role in ocular metabolism, volume regulation, and temperature control. Abnormalities of the choroid have been implicated in several ophthalmic diseases, most importantly in the pathophysiology of retinal conditions [30, 31]. In addition, changes in choroidal structure and function have also been postulated to play a role in glaucoma [32, 33]. Study of this structure was until recently limited to postmortem histological studies [32, 34] and ultrasonography [35]. An important source of uncertainty about the cause-and-effect relationship of choroidal changes and disease processes arises from the lack of precision and effect of artifacts on these methods [36].

Imaging with SS-OCT has been shown to enhance the visualization of the choroid (Fig. 18.4). The use of SS-OCT has been evaluated in choroidal and retinal thickness measurements. Automated measurements of choroidal and retinal thickness have been shown to be highly repeatable [37]. The frequency and type of artifacts have also been assessed: the most frequent image artifact was signal loss resulting from blinking. Other artifacts that occurred were segmentation failure and motion artifacts. SS-OCT has been used to measure choroidal thickness in healthy and glaucomatous eyes [38]. A relationship was demonstrated between increasing age, longer axial length, and thinner choroid. No association was found between glaucoma and choroidal thickness after accounting for differences in age and axial length. Choroidal thickness has also been evaluated using SS-OCT after the water-drinking test [39]. The water-drinking test has been used to estimate the magnitude of peaks in IOP, and consists of ingestion of 1000 mL of water within a period of 5–15 min. A consistent and statistically significant increase of peripapillary and macular choroidal thickness and volume after the water-drinking test has been demonstrated. However, this phenomenon was of small magnitude and unlikely to explain the observed IOP increase by itself.

To determine whether the choroid plays a role in glaucoma, longitudinal studies are needed to evaluate the correlation between choroidal changes and glaucoma progression.

Less attention has been paid to the ability of SS-OCT to evaluate the RNFL, as it is widely presumed that no additional benefit for this parameter is provided compared to SD-OCT. Yang et al. assessed the ability of SS-OCT RNFL measurements to differentiate glaucomatous eyes from healthy eyes [40]. Wide-angle and peripapillary RNFL thickness were assessed. The diagnostic accuracy of the RNFL thickness measurements by SS-OCT was similar to that of peripapillary RNFL measurements by SD-OCT. The same group also evaluated the diagnostic ability of macular ganglion cell and inner plexiform layer measurements using SS-OCT [41]. The thickness of the macular ganglion cell complex (mGCC) and the macular ganglion cell inner plexiform layer (mGCIPL) were shown to be significantly reduced in glaucomatous compared to healthy eyes. Diagnostic accuracies for both parameters from both SS-OCT and SD-OCT showed a similar ability to detect glaucoma as circumpapillary RNFL. Good diagnostic performance was also shown for early glaucoma.

Evaluation of the anterior chamber angle is an important part of glaucoma management and diagnosis. Although gonioscopy remains the clinical gold standard for the visualization of the angle structures, anterior segment optical coherence tomography (AS-OCT) has become available for the evaluation of the anterior chamber angle. This imaging technique enables the obtention of objective and reproducible measurements of the anterior chamber angle, and has improved the diagnostic performance to detect angle closure [42].

Studies generally use the scleral spur as a reference point for evaluation of the angle. Parameters to measure the angle dimensions including the angle-opening distance, trabecular iris angle, angle recess area, and trabecular iris space area are measured with reference to the scleral spur [42]. The scleral spur may not always be visible with time domain AS-OCT [43, 44]. Using the Casia SS-OCT (Tomey, Nagoya, Japan), McKee et al. [45] visualized the scleral spur in over 95% of all examined quadrants in HD scan images. In addition to the scleral spur, Schwalbe’s line and the Schlemm’s canal could also be visualized. Their visibility was influenced by the scan location and the scan density: images obtained with HD scan in the nasal and temporal quadrants had superior visibility of the angle landmarks compared with those obtained with LD scan in the superior and inferior quadrants. The ability to visualize both the scleral spur and Schwalbe’s line by SS-OCT makes it possible to measure the length of the trabecular meshwork. This may improve the precision of angle measurements and detection of angle closure [42].

Anterior chamber angle measurements using SS-OCT were shown to have low variability. Liu et al. [46] measured the angle opening distance, the trabecular iris space area, and the trabecular-iris angle at four quadrants, and found intraclass correlation coefficients of all angle parameters to be ≥0.83. Iris thickness, scan location, angle dimension, and axial length were associated with increased variance of angle measurements, but overall SS-OCT was found to be a reliable technique for the evaluation and measurement of the anterior chamber angle.

Peripheral anterior synechiae (PAS) are adhesions between the peripheral iris and cornea that can be found in different forms of angle-closure glaucoma. The fast scan speed of SS-OCT enables the acquisition of multiple high-resolution cross-sectional images of the angle, and facilitates evaluation of PAS. Lai et al. used SS-OCT to evaluate the area and degree of PAS involvement in patients with angle-closure glaucoma [47]. The measurements of the area and degree of PAS involvement were found to be reproducible. Synechial and appositional angle closure could be discriminated by variation of the lighting condition during OCT imaging: in appositional angle closure the angle is closed in the dark and open in the light, whereas in synechial closure the angle remains closed.

OCT evaluation of PAS has an advantage over gonioscopy in that it is a non-contact method and provides precise quantitative measurements of PAS. These measurements could be useful in the monitoring of PAS progression over time. Another group evaluated the use of SS-OCT in detection of iridotrabecular contact (ITC) in eyes with shallow peripheral anterior chamber and compared the results to these obtained with ultrasound biomicroscopy (UBM) [48]. The prevalence of ITC, which involves both PAS and appositional angle closure, in eyes with shallow peripheral anterior chambers was found to be significantly higher with SS-OCT as compared to UBM under light conditions, but not under dark conditions. The ITC range evaluated by SS-OCT was greater in the dark than in the light. PAS-positive eyes had a greater range of ITC than PAS-negative eyes under light conditions.

The high scan speed of SS-OCT also facilitates a more complete imaging of the iris and measurement of iris volume as compared to time-domain OCT. Mak et al. measured iris volume using SS-OCT and assessed the relationship with primary angle closure [49]. A larger iris volume was found to be associated with a smaller angle width, a smaller pupil diameter, smaller anterior chamber volume, and a longer axial length. After pharmacological dilatation, mean iris volume significantly decreased in all eyes independent of angle status (angle closure, POAG, and normal eyes). From light to dark 19.4% of angle closure eyes, 16.7% of normal eyes, and 19.4% of POAG eyes showed an increase in iris volume. SS-OCT iris volume measurements can provide useful information for a better understanding of the pathophysiology of angle-closure glaucoma.