24 Optical Coherence Tomography for Imaging the Sub-Tenon Space, Sclera, and Choroid
Optical coherence tomography (OCT) has been extensively used for cornea and angle evaluation in anterior-segment (AS) and macula and retinal examination in posterior segment. With advancements in OCT image-processing software, more refined details of the posterior segment can be appreciated and characterized in vivo. Newer-generation OCT devices like spectral-domain (SD) OCT and enhanced depth imaging (EDI) OCT have revolutionized ocular imaging of posterior segments like the choroid. In this chapter, we discuss the imaging of structures like the sub-Tenon space, sclera, and choroid. We analyze the applications and use of OCT in imaging these structures.
24.1 OCT for Imaging the Sub-Tenon Space
The Tenon capsule is a tenuous tissue layer composed of dense collagen that lies between the episclera and the substantia propria. It extends forward from the rectus muscle insertions, becoming thinner as it moves anteriorly. Sub-Tenon injections have been used for decades for drug delivery in uveitis or macular edema. 1 , 2 , 3 The sub-Tenon space is preferred for drug administration because of its prolonged effect and minimal systemic side effects. 4 , 5 Although sub-Tenon injections have been given by various methods, the position of the cannula or the drug has not been imaged in vivo until now. 4 , 5 We (idea conceived by D.A.K.) have used high-speed AS OCT (Carl Zeiss Meditec, Dublin, CA) for imaging the sub-Tenon space in vivo.
24.2 OCT-Assisted Sub-Tenon Injection
The periocular skin is cleaned with povidone iodine 5%, and a few drops are instilled into the conjunctiva. A drop of proparacaine 0.5% is then instilled in the eye. A sterile cotton-tipped applicator soaked in 4% lidocaine is placed over the superotemporal quadrant for 2 minutes as the patient looks inferonasally. The patient is made to sit in a comfortable position in a sliding chair and asked to fixate on an inferonasal target. The eyelids are retracted. Under sterile conditions, the superotemporal conjunctiva is lifted with a forceps and an intravenous polytetrafluoroethylene (PFTE) cannula of 22 G, 0.8/25-mm size, is introduced superotemporally 10 mm from the limbus. The needle is then removed, and the cannula alone is pushed 3 mm into the sub-Tenon space. The patient’s head is positioned on the chin rest in the AS OCT machine, and the patient is asked to fixate on the inferonasal target. Then the tuberculin syringe with 1 mL of triamcinolone acetonide (Kenalog-40, Bristol-Myers-Squibb, NJ) suspension is injected into the cannula. Cross-sectional (180–0 axis) images centered on the cannula are taken with AS OCT (Fig. 24.1). Corneal high-resolution single-scan mode is used. Scan is taken at the posteriormost cannula visible on the OCT screen (Fig. 24.1).
24.3 OCT Features in Subtenon Injection
The mean conjunctiva-Tenon complex is 0.38 ± 0.08 mm. The mean thickness of the Tenon layer is 0.21 ± 0.07 mm. The drug is tracked behind the Tenon capsule as bright white fluid during the injection (Fig. 24.2). There is no conjunctival chemosis or scleral perforation during the procedure (Fig. 24.3). The drug is localized in 100% of the eyes. Sub-Tenon space of about 10 to 13 mm from the limbus is visualized. Because the underlying sclera is always visualized in OCT-assisted posterior sub-Tenon triamcinolone in vivo, the risk of scleral perforation is decreased. OCT has been shown to have significant role in various clinical situations. 6 , 7 , 8 This noncontact and noninvasive imaging method gives an axial resolution of 18 µm and a transverse resolution of 60 µm. The 1310-nm wavelength reduces the amount of signal scattering, which allows better penetration into turbid tissue, such as the sclera. Because of its high speed, it minimizes patient motion artifacts and improves the quality of captured images. Thus, high-speed anterior segment OCT can be used to visualize the sub-Tenon space, and clinically it can be used for imaging the periocular or depot delivery of drugs in the potential space.
24.4 OCT for Imaging Sclera
Sclera is not routinely seen in time-domain or SD OCT. However, swept-source OCT can image scleral tissue. Sclera has the tendency of scattering light, and hence lower wavelength may be suboptimal in delineating sclera. OCT imaging of the AS with a longer wavelength of 1310 nm had the advantages of better penetration through sclera. Recent advances in OCT have enabled investigators to image the tissues deeper than the neural retina, such as the choroid and sclera. Especially in eyes with pathological myopia, the retina and choroid of which are very thin, it is possible to observe the entire thickness of the sclera. Normal sclera in OCT is observed as a relatively uniform, hyperreflective structure exterior to the thin choroid on swept-source OCT images. Grulkowski et al has imaged scleral and limbal vasculature in swept-source OCT with 1050-nm wavelength and 100-kHz A-scan rate. 9
24.4.1 High Myopia
Imamura et al applied EDI OCT for imaging sclera in myopic eyes. 10 Patients with dome-shaped macula, a condition defined as convex elevation of the macula compared with the surrounding staphylomatous region in a highly myopic eye, were identified through routine examinations using OCT. 9 The scleral thickness was measured from the outer border of the choroid to the outer scleral border under the fovea and 3000 μm temporal to the fovea. The mean subfoveal scleral thickness reported by the study in 23 eyes with dome-shaped macula was 570 (± 221) μm. 10 These investigators concluded that the dome-shaped macula was the result of localized thickness variation in the sclera.
The mean subfoveal scleral thickness as noted by Ohno-Matusi et al was 227.9 ± 82.0 μm in highly myopic eyes. 11 The sclera was thickest at 3000 μm nasal to the fovea. They were able to divide the curvatures of the inner scleral surface of highly myopic eyes into curvatures that sloped toward the optic nerve, those that were symmetrical and centered on the fovea, those that were asymmetrical, and those that were irregular. 11
24.4.2 Scleral Spur
Scleral spur location represents an important anatomical landmark in imaging the anterior-chamber angle because it is a reference point for the relative position of the trabecular meshwork. The probability of time-domain anterior-segment OCT showing the scleral spur is 70%. 12 Swept-source OCT can be used for visualizing scleral spur (Fig. 24.4). 13
24.4.3 Spectral-Domain OCT
Optical coherence tomography can be used to assess the prognosis and pathophysiology of scleral inflammation. In a study conducted in our center, cross-sectional imaging of the eye centered on the nodular lesion was taken with a high-speed AS OCT (Visante, Carl Zeiss Meditec) in nodular scleritis. In chronic cases, there was apparent nodular elevation seen from the limbus. There was thickening of the anterior sclera with hyperreflectivity compared with the sclera of the normal fellow eye (Fig. 24.5). A few spaces of hyporeflective spots and focal thinning were seen in the deep sclera (Fig. 24.6). In early cases, the anterior AS OCT showed marked thickening of conjunctiva, episcleral, and sclera over the nodule. A prominent elevation was noted starting from the limbus (Fig. 24.7), and there was a prominent line or hyporeflective zone seen longitudinally in the scleral surface, which is suggestive of the cleavage seen between the scleral lamellae in acute inflammation (Fig. 24.7); this is due to inflammatory fluid accumulation.
Miura et al showed that polarization-sensitive OCT is useful as a contrast engine of the anterior eye segment and for the evaluation of pathological change in the sclera. 14 Neema et al (www.abstractsonline.com/plan/ViewAbstract.aspx, ARVO2011 ABSTRACT) has used AS OCT in scleral inflammation. Transcleral high-speed OCT for imaging intraocular structures has been tried ex vivo in cadaveric eyes. 15
In inflammation, tissue edema is seen as thickening of the involved sclera, and the hyperreflectivity is due to tissue infiltration. Focal hyporeflective spots seen in deep sclera may represent the tissue damage, which can lead to scleral thinning in later stages. One can also see the prominent line or hyporeflective zone seen longitudinally in the sclera, which is suggestive of the cleavage seen between the scleral lamellae in acute inflammation and may be due to fluid accumulation in the acute stage. Resolution of the scleral edema, increase in homogeneous reflectivity of the sclera, and disappearance of the cleavage line or fluid accumulation are the OCT indicators of the response to treatment.