12 Optical Coherence Tomography in Endothelial Keratoplasty
First described by Huang et al 1 in 1991, the application of optical coherence tomography (OCT) in the measurement and imaging of ophthalmic systems has revolutionized the approach to many aspects of eye disease. As a noncontact imaging modality, OCT confers a distinct advantage over ultrasound and confocal biomicroscopy. The rapid, noninvasive, and high-resolution qualities of modern OCT make it an excellent adjunct to the diagnostic modalities available to the modern corneal surgeon. In this chapter, we use a case-based approach to demonstrate the use of OCT in the management of endothelial keratoplasty (EK).
The initial iteration of anterior-segment (AS) OCT was labeled time-domain (TD) OCT. Carl Zeiss Meditec (Dublin, OH) introduced the Visante TD-OCT as a tool for precise AS imaging in 2005. The Visante uses a 1310-nm wavelength that can penetrate the cornea and some limbal structures to give a limbus-to-limbus view of the AS with spatial resolution as fine as 15 µm. The Visante can demonstrate two-dimensional corneal shape, corneal opacities, corneal pachymetry, anterior-chamber depth, iris-corneal angle anatomy, iris anatomy, and structures adjacent to the anterior lens capsule, such as phakic intraocular lens (IOLs). Newer Fourier-domain (FD) OCT devices (e.g. Avanti, OptoVue, Freemont, CA; Cirrus, Carl Zeiss Meditec) use an 830-µm wavelength that is popular for retinal imaging. Although resolution improves up to 5 µm, the scan length is more limited. Special attachment lenses are required to image the AS with most models of FD OCT. Although current technology does not allow FD OCT to span the entire corneal diameter, the improved resolution of corneal and angle structures is quite useful.
In recent years, EK has rapidly gained in popularity worldwide for the treatment of cornea endothelial dysfunction. First described by Melles et al, 2 EK confers significant advantages over penetrating keratoplasty (PK), such as stronger postoperative integrity of the globe, less induced astigmatism, faster visual recovery, and significantly reduced episodes of immune graft rejection. 3 With the advances in EK technique have come challenges in securing the posterior corneal graft to the host cornea. Preoperative, intraoperative, and postoperative imaging of the AS with OCT has proven critical in many cases. Currently, the two most popular iterations of EK are Decsemet stripping automated endothelial keratoplasty (DSAEK) and Descemet membrane (DM) endothelial keratoplasty (DMEK). In our practice, DMEK is the preferred EK technique because of the vastly lower rejection rates, smaller incision size, rapid visual recovery, and better visual potential. DMEK is technically more difficult than DSAEK and thus is not suitable for eyes with a discontinuous iris-lens diaphragm, aphakia, aniridia, or extensive posterior corneal irregularity that may preclude DMEK graft adhesion. Unless a surgeon is highly experienced in DMEK technique, DSAEK may also be preferable in eyes with a history of filtering glaucoma surgery, PK, vitrectomy, and significant corneal edema that limits the view of the anterior chamber.
12.1 Preoperative OCT Imaging
In the presence of significant corneal edema, AS OCT may provide critical information regarding the anatomy of the AS and the suitability for EK surgery in a particular eye. Significant peripheral anterior synechia, a shallow anterior chamber, large iris defects, and posterior corneal irregularities may all increase the complexity of EK surgery. In eyes where slit-lamp examination is limited by media opacities, an AS OCT can demonstrate these potential problems preoperatively and assist in proper surgical planning. When treating endothelial failure of a previous PK graft, AS OCT provides valuable information about the potential for posterior apposition of the graft to the host, which can guide surgical planning and help predict postoperative difficulties with adherence. Some authors have suggested a role for AS OCT to determine the DSAEK graft diameter. 4 In cases of failed PK we prefer DMEK in most cases but will choose DSAEK if there is a significant graft-host mismatch as evidenced on AS-OCT.
12.2 Intraoperative OCT Imaging
The use of OCT during ophthalmic surgery has been previously reported by using a handheld OCT device or an OCT device attached to a C-arm in the operative suite. Use of OCT in this manner required a pause in surgery and repositioning of the surgical microscope to allow for OCT imaging. 5 , 6 Another approach to intraoperative OCT was described by Geerling et al, who coupled a TD OCT unit with a dielectrical mirror to a surgical microscope, producing two-dimensional images with a number of limitations, including difficulty orienting the image and poor light penetration, but it served as a proof of concept. 7 More recently, OCT was incorporated into a commercially available surgical microscope in the Haag-Streit iOCT system (Haag-Streit AG, Bern, Switzerland). This device allows for real-time images of the anterior or posterior segment during surgical maneuvers as the OCT scanning beam is incorporated into the microscope and projected though the main objective lens. A small liquid crystal display (LCD) screen near the surgeon allows for easy viewing of the OCT image with minimal head movement during surgery. Intraoperative OCT confers advantages in many different aspects of ophthalmic surgery. Specific advantages during EK include visualization of interface fluid during DSAEK, determination of proper graft orientation in DMEK, and visualization of posterior corneal deformities that may preclude proper graft positioning or adherence in either iteration of EK. 8
12.3 Postoperative OCT Imaging
The most popular use for OCT in EK is evaluation of the graft postoperatively. Corneal edema may preclude a detailed slit-lamp view of graft position or adhesion. OCT is commonly used to evaluate the cause of graft detachment, 9 confirm proper graft orientation, 10 and guide postoperative decision making in cases of graft malfunction. OCT can also accurately check graft and host thickness during the immediate postoperative period 11 , 12 or later during episodes of graft rejection or graft failure. Other potential complications, including epithelial ingrowth, 13 interface opacification, 14 and retained DM, 15 may also be elucidated with AS OCT.
A recent study by Melles et al 2 , 16 evaluated the predictive value of AS OCT in DMEK graft attachment, comparing graft attachment on AS OCT at 1 hour, 1 week, and 1 month postoperatively with attachment at 6 months. This study, in which no air reinjection was performed at any time point, concluded that DMEK graft attachment at 1 week had excellent predictive value for continued attachment through 6 months. Grafts that were attached at 1 hour but significantly detached at 1 week were likely to reattach spontaneously, whereas grafts detached at both time points were less likely to undergo spontaneous reattachment. 16 These findings provide insight into the evolution of graft adherence and may guide decision making in certain circumstances. It is worth noting that in our practice we reinject air for any significant detachment rather than awaiting spontaneous clearance. In our analysis of 673 eyes with at least 6 months’ follow-up, this practice achieves visual improvement sooner and is not associated with decreased endothelial cell counts or increased incidence of any complications. 17
12.4 Case Studies of OCT in Epithelial Keratoplasty
12.4.1 Case 1: Uncomplicated Normal DSEK
In this case, a normal postoperative DSEK is demonstrated (Fig. 12.1). The central cornea is clear, and there is a thin, well-centered graft without folds or detachment. The graft edge is clearly seen by slit lamp and on OCT. Graft and host thickness can be measured individually on OCT. Ultrasonic pachymetry will yield only the total thickness of the graft-host complex. The Visante TD-OCT demonstrates the entire width of the graft and easily penetrates deep into the anterior chamber with the longer 1310-nm wavelength. Some detail is lost in the lower resolution with the longer wavelength. Graft and host thickness can be easily measured at any point using the “Flap Tool” function (not shown) that was originally designed to measure LASIK (laser-assisted in-situ keratomileusis) flap thickness.
12.4.2 Case 2: Uncomplicated DMEK
In this case, a postoperative day 1 DMEK (Fig. 12.2) is still supported by an air bubble and fully attached as demonstrated by the Avanti FD OCT (Optovue, Freemont, CA). Note that the maximum scan diameter is 8 mm instead of 12 mm as in a Visante scan, accompanied by a corresponding decrease in scan depth. The FD OCT imparts higher resolution as a result of the shorter 830-nm wavelength of the scanning source. The DMEK graft adheres in such a seamless manner that it is nearly impossible to notice a difference between a graft and a virgin cornea.
This Visante TD-OCT demonstrates the normal appearance of a postoperative DMEK graft with good graft adhesion, centration, and function. This patient has no air left in the anterior chamber and a contact lens still in place, consistent with a postoperative day 5 OCT. The faint edge lift noted superiorly is inconsequential and may be observed in the course of normally scheduled postoperative visits. The surface hyperreflectivity in the AS OCT is due to the bandage contact lens placed at the conclusion of surgery because the patient underwent simultaneous superficial keratectomy.
12.4.3 Case 3: Small DMEK Detachment Requiring Observation Only
Two days after uncomplicated DMEK, this patient was found to have a localized inferior detachment of the graft (Fig. 12.3). Small detachments such as this one are common and typically inferiorly where the air bubble spends less time, even with good reported compliance to supine positioning. The detachment was fully resolved by postoperative day 5. TD OCT provides visualization of even subtle detachments and may be used to objectively follow their extent. Note the gentle curve of the detached portion of the DMEK graft toward the stroma, confirming that the graft is in the proper orientation.
12.4.4 Case 4: DMEK Detachment Requiring Air
In this example, 3 days after DMEK, a large undulating DMEK graft detachment is demonstrated under an area of corneal edema (Fig. 12.4a). This patient had an immediate benefit from an additional air injection and supine positioning. Careful inspection of the detached graft edge reveals a slight upward curl toward the corneal stroma. This finding confirms that the graft is in the correct orientation with endothelium facing the iris. In general, our criteria for air reinjection for DMEK are the following:
An absence of adequate chamber air bubble to cover completely a detachment (typically < 30 to 40% air bubble)
Graft detachment greater than two clock hours or an expanding detachment
Significant edema over a graft detachment
Graft detachments that threaten the visual axis
Small, nonprogressive, peripheral detachments without overlying stromal edema that do not threaten the visual axis may be safely observed.
In another example, a DMEK was done in a patient (Fig. 12.4b) who had a history of pars plans vitrectomy and a well-functioning trabeculectomy. The inability to shallow the anterior chamber during surgery in a patient without a vitreous body greatly increases the complexity and operating time of DMEK. In addition, a well-functioning trabeculectomy can quickly cause dissipation of even a full air fill, which can leave little air contact time for the graft and increase the chance of a need for repeat air injection. The use of 16% sulphur hexafluoride (SF6, a nonexpansile concentration) helps to increase the graft contact time of the inert gas, but caution must be used in the setting of a trabeculectomy. The inert gas absorbs much more slowly and can occlude the sclerostomy or fill the bleb with gas, resulting in a dangerous elevation of intraocular pressure. Inferior graft detachments are the most common, as patients often have trouble abiding by a strict supine positioning regimen. When upright, the bubble will continue to support the superior graft, but the inferior graft will be supported only when the patient is supine. Occasionally, supine positioning with a chin-up posture is required to ensure that the inferior aspect of the graft is fully supported by the air bubble. Face-down positioning should be avoided because it can lead to air migrating posterior to the iris.