Optical Coherence Tomography in Endothelial Keratoplasty

19 Optical Coherence Tomography in Endothelial Keratoplasty


Yuri McKee, Evan D. Schoenberg, and Francis W. Price, Jr.


19.1 Introduction


First described by Huang et al1 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 for an excellent adjunct to the diagnostic modalities available to the modern corneal surgeon. This chapter uses a case-based approach to demonstrate the utility of OCT in the management of endothelial keratoplasty (EK).


The initial iteration of anterior segment OCT (AS-OCT) was labeled time-domain OCT (TD-OCT). Carl Zeiss Meditec introduced the Visante TD-OCT as a tool for precise anterior segment 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 anterior segment 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 a phakic intraocular lens (IOL). Newer Fourier-domain OCT (FD-OCT) devices (e.g., Avanti, OptoVue; Cirrus, Carl Zeiss Meditec) use an 830 µm wavelength that is popular for retinal imaging. Although resolution improves to as good as 5 µm, the scan length is more limited. Special attachment lenses are required to image the anterior segment 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.


EK has been rapidly growing in popularity worldwide in the past several years 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 anterior segment with OCT has proven critical in many cases. Currently, the two most popular iterations of EK are Descemet stripping automated endothelial keratoplasty (DSAEK) and Descemet membrane endothelial keratoplasty (DMEK). In our practice DMEK is the preferred EK technique due to 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 in 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, penetrating keratoplasty, vitrectomy, and significant corneal edema that limits the view of the anterior chamber.


19.2 Preoperative OCT Imaging in EK


In the presence of significant corneal edema AS-OCT may provide critical information regarding the anatomy of the anterior segment and the suitability for EK surgery in a particular eye. Significant peripheral anterior synechiae, 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 regarding 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 DSAEK graft diameter.4 We prefer DMEK in most cases but will choose DSAEK if there is a very significant graft–host mismatch as evidenced on AS-OCT.


19.3 Intraoperative OCT Imaging in EK


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, coupling a TD-OCT unit with a dielectric mirror to a surgical microscope. This produced 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 Recently OCT has been incorporated into a commercially available surgical microscope in the Haag-Streit intraoperative OCT (iOCT) system. 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 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


19.4 Postoperative OCT Imaging in EK


The most popular use for OCT in EK is postoperative evaluation of the graft. 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 follow graft and host thickness during the immediate postoperative period11,12 or later during episodes of graft rejection or graft failure. Other potential complications, including epithelial ingrowth,13 interface opacification,14 and retained Descemet membrane,15 may also be elucidated with AS-OCT.


A recent study by Dr. Gerrit Melles’ group (Yeh et al.) 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 to 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 spontaneously reattach, whereas grafts detached at both time points were less likely to undergo spontaneous reattachment.16 This provides 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 achieves visual improvement sooner and is not associated with decreased endothelial cell counts nor increased incidence of any complications.17


19.5 Case Studies of OCT in EK


19.5.1 Case 1: Uncomplicated Normal DSEK


In this case a normal postoperative DSEK is demonstrated ( Fig. 19.1). The central cornea is clear, and there is a thin, well-centered graft without folds or detachment. The graft edge is clearly seen at the slit lamp and on OCT. Graft and host thickness can easily be measured individually on OCT. Ultrasonic pachymetry will only yield 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 1310nm 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 flap thickness.



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Fig. 19.1 (a) Color photo of a normal Descemet stripping automated endothelial keratoplasty (DSAEK) after complete healing. (b) Time-domain optical coherence tomography of a normal DSAEK.


19.5.2 Case 2: Uncomplicated DMEK


Here a postoperative day 1 DMEK ( Fig. 19.2) is still supported by an air bubble and is fully attached as demonstrated by the Avanti FD-OCT (Optovue). Notice 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 due to 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 is still in place, consistent with a postop 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 (BCL) placed at the conclusion of surgery because the patient underwent simultaneous superficial keratectomy.



image

Fig. 19.2 (a) A normal postoperative day 1 Descemet membrane endothelial keratoplasty (DMEK) as seen on Fourier-domain optical coherence tomography (OCT). The air bubble is not visible on this image of the OCT, but the edge of a bubble can be seen in certain frames. Do not confuse the edge of an air bubble with an inverted graft or a graft detachment. (b) A normal DMEK on time-domain OCT at postoperative day 5.


19.5.3 Case 3: Small DMEK Detachment Requiring Observation Only


Two days status post–uncomplicated DMEK, this patient was found to have a localized inferior detachment of the graft ( Fig. 19.3). 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.


19.5.4 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. 19.4a). This patient demonstrated 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 as follows:



  1. Absence of adequate anterior chamber air bubble to completely cover a detachment (typically < 30–40% air bubble)
  2. Graft detachment > 2 clock hours or an expanding detachment
  3. Significant edema over a graft detachment
  4. 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. 19.4b) 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. This can leave very little air contact time for the graft and increase the chance of a need for repeat air injection. The use of 16% sulfur hexafluride (SF6), a nonexpensive concentration of an inert gas, 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 this can lead to air migrating posterior to the iris.


19.5.5 Case 5: Postoperative Bullae after DSAEK


Bullae and stromal edema can make evaluation of graft position difficult, especially with a thin-cut DSAEK or DMEK. If the involved area is covered by the graft, this may indicate graft dysfunction or failure. On the other hand, if the involved area is outside the graft, observation and temporizing measures, such as hypertonic saline drops, should be advised because endothelial cells may migrate from the graft over time. AS-OCT can demonstrate the graft position even through a hazy cornea while also visualizing the bullae themselves ( Fig. 19.5). In this case inferior corneal edema after a DSAEK proved to be related to a superior displacement of the graft but no graft detachment. Observation and conservative measurements eventually resulted in resolution of the edema.


May 28, 2018 | Posted by in OPHTHALMOLOGY | Comments Off on Optical Coherence Tomography in Endothelial Keratoplasty
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