Ординатура / Офтальмология / Английские материалы / Corneal Endothelial Transplant (DSAEK, DMEK & DLEK)_John_2010

easiest to handle and unfold in the anterior chamber.6 OCT corneal imaging may have a place in the preand post-cut analysis of the donor corneal dimensions. As more eye banks cut and prepare DSEK buttons for the surgeon’s use, this additional information relating to the donor disk may be helpful to the end-user surgeons (Editorial note: “Surgery by surgeons,” meaning, surgeon-cut tissue in the operating room possibly provide the best surgeon controlled environment, as it relates to the donor corneal tissue.) (See also Chapter 19, Eye Banking and Donor Corneal Tissue Preparation in DSAEK, and Chapter 30, Use of Eye Bank Pre-cut Donor Tissue in DSAEK). The corneoscleral button is then trephined with or without the anterior corneal portion to a desired diameter and placed aside in the preservation medium from the eye bank, namely, Optisol GS. (Editorial note: Trephination of the donor disk with the anterior, cut, corneal cap in place, prevents the introduction of potential debris into the donor-host interface).

Then the recipient cornea is prepared for transplantation. Paracenteses are made in 4 oblique quadrants to provide access to the anterior chamber. Paracentral vents may be placed with a diamond knife for future removal of fluid or air in the interface (See also Chapter 27, Techniques to Facilitate Disk Adherence to Recipient Cornea in DSAEK). The epithelial surface is then marked with an 8.00 – 9.00 mm diameter circular mark to delineate the diameter of the descemetorhexis. Viscoelastic or continuous irrigation may be used to maintain the anterior chamber during recipient corneal surgery. Through a scleral tunnel or limbal incision, an instrument is then used to score the descemetorhexis and a scrapping instrument is used to strip off the Descemet’s membrane (See also Chapter 11, New/Useful Surgical Instruments in DSAEK). Alternatively a clinical femtosecond laser (Intralase, Irvine, CA) may be used to create the descemetorhexis at the desired position and diameter [See also Chapter 26, Femtosecond Laser (Intralase®)–Descemet’s Stripping Endothelial Keratoplasty (Femto-DSEK): Initial Studies of Surgical Technique in Human Eyes] prior to bringing the patient into the operating room. Attention is then redirected to the donor lamellar button, namely, the posterior lamellar button is carefully separated from the anterior cut-corneal cap and the viscoelastic is placed on the endothelial surface. (Editorial note: Avoid excess use of viscoelastic, as this can potentially interfere with donor disk attachment to the recipient cornea). The lamellar button is then folded endothelial side inward, grasped with forceps (See also Chapter 11, New/Useful Surgical Instruments in DSAEK) and then pulled or pushed into the anterior chamber. The button is unfolded endothelial side down (stromal side up) using air or balanced salt solution and attach to the bare stroma. Irrigation through the paracentesis may further position the disc. Residual viscoelastic is removed and air

is injected into the anterior chamber to force the graft to appose to the stromal surface. Fluid or air in the graft interface may be expressed through the paracentral vents.

The AC-OCT clearly demonstrates the postoperative image of the DSEK button and progression of stromal deturgescence. Figure 6-3 shows an image of the meniscusshaped DSEK button that has a tapered flange and is well apposed to the recipient stromal surface. Postoperative corneal deturgescence can be monitored with serial ACOCT (Figures 6-4A to F). Figures 6-4A and B is at postoperative month 5, while, Figures 6-4C and D is at postoperative month 8, and Figures 6-4E and F is at postoperative month 11. Note the progressive apical thinning of the cornea. Often, the host cornea becomes optically clear within about 3 to 4 weeks (See also Chapter 20, Endothelial Keratoplasty: A Step-by-Step Guide to DSEK and DSAEK Surgery, and Chapter 22, Endothelial Keratoplasty: Visual and Refractive Outcomes). At that point, refraction can be performed for spectacle correction. Unlike the high astigmatism encountered after PKP, there is minimal change in the refractive error after DSEK (See also Chapter 22, Endothelial Keratoplasty: Visual and Refractive Outcomes). Price et al has shown that postoperative corneal thickness does not necessarily correlate to optical clarity and as such, the thickness of the graft may not be as important for the overall success of the procedure.7

Figure 6-3: Graft of the lamellar button is nicely apposed to the recipient stroma. The lamellar button is meniscus-shaped with tapered flanges.

AC-OCT and DSEK Complications

The most common complication encountered in DSEK is detachment of the lamellar button, which has been reported in 15-30% of cases in the early postoperative period.7 This complication, however, can be addressed by repositioning the button and/or injecting more air into the anterior chamber (called rebubbling). Figure 6-5A shows a detached DSEK button. The button was repositioned by manipulation through one of the paracenteses and rebubbled with air. Figure 6-5B shows the reattached button at postoperative month 1 that remained attached at postoperative month 3 (Figure 6-5C). Figures 6-5D to F shows the progressive deturgescence of the cornea.

Another complication is the presence of air or fluid in the graft interface. Figures 6-6A and B shows a thickened

Figures 6-6A to C: (A-B) Example of thickened recipient cornea secondary to (C) fluid in interface.

cornea after DSEK. In this patient, fluid was noted in the graft interface, shown in some sections on AC-OCT (Figure 6-6C). Fluid or air can be evacuated postoperatively through the paracentral vents made during surgery.

Epithelium or blood may also be present in the graft interface after DSEK. On AC-OCT the presence of such material is highly reflective. Figures 6-7A and B shows a highly reflective area in the graft interface. On examination, the patient was noted to have epithelial ingrowth in the nonvisual axis (Figure 6-7A), presumably from the recipient

Figures 6-7A and B: (A) Epithelial ingrowth in the interface presumably from recipient at postoperative month 9. This layer is highly reflective on AC-OCT (B, arrows).

cornea, introduced during surgery. Figure 6-8A shows blood at the donor-recipient interface border and on ACOCT (Figures 6-8B and C) there is a highly reflective area (arrow) correlating with the blood in the interface seen clinically (Figure 6-8A).

Although the presence of epithelium or blood may not ultimately affect attachment of the donor button, other interface abnormalities can lead to persistent detachment. Figure 6-9A shows a detached button and upon closer examination, there is a thin highly-reflective membrane at the interface (Figures 6-9B and C). Despite repositioning and rebubbling, the button in this patient remained detached. Histopathology showed the presence of PASstaining Descemet’s membrane at the interface (Figure 6-9D, arrows). In another patient, the graft was detached and free-floating on the first postoperative day (Figure 6-10A) and on AC-OCT, the graft was noted to be thickened on one end (Figure 6-10B). Histopathology revealed that

the button had full-thickness epithelium, Bowman’s layer, stroma, and Descemet’s membrane at one margin with epithelial ingrowth into the interface (Figure 6-10C). This complication occurred most likely from uneven lamellar dissection with the microkeratome. Both these cases demonstrate the usefulness of AC-OCT images that are highly correlative to the histopathological findings.

Conclusion

Insummary,therearemanyapplicationsfortheAC-OCTin DSEKsurgery.Thepreoperativethicknesscanbegaugedby the AC-OCT and compared to postoperative thicknesses. Theplanecreatedbythemicrokeratomeinthedonorbutton preparation can be shown by AC-OCT. On AC-OCT the donor lamellar button is meniscus-shaped with tapered flanges. Progressive deturgescence of the recipient cornea canbefollowedbyAC-OCT.Finally,complicationsofDSEK surgery, such as detachment and presence of interface material can be easily imaged with AC-OCT. Clearly, ACOCT is an invaluable tool in DSEK surgery.

References

1. Huang D, Swanson EA, Lin CP, et al. Optical coherence tomography. Science 1991;254:1178-81.

2. Baikoff G, Lutun E, Ferraz C, Wei J. Static and dynamic analysis of the anterior segment with optical coherence tomography. J Cataract Refract Surg 2004;30:1843-50.

3. Dawczynski J, Koenigdoerffer, Augsten R, Strobel J. Anterior optical coherence tomography: a non-contact technique for anterior chamber evaluation. Graefe’s Arch Clin Exp Ophthalmol 2006; Epub ahead of print.

4. Maldonado MJ, Ruiz-Oblitas L, Munuera JM, Aliseda D, GarciaLayana A, Moreno-Montanes J. Optical coherence tomography evaluation of the corneal cap and stromal bed features after laser in situ keratomileusis for high myopia and astigmatism. Ophthalmology 2000;107:81-87.

5. Baikoff, G, Bourgeon G, Jodai HJ, et al. Pigment dispersion and Artisan phakic intraocular lenses: crystalline lens rise as a safety criterion. J Cataract Refract Surg 2005;31:674-80.

6. Culbertson WW. Descemet stripping endothelial keratoplasty. Int Ophthalmol Clin 2006;43:155-68.

7. Price FW, Price MO. Descemet’s stripping with endothelial keratoplastyin 200 eyes: early challenges andtechniques to enhance donor adherence. J Cataract Refract Surg 2006;32:411-8.

Introduction

Ultrasound is a widely used technique for clinical diagnostic imaging. It is advantageous because it does not involve use of ionizing radiation, is relatively inexpensive, provides real-time imaging and is very portable. Ultrasonic imaging offers a view of tissue structures that are otherwise hidden by optically opaque overlying structures (Editorial Note: With optical coherence tomography (OCT) structures behind pigmented tissue are not visible.) [See also Chapter 4, Optical Coherence Tomography (OCT) of the Anterior Segment, Chapter 5, Optical Coherence Tomography in Corneal Implant Surgery, and Chapter 6, Use of Optical Coherence Tomography in Descemet’s Stripping with Endothelial Keratoplasty (DSEK) and Descemet’s Stripping Automated Endothelial Keratoplasty (DSAEK)]. Its application to the eye can provide images with almost microscopic resolution.

Sound waves are propagating disturbances in the density of a medium. These waves are characterized by their wavelength and frequency, the product of which is equal to the speed of sound. The speed of sound varies with the composition of the medium and other factors, such as temperature. Ultrasound is defined as any sound of frequency above the range of human hearing, i.e. approximately 25,000 cycles/second or more. In diagnostic ultrasound, frequencies in the megahertz (MHz = millions of cycles per second) range are used. Ultrasonic imaging systems require a piezoelectric transducer that converts a voltage transient into an acoustic pulse. When the propagating ultrasonic pulse encounters a density transition, reflections occur. When these reflections, or echoes, reach the piezoelectric material of the transducer, small voltages are generated that are then amplified and displayed by the ultrasound system. The time between pulse transmission and echo return is used to determine the range from the transducer to tissue interfaces. Echo amplitude is proportional to the change in acoustic impedance (the product of density and speed-of-sound) across a tissue interface. A plot of echo amplitude as a function of range along one line-of-sight is called an A- scan. Two-dimensional B-mode images are produced by scanning the transducer orthogonally to the beam axis. Since the transducer orientation and range to each echo are known, a two-dimensional image can then be formed in which pixel brightness is proportional to echo amplitude.

In soft tissues, ultrasound travels at about 1540 meters/ second. Thus, an ultrasound pulse takes approximately 31 microseconds to go from the cornea to the retina and back (2 × 24 mm/1.54 × 106 mm/sec). The wavelength of an ultrasound pulse relates directly to obtainable resolution, and is defined as speed-of-sound/frequency. Thus, as

frequency increases, wavelength decreases and resolution improves. Most medical ultrasound systems operate at 1-10 MHz, equivalent to wavelength ranging from 1540 microns at 1 MHz to 154 microns at 10 MHz. While we always wish to obtain the best possible resolution in medical imaging, attenuation of ultrasound increases exponentially with frequency. Thus, abdominal exams would be performed at the lower end of the above range, while examination of superficial tissue structures, such as the eye, can be performed at 10 MHz or above.

Ophthalmic ultrasonography was first described by Mundt and Hughes in 19561 with many other papers and books regarding this technique appearing in succeeding years.2-8 Almost from its beginnings, B-mode ophthalmic ultrasonography was performed at 10 MHz, a frequency that provides a good balance between resolution and sensitivity for examination of the vitreous, retina and orbit. However, the best-case 150 micron resolution obtainable at 10 MHz is inadequate for imaging of the structures of the anterior segment, including the ciliary body, iris and cornea. In the early 1990’s, instruments employing much higher frequencies (35-50 MHz) for imaging of the anterior segment became available.9,10 This frequency range is referred to as very high-frequency ultrasound (VHFU). While the eye can be scanned using coupling gel through a closed eyelid at 10 MHz, VHFU scans must be performed with open lids and a fluid coupling medium to prevent attenuation by the lids. Attenuation also limits scan depth to the anterior segment. However, VHFU can provide an axial resolution as fine as 30 microns. This has allowed VHFU systems to provide superbly detailed images of anterior segment structures, even in the presence of optical opacities such as hyphema or corneal scarring, and allows imaging of structures such as the ciliary body that are otherwise hidden by the sclera or iris.

VHFU instruments are often called ultrasound biomicroscopes (UBM) after the first commercial system developed by Zeiss-Humphrey (Dublin, CA) and later Paradigm Medical Industries (Salt Lake City, UT). At the present time, handheld ophthalmic UBM systems are available from numerous companies.

VHFU imaging of the cornea has long been an area of special interest. The short wavelength of VHFU systems allowed resolution of Bowman’s membrane, and hence provided a means for layered corneal biometry. VHFU ultrasound was also found capable of readily visualizing the interface between the flap and residual stroma following laser in situ keratomileusis (LASIK).11 In the mid-1990’s, our laboratory developed signal processing strategies suitable for imaging and automated detection of these interfaces.12-14 By scanning the cornea in a series of parallel

planes, the depths of Bowman’s membrane, the flap and the posterior corneal surface could be measured and color maps representing the thickness of each layer produced. A shortcoming of that technology, however, arose from the specular nature of the corneal surface: outside the 3 mm central zone, echoes are deflected due to the curvature of the corneal surface and echo data not detected. To address this, we developed an arc-scan mechanism that maintained approximate normality and constant range between the ultrasound beam axis and the corneal surface.15 Using the arc-scan, the eye could be scanned in a sequence of meridians and layered pachymetry mapped over virtually the entire cornea. The first commercial system using the arc-scan for corneal imaging is the Artemis-2, developed by Ultralink, LLC (now merged with ArcScan Inc., Morrison, CO). A feature of this system is that unlike conventional B- scanners, the Artemis-2 incorporates optical means for eye fixation and visualization of eye position during scanning, a feature that is crucial for obtaining reproducible measurements.

Clinical Ultrasonic Imaging of the

Anterior Segment

In conventional 10 MHz B-mode imaging, the probe is placed in contact with the eyelid, or sometimes directly upon the globe after administration of a topical anesthetic. Because the transducer has a fixed focal length that is generally designed to fall near the retina, the anterior segment falls in the defocused near-field of the transducer. As demonstrated in Figure 7-1, the resulting image is nearly useless for evaluation of anterior segment structures. However, by standing off the transducer from the eye with a normal-saline waterbath, the focus can be placed on the anterior segment (Figure 7-2). This results in significant improvement, allowing depiction of anterior segment structures, although the resolution at 10 MHz limits the amount of detail that can be observed. Figure 7-3, a 20 MHz immersion scan image of the anterior segment, demonstrates a comparative improvement in resolution. The appearance of a normal anterior segment at 35 MHz is shown in Figure 7-4. Notice that while the anterior surface of the lens is sharply depicted, the equator is not seen. This is a consequence of the specularity of the lens surface and the oblique presentation of this surface at the equator. The anterior segment shown in Figure 7-5 has a somewhat shallow anterior chamber (1.71 mm from endothelium to lens surface) as well as an area of atrophic iris with narrowed angle temporally. While the lens equator is generally not visible, it can sometimes be seen if blood or

Figure 7-1: Image of the normal eye acquired using a 10 MHz sector B-scan in contact mode. Note the relatively poor definition of the anterior segment.

Figure 7-2: Image of the whole eye of a normal subject acquired using a 10 MHz sector B-scan in immersion mode.

inflammatory material is present, as shown in Figure 7-6. The appearance of the anterior segment with post-traumatic hyphema is shown in Figure 7-7.

Very high frequency ultrasound is of great value in evaluation of tumors of the iris and ciliary body. In many cases, a patient will present with a pigmented lesion in the angle (Figure 7-8), and the degree of involvement with the ciliary body is not apparent. Ultrasound can readily visualize the ciliary body and determine if melanoma is

Figure 7-3: Image of the anterior segment of a normal subject acquired using a 20 MHz sector B-scan in immersion mode.

Figure 7-4: Anterior segment view with a 35 MHz immersion arc-scan.

Figure 7-6: Imaging with a 35 MHz arc-scan axial (top) and temporal (bottom) of eye with endophthalmitis. Inflammatory material outlines the boundaries of the lens. The ciliary body is detached (arrow).

Figure 7-7: Anterior segment view with a 35 MHz arc-scan in a posttraumatic eye demonstrating hyphema and conjunctival thickening. Also note echoes just beneath anterior lens surface, suggesting cataractous change.

Figure 7-5: High resolution 35 MHz arc-scan of an anterior segment with shallow anterior chamber and area of atrophic iris (arrow).

Figure 7-8: Pigmented lesion seen in angle superiorly (arrow) is shown ultrasonically to involve both the iris and ciliary body.