Microscope-integrated intraoperative OCT facilitates PDEK
Real-time images delivered by i-OCT can improve the intraoperative profile of PDEK, thereby enhancing its anatomical and surgical success.
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Pre-Descemet’s endothelial keratoplasty is the latest entrant in the field of endothelial keratoplasty.
The advantages are mainly that we can use any donor age, and the manipulation of the graft inside the eye is much better than other procedures such as Descemet’s membrane endothelial keratoplasty. The guest authors of this column are Rinky Agarwal, MD, DNB, and Namrata Sharma, MD, DNB, MNAMS, of Dr. Rajendra Prasad Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi.
Amar Agarwal, MS, FRCS, FRCOphth
OSN Complications Consult Editor
Endothelial keratoplasty allows the replacement of dysfunctional host endothelium along with its Descemet’s membrane by donor endothelium and Descemet’s membrane with or without inclusion of additional stromal layers. Constant evolution of endothelial keratoplasty has occurred from deep lamellar endothelial keratoplasty at the advent to Descemet’s stripping endothelial keratoplasty and Descemet’s membrane endothelial keratoplasty in the present times.
While DLEK is considered an obsolete procedure, DSEK and DMEK are practiced extensively by corneal surgeons. PreDescemet’s endothelial keratoplasty is a newer variant of endothelial keratoplasty described by Agarwal and colleagues. As the name indicates, it involves the inclusion of the pre-Descemet’s layer, or Dua’s layer, along with Descemet’s membrane as a donor lenticule. Unlike DSEK, the harvested donor lacks a stromal component in this procedure and is therefore expected to cause fewer postoperative optical aberrations. In contrast to DMEK, PDEK involves less stringent donor criteria and a relatively less tricky donor lenticule preparation and manipulation inside the host anterior chamber.
Microscope-integrated intraoperative OCT (i-OCT) is a recently introduced modality for objective imaging of trans-surgical details. The instrument has a heads-up display unit, which enables simultaneous visualization of the anatomical specifics while operating on tissue. An incorporated OCT unit in the system provides real-time dynamic feedback of these tissue planes along with ocular structure-instrument interaction during surgical maneuvering. These high-resolution OCT scans play a pivotal role in surgical decision-making during various lamellar corneal surgeries, either anterior or posterior. We believe that this apparatus can flatten the learning curve of PDEK. We presently discuss our experience with this modality during PDEK (Figures 1 to 6).
Source: Rinky Agarwal, MD, DNB, and Namrata Sharma, MD, DNB, MNAMS
Role of i-OCT during donor preparation
While every step plays a significant role in anatomical and visual success of PDEK, the most crucial step remains the preparation of the donor. For this, an optical grade donor corneoscleral button of any age is selected and placed on a Teflon block with its endothelial side up. A 30-gauge needle attached to an airfilled 5 mL syringe is introduced from the edge of the button to the midperiphery of the graft. Air is then injected until a central shiny, domeshaped type 1 big bubble (BB) is formed.
Too deep a needle during this step might bloat the stroma, making it emphysematous enough to block further visualization, while a superficial needle passage may perforate the overlying tissues, thereby compromising antecedent BB formation by permitting an air leak. i-OCT scans can aid in overcoming this unpredictability by favoring precise estimation of needle depth and bevel orientation during its passage. In case multiple bubbles are formed, their depth and planes can also be well appreciated on this modality, irrespective of an emphysematous stroma. This guides accurate needle advancement toward the desired bubble, thereby allowing successful formation of a type 1 BB.
All of these maneuvers prevent inadvertent perforation of donor tissue by allowing a safe and controlled manipulation of needle depth, direction and movement during BB formation. Once formed, i-OCT can also aid in identification of the type of the bubble (type 1, 2 or 3) by focusing on its exact location in the donor tissue. While puncturing the extreme periphery of the BB with a side-port blade and further harvesting with corneoscleral scissors, correct position of instruments and any micro-attachments can be aptly shown by this imaging modality.
During automated preparation of the preDescemetic graft using a novel vacuumassisted microkeratome, similar to automated DSEK, i-OCT measured donor and lenticule thickness may guide selection of the appropriate microkeratome head and need for a second pass, respectively.
Role of i-OCT during recipient preparation
Host preparation during PDEK is similar to other endothelial keratoplasty procedures. In individuals uncooperative for a preoperative clinical evaluation, corneal clarity can be ascertained on the table before planning for the PDEK procedure. i-OCT particularly offers increased sensitivity with highly opacified corneas. The apparatus facilitates visualization of intracameral details such as iris defects, position and stability of the IOL, and presence of preexisting posterior capsule defect, pigments or vitreous in the anterior chamber without compromising the surgical sterility. Assessment of morphological features as well as present surgical wounds, such as site, size, intrastromal passage and amount of gaping, may guide the size and the position of the trephine chosen for superficial host marking as well as for punching the donor lenticule. Recognition of preexisting Descemet’s membrane tears or detachments as imaged on i-OCT can aid in averting any undue stromal manipulation during descemetorhexis. Any residual Descemet’s membrane tags analyzed by this imaging modality can facilitate their intraoperative removal, thereby improving the rate of surgical success.
Role of i-OCT during graft insertion
For injecting a PDEK graft inside the host anterior chamber, it is loaded on an IOL cartridge that is stripped of its spring to prevent any back suction of the donor lenticule. On insertion inside the host anterior chamber, the graft rolls like a DMEK graft with its endothelial side facing outward. i-OCT-assisted visualization of these rolled edges allows smooth unfolding of the lenticule in its correct orientation, thereby permitting minimal endothelial cell loss. Once unrolled, i-OCT can also back clinical assessment of residual interface fluid and the need for further superficial massage or stab incisions. Graft centration and the amount of air fill can also be ascertained simultaneously, thereby promoting further graft-related maneuvers. The adequacy of stromal hydration and wound apposition can also be established at the end of procedure on i-OCT, thus decreasing the incidence of postoperative wound leak.
Role of i-OCT during postoperative phase
In the postoperative phase, graft attachment is difficult to determine clinically and by use of conventional OCT unit in noncompliant individuals and infants who have difficulty with sitting position. When these patients are examined under general anesthesia, an i-OCT-equipped microscope can be employed to provide a comprehensive and magnified view of anterior segment structures. Any graft detachment/folds and interface fluid/debris can be documented, and rebubbling can be planned accordingly. If undertaken, a successful rebubbling can be achieved in a controlled fashion under the guidance of i-OCT, and donor folds, if needed, can be straightened mechanically at the same time. Not only this, i-OCT-assisted measurement of sequential host and donor thickness can be used to monitor primary donor failure in the immediate postoperative phase.
Limitations
Despite its inherent advantages, the practical use of i-OCT is hindered by many factors. Shadowing from metallic instruments may limit visualization of underlying structures, and surgery has to be halted for each required step. Changing the mode of microscope may reduce the field of vision, disturbing the surgical continuity. Cost and availability are other confines associated with the equipment. However, with worldwide acceptance and further advancements in OCT-compatible instrumentation, a new dimension to this technology is expected in the future.
Conclusion
To conclude, real-time images delivered by i-OCT can improve the intraoperative profile of PDEK, thereby enhancing its anatomical and surgical success. Successful achievement of surgical objectives with the aid of this technology can prevent inadvertent loss of a scarce and precious human donor cornea due to surgery-related complications. Continuous video monitoring and recording feature of this instrument can be further used for reviewing at a later date of the critical steps of the surgery. All of these can decrease the learning curve of PDEK by facilitating the surgeon’s judgment, technique and knowledge during procedural maneuvers.
- References:
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- For more information:
- Edited by Amar Agarwal, MS, FRCS, FRCOphth, director of Dr. Agarwal’s Eye Hospital and Eye Research Centre. Agarwal is the author of several books published by SLACK Incorporated, publisher of Ocular Surgery News, including Phaco Nightmares: Conquering Cataract Catastrophes, Bimanual Phaco: Mastering the Phakonit/MICS Technique, Dry Eye: A Practical Guide to Ocular Surface Disorders and Stem Cell Surgery and Presbyopia: A Surgical Textbook. He can be reached at 19 Cathedral Road, Chennai 600 086, India; email: aehl19c@gmail.com; website: www.dragarwal.com.
- Namrata Sharma, MD, DNB, MNAMS, can be reached at Dr. Rajendra Prasad Centre for Ophthalmic Sciences, AIIMS, New Delhi, India; email: namrata.sharma@gmail.com.
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