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Intraoperative Optical Coherence Tomography in DALK
Kristin E. Hirabayashi, MD and Charles C. Lin, MD
Deep anterior lamellar keratoplasty (DALK) is an excellent surgical option in patients with corneal pathology that have preserved endothelial function, such as keratoconus, stromal scarring due to infectious and noninfectious conditions, and corneal dystrophies. Compared to penetrating keratoplasty (PK), it offers a variety of immunological and mechanical advantages. Preservation of the host endothelium decreases the risk of allograft rejection and can also lessen the reliance on postoperative topical steroids.1,2 This can be a critical advantage in patients who are phakic, steroid responders, or poorly compliant. DALK has also been shown to have less endothelial cell loss after surgery compared to PK.1,3–6 There are obvious safety advantages to avoiding the open-sky component of PK, especially mitigating the risk of suprachoroidal hemorrhage. Furthermore, DALK offers the potential for faster visual rehabilitation as sutures can be removed earlier in the postoperative course compared to PK.5 Finally, in terms of graft survival, DALK has been shown to have fewer rejection episodes compared to PK.1,7
Although DALK carries significant advantages over PK, the technical challenges of DALK have limited more widespread adoption. Prior studies have reported mixed conclusions regarding the visual outcomes of DALK compared to PK and surgeon experience plays a significant role.1,2,4,7–9 The American Academy of Ophthalmology does recommend DALK as a preferred technique.10 Baring Descemet’s membrane (DM), or leaving minimal residual posterior stroma, underlies successful DALK surgery. In fact, leaving a residual stromal thickness of 20 μm or less correlates with superior visual outcomes.11,12 Intraoperative optical coherence tomography (iOCT) can help achieve this goal by providing visualization of cross-sectional anatomy to guide cornea stromal dissection.
Figure 8-1. A microcyclodialysis spatula partially opens the trephination groove to reveal a penetration depth approximately two-thirds of the corneal stroma (red arrow), best appreciated on the horizontal section of the OCT cross-hair image.
INTRAOPERATIVE OPTICAL COHERENCE TOMOGRAPHY APPLICATIONS FOR DALK
OCT is a non-contact imaging modality that uses light to discriminate fine details in anatomical structures on the order of microns. iOCT enables a real-time anatomical cross-sectional view of the cornea, which can provide invaluable information to a surgeon performing DALK. There are currently 2 iOCT models on the market. First, the Zeiss Rescan (Carl Zeiss Meditech) has an OCT built into the operating microscope, and the OCT images can be viewed on an attached screen or through a heads-up display in the oculars. It is a spectral domain OCT that uses light with a wavelength of 840 nanometers and has an axial resolution of 5.5 μm in tissue. It provides real-time high resolution 1-line, 5-lines, and cross-hair images and can capture 1-line, 5-lines, and cube scans. Second, the Leica EnFocus OCT (Leica Microsystems) comes in 2 forms: Ultra-HD OCT and Ultra-Deep OCT. The Ultra-HD OCT uses light with a wavelength of 860 nanometers and has an axial resolution of 4 micrometers, and the Ultra-Deep OCT uses light with a wavelength of 880 nanometers and has an axial resolution of 9 micrometers. It can perform line, rectangular, annular, and radial scans. Similar to the Rescan OCT, it is integrated into an operating microscope, and the OCT images can be viewed through the oculars.
iOCT may be helpful in several steps of DALK surgery (Video 8-1). First, OCT can image the depth of the initial trephination. Using preoperative pachymetry, the trephination depth can be targeted to the deep stroma with caution to avoid premature perforation. iOCT allows real-time visualization of the depth of penetration and can help the surgeon determine if deeper trephination is needed13 (Figure 8-1).
Figure 8-2. At the trephine edge (red arrow), the iOCT image shows that the anterior one-third of the stroma has been removed.
Figure 8-3. The view through the operating microscope on the left shows severe stromal emphysema that makes assessing the thickness of the residual stroma challenging. The OCT image shows residual posterior stroma of varying thickness with intrastromal bubbles.
Following removal of the anterior stroma, OCT images can be used to assess thickness of the residual stromal bed, a task that is more difficult to ascertain with the usual en face microscope perspective (Figure 8-2). This can be particularly useful in cases with decreased visualization (Video 8-2), such as a stromal scar or stromal emphysema (Figure 8-3), as well as in severely ectatic keratoconic eyes with significant apical thinning and a higher perforation risk. It may also be useful in the Jacob modified technique of primary management of acute hydrops with pre-Descemetic DALK (described in detail elsewhere in this book) to determine depth of dissection through the edematous hydrops tissue.14,15
A critical step in DALK surgery is attaining a deep stromal tunnel prior to attempting a big bubble air injection. Regardless of whether the big bubble is attempted with a needle or Fogla cannula, positioning the tunnel in the deep posterior stroma increases the chance of a successful big bubble. Prior reports indicate that placement of the cannula less than 100 μm from DM maximizes the chance of successful big bubble formation.16,17 A superficial placement, as shown in Figure 8-4, often results in stromal emphysema, which may impair further visualization. Real-time OCT imaging during stromal tunnel creation can provide guidance for a more posterior position of the needle or cannula while preventing excessive tunneling and perforation (Figure 8-5 and Video 8-1).
Figure 8-4. The iOCT image locates the Fogla probe–dissected tunnel within the anterior half of the residual stromal bed (red arrow).
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