Intra-operative optical coherence
tomography (iOCT) has the ability to enhance visualization and depth
appreciation during glaucoma surgery, especially with the gaining popularity of
Minimally Invasive Glaucoma Surgery (MIGS)—to facilitate targeted device
placement and fine surgical maneuvers in the angles, the sub-conjunctival layer
and the suprachoroidal space.
The iOCT allows the surgeon during trabeculectomy,
to ascertain the depth of the initial scleral incision, as well as to visualize
the regularity of the dissection and thickness of the scleral flap. While the
current state of technology does not allow precise depth identification, iOCT alters
the nature of the procedure from a ‘blind’ approach to a ‘visually guided’
approach. This is particularly important in eyes with high axial myopia, where
the thin sclera increases the risk of full-thickness dissection as well as
subsequent scarring, contributing to a higher rate of failure. The iOCT also
allows more accurate visualization of the sclerostomy and iridectomy during
trabeculectomy. iOCT may also be used as a tool for training residents to
perform glaucoma filtering surgery.
The iOCT allows the surgeon to perform
goniosynechiolysis without the use of intra-operative gonioscopy.
Post-synechiolysis, the iOCT may enable the surgeon to more objectively assess
the adequacy of synechiolysis and the effectiveness of the procedure in opening
the angle. However, in eyes with hazy corneas, the iOCT may be of limited
advantage, given that it shares similar limitations as a table-top SDOCT in
these scenarios.
The iOCT may be useful in bleb needling by
allowing the visualization of deeper tissue and structures within the bleb,
demonstrating the extent and location of adhesions and loculations, thereby
allowing targeted release of these areas of fibrosis. The iOCT further allows
objective assessment of surgical success, by identifying multiple cystic spaces
within a rising bleb. The iOCT also allows visualization of the often
concomitantly performed flap lift, which could be obscured by significant
encapsulation and tenon’s proliferation.
Swaminathan et al. described the use of
iOCT in guiding the GDD tube into the eye during surgery, demonstrating its
particular usefulness in the sulcus-placement of tubes. The direct visualization
of the tunnelling needle into the anterior chamber (AC) provides the surgeon
certainty of its entry into the AC. Conversely, the anterior movement of the
iris over the needle on iOCT indicates the entrance of the needle into the
sulcus. iOCT visualization may aid in reducing the risk of tube erosion after
GDD implantation by ensuring adequate coverage of conjunctiva and
partial-thickness sclera or donor tissue over the tube.
Siebelmann et al. reported the use of iOCT
as an adjunct to ab-externo canaloplasty surgery. The iOCT allows the visualization
of intra-operative structures, facilitating precise microcatheter passage
through the Schlemm’s canal. The iOCT may also allow direct visualization of
the suture tightening against the Schlemm’s canal—altering the shape of the AC,
from a concave, to an M-shape. The appreciation of the degree between the AC
and Schlemm’s canal upon suture tightening may allow the surgeon to titrate the
tension of the suture. Furthermore, the iOCT may help the surgeon minimize the
risk of complications from this procedure by reducing the risk of complete
penetration during dissection and minimizing the incidence of DM detachments.
Trabecular aspiration is an angle-based
MIGS which acts by clearing the inter-trabecular spaces of extracellular
debris, thereby improving aqueous drainage through the angle. iOCT-use would
theoretically facilitate more accurate siting of the aspiration cannula.
However, Heindl et al. has reported the limitations of the iOCT in its
application here—given the 840-nm wavelength of the iOCT, visibility of the
angle remains limited. In addition, the iOCT scans only vertically or
horizontally and may not allow precise imaging of the aspirator tip in relation
to the TM. OCT-guided trabecular aspiration using 1310 nm wavelengths has also been attempted in porcine eyes.
The ab-interno trabectome is another
angle-based MIGS device which may benefit from adjunct iOCT use, involving the
removal of trabecular tissue to increase aqueous outflow. The concurrent use of
a gonioprism or Swan–Jacobs gonioscopy lens and the iOCT has been shown to aid
the surgeon in visualizing the angle and in identifying the opening of the
inner wall of Schlemm’s canal. Tilting the microscope and the iOCT from the
regular 60° to a more horizontal position may provide better visualization for
the surgeon.
The iOCT may be useful during XEN45
implantation as it may help determine the final placement of the implant during
surgery. The use of iOCT may decrease intra-operative risks and optimize
placement of the implant. The iOCT may also be useful in visualizing the XEN45
implant in the event of sub-conjunctival hemorrhage, which obscures the
surgical field and hinders accurate placement of the implant, at times
requiring the surgeon to abort the procedure entirely.
Limitations of iOCT:
Firstly, as both stand-alone iOCTs and
microscope-integrated iOCT systems are expensive, therefore, cost and
accessibility remain a limitation in the use of this technology.
Secondly, adopting iOCT in one’s surgical
practice may involve a steep initial learning curve. The microscope-integrated
iOCT platforms require simultaneous viewing of both the surgical field and the
OCT image during surgery, through both oculars of the microscope.
Thirdly, the scanning raster of current
iOCT platforms has a restricted scanning area and this may require the surgeon
to constantly move the scanning zone during surgery to visualize the instrument
tip or area of interest.
Fourthly, optimal iOCT images may be
difficult to obtain. Current iOCT platforms do not respond well to movements of
the eye during surgery—motion artefacts are generated, and there is a
noticeable time lag in between real-time movement and the displaying of images.
A dry corneal surface, incorrect magnification and improper focus may also
compromise image quality.
Fifthly, structures posterior to metallic
instruments may not be visualized due to impedance of OCT light source
wavelength.
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