Real-time image-guided surgery is spreading in different surgical fields for its potential to improve surgical outcome and numerous types of imaging devices are available today [1,2,3,4,5]. The purpose of this study is to evaluate the role of intraoperative ultrasound biomicroscopy (iUBM) and optical coherence tomography (iOCT) for monitoring morphological changes of anterior chamber (AC) structures during ab externo canaloplasty for treating open angle glaucoma (OAG).
Material and methods
Consecutive patients affected by OAG refractory to topical therapy were enrolled and underwent ab externo canaloplasty (iTrackTM, Ellex iScience, Inc., Fremont, CA, USA) between January and June 2017. The study was approved by hospital ethics committee and informed consent from all patients was obtained. Surgery was performed by one surgeon (R.F.). Demographic and preoperative parameters were as follows: age, gender, eye, glaucoma type, intraocular pressure (IOP), best-corrected visual acuity (BCVA), lens status, and axial length. IOP and BCVA were monitored at 1, 3 and 6 postoperative months. iOCT integrated on to a microscope (OPMI Lumera 700, RESCAN 700 spectral domain-OCT wavelength 840 nm, Carl Zeiss Meditec, Jena, Germany) and iUBM (iUltrasound iscience interventionalTM, Menlo Park CA, 80 megaHertz transducer frequency) were used for evaluating the morphological changes of AC structures and for measuring the diameters of Schlemm canal (SC), trabecular meshwork (TM) thickness and AC angle during the surgery.
Twenty eyes (20 patients) were recruited (Table 1). IOP decreased significantly during follow-up (from 27.3 ± 3.9 to 14.5 ± 2.5 mmHg). BCVA improved only in patients who underwent cataract surgery (6 out of 20).
iOCT allowed the surgeon to monitor SC and AC angle changes during microcatheter passage and viscodilation. During viscodilation SC changed its profile from oval to round-shape with hyporeflective content (Fig. 1a–c). The poor quality of iOCT imaging did not allow the observer to obtain accurate measurements.
TM thickness, SC diameters and AC angle were measured by iUBM. TM thickness mean was 92 ± 20.6 µm. Horizontal and vertical diameters mean of SC were, respectively, 410 ± 56 and 110 ± 23 µm during viscodilation. AC angle mean was reduced from 25.8° ± 11° to 24.5° ± 14° after tensioning suture and its profile changed from concave to acute. SC appeared as an anechogenic oval-shaped space. Microcatheter into SC appeared hyperechogenic. In one case endothelial detachment was detected during viscodilation (Fig. 1d–n).
iOCT allowed the surgeon to monitor morphological changes of AC structures during surgery but scanning acquisition was not immediate requiring continual adjustment of the focus and scanning point due to the eye movements during surgery that changed the depth of the plan of visualization. The limit of iOCT was the short penetration depth, responsible for shadowing of scleral tissue and of overlapping and inverted images. Anyhow, iOCT was the best option to obtain a high resolution for imaging superficial areas. Instead iUBM was useful in detecting deeper structures, giving sharper images to measure morphological changes of AC structures. However, the image acquisition required interrupting surgery and the contact of the probe with the eye.
Intraoperative real-time image-guided surgery represents the future even if it still requires further efforts to make the instrumentation functional and simple to use.
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Conflict of interest
The authors declare that they have no conflict of interest.
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Frisina, R., Meduri, A. Intraoperative real-time image-guided ab externo canaloplasty. Eye 33, 1510–1513 (2019). https://doi.org/10.1038/s41433-019-0447-6