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Nalin J. Mehta; Analysis of Choroidal Changes resulting from Intraoperative IOP Variations during Vitreoretinal Surgery using Automated Intensity Plotting of C-Scan (en face) Spectral-Domain Optical Coherence Tomography (SD-OCT) Images. Invest. Ophthalmol. Vis. Sci. 2012;53(14):2140.
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To evaluate intra-operative choroidal changes as a function of intraocular pressure (IOP) from SD-OCT scans derived from a portable OCT device mounted on a microscope stand configured for intra-operative use, utilizing en face images derived from 3-D data acquisition during vitreoretinal surgery.
Using the iVue® SD-OCT in combination with the iStand®, a novel floor stand system (Optovue, Fremont CA) and a sterile commercial microscope drape, patient eyes undergoing pars plana vitrectomy under local anesthesia were scanned intra-operatively in the supine position at various times during surgery, with corresponding intraocular infusion pressures recorded from an Accuris® vitrectomy unit (Alcon, Ft. Worth TX). All patients were consented with an IRB-approved protocol. Multiple 6mm x 6mm 3-D scans, each consisting of 128 horizontal lines, were taken, and the scans with the best overall image quality and comparable centration were included. Three commonly-encountered intraocular pressure (IOP) levels were chosen for analysis: 25 mm Hg, 35 mm Hg and 45 mm Hg. En face images of the choroidwere derived from sections analyzed immediately below the retinal pigment epithelium (RPE) using Astra Image 3.0 Pro (Phase Space Technology, Victoria AU), image-processing software which automatically quantifies voxel brightness on a 0-256 grayscale, correlating this with corresponding frequencies, creating 3-D and histogram representations of the image’s brightness spectrum.
Five patient eyes were evaluated. Ages ranged from 68 to 84 years. Four eyes were pseudophakic, one with a crystalline lens. Surgical indications included macular puckering (three), macular hole (one) and vitreous hemorrhage (one). In all eyes, logarithmic histograms of the frequency of relative voxel brightness demonstrated a trend toward increased reflectivity from en face images with increasing IOP, consistent with decreased sub-RPE choroidal vascular density.
The iVue® with iStand® proves to be a practical method of both qualitative and quantitative intraoperative evaluation of the choroid as a function of IOP. Comparing intraoperative en face SD-OCT at various IOP levels shows a choroidal pattern consistent with a decrease in choroidal perfusion with increased IOP. These hemodynamic changes should therefore be a consideration during vitreoretinal surgery, especially in patients prone to ischemic chorioretinal damage.
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