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SD-OCT used to evaluate anterior chamber inflammation
This method can be a valuable tool for objective calculation of anterior chamber reaction in the treatment and prognosis of uveitis.
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Anterior chamber inflammation can occur after cataract surgery. Slit lamp biomicroscopy has been the conventional method of visualizing the aqueous, but this method is limited because it is subjective. In addition, it is difficult to grade the severity of anterior chamber reaction in corneal edema or opacification. Lowder and associates were the first to visualize the anterior chamber reaction by optical coherence tomography. We were the first to do anterior chamber cell grading in uveitis using time-domain OCT.
Optical coherence tomography
Recently, OCT machines with sophisticated spectral-domain or Fourier-domain optics, which enable examination of both the anterior and posterior segments in a single setup, have been developed. This enables high-definition scanning of the anterior chamber. In SD-OCT, by reading the information of an A-scan simultaneously and analyzing the frequency spectrum of the interference between the reflected light and a stationary reference mirror, scan speed higher than time domain is possible. This permits greater imaging resolution and less distortion caused by patient movements. Moreover, its faster acquisition time enables volume-scanning and three-dimensional reconstruction. We used high-resolution SD-OCT for imaging anterior chamber cells in eyes with postoperative uveitis.
Technique
Cirrus HD-OCT (Carl Zeiss Meditec) imaging of the anterior chamber was performed on the first postoperative day. OCT images were taken with the patient in the sitting position. Anterior segment raster scan mode was used to visualize the anterior chamber. The line-scanning ophthalmoscope illumination of the retina mode was turned off, and the internal fixation target was centered. The scan was centered at the corneal vertex, and three scans were taken in each eye, in the central, superior and inferior quadrants (Figure 1). Kumar performed all the images.
Figure 1. SD-OCT imaging by anterior segment raster scan for chamber visualization (A: superior, B: central, C: inferior).
Images: Agarwal A
Figure 2. Clinical picture (A) and SD-OCT image (B) of a patient with postoperative corneal edema.
Figure 3. Slit lamp biomicroscopy (A) and SD-OCT image (B) of an eye with grade 4 cellular reaction and clear cornea.
Anterior chamber cells were seen as hyperreflective particles in the anterior chamber. The cells were quantified by an automated software algorithm. SD-OCT was able to identify anterior chamber cells in corneal edema (Figure 2), ranging from 496 µm to 852 µm. Slit lamp grading was possible in eyes with central corneal thickness ranging from 496 µm to 668 µm (Figure 3).
Discussion
Non-visualization of the anterior chamber postoperatively can occur due to varying degrees of corneal edema. Corneal edema can be epithelial edema due to high IOP or intraoperative endothelial cell damage in phacoemulsification or stromal edema due to stromal hydration after a prolonged surgery. Whatever might be the cause, the assessment of anterior chamber inflammation becomes difficult under such circumstances. Although laser flare meter has been used to count photons per square mm in the evaluation of anterior chamber flare, it may not help in an edematous cornea. Moreover, if the severity of corneal edema is high, anterior chamber visualization will be worse with slit lamp biomicroscopy. TD-OCT has been used for seeing anterior chamber cells in uveitic eyes. However, SD-OCT has not been used for grading anterior chamber cells. To our knowledge, there are no reports of anterior chamber reaction evaluation with SD-OCT in postoperative eyes.
SD-OCT has been shown to have greater sensitivity than TD-OCT. With conventional anterior segment TD-OCT, the axial resolution is 18 µm and the transverse resolution is 60 µm. Cells that are smaller than the resolution size may not be detectable in TD-OCT. The SD-OCT system used in the current study employs a higher-bandwidth light source, which provides a finer axial resolution of 5 µm. The sensitivity and higher resolution could lead to higher reflectivity of the cellular reaction. Thus, we were able to detect cells smaller than 18 µm.
Because there was no significant difference in anterior chamber cell grading between the slit lamp method and SD-OCT, the method can be a valuable tool for objective calculation of anterior chamber reaction in the treatment and prognosis of uveitis. Moreover, institutions that have a posterior segment SD-OCT system can use it for this purpose just by changing the scan mode option.