Although the current study assessed only phase retardation, the local birefringence is another emerging readout of PS-OCT. Local birefringence measurements of blebs using PS-OCT were recently introduced.
28–30 Because the phase retardation does not accurately provide the depth-localized polarization property of the bleb, local birefringence analysis provides more accurate localization of the birefringence change than does phase retardation analysis and shows the precise location of scar tissue.
22,31 Postoperative surgical manipulation against the trabeculectomy bleb has become a recognized factor for increasing surgical success. Bleb needling provides a relatively simple approach to disrupt subconjunctival scar tissue and restore bleb function. The first needling revision against the bleb was described by Ferrer in 1941,
32 and several studies regarding the successful restoration of failing blebs have been performed.
33,34 Although previous AS-OCT measurements focused on various causes of obstruction of aqueous humor flow in the bleb (low reflectivity in the bleb walls, microcysts, fluid-filled cavities, and filtration scleral flap opening), a clear strategy to ensure successful needling revision or repeated filtration surgery has not been established.
12,24,27,35 Local birefringence imaging was recently used to monitor blebs before and after needling revision and remove the scar tissue.
28 A narrow cleft of the filtering bleb and highly birefringent scar tissue were found at the location of partial adhesion of the scleral flap before reconstruction of the bleb. The scar tissue was extracted and the region of the cleft was enlarged after bleb reconstruction. Because local birefringence imaging selectively visualizes the fibrosis tissue, it is useful to detect localized scar tissue surrounding the drainage path of the aqueous flow. Tsuda et al.
29 reported that a high birefringence region was identified in nonfunctional scarred bleb tissue with local birefringence analysis using PS-OCT. They also confirmed this finding with polarization microscopy of scar tissue resected during revision of the bleb.
29 On the other hand, phase retardation is a cumulative quantity of polarization information along depth, and it is therefore sensitive to small changes in polarization and useful to detect early changes in birefringence. For example, the average en face phase retardation of the posterior corneal surface was increased in patients with very early keratoconus due to changes in the lamellar structure of collagen fibers.
20 Phase retardation analysis emphasizes small changes by cumulative information and may be useful for small changes in birefringence. In the current study, deterioration of bleb function was preceded by alteration in phase retardation in the early postoperative period. We are planning to compare local birefringence with phase retardation in a future study.