Identification of a Single Zonular Fiber Preserving Aqueous Outflow Under Extensive Posterior Synechiae in a Bitch: The Potential Diagnostic Value of Ultrasound Biomicroscopy

Abstract

Background: This report describes a rare postoperative situation in which nearly circumferential posterior synechiae developed secondary to postoperative uveitis, yet pupillary block and intraocular pressure elevation were prevented. This outcome was attributed to a single anteriorly displaced zonular fiber that maintained focal separation between the iris and the anterior lens capsule, thereby preserving localized aqueous humor outflow. This study aimed to use ultrasound biomicroscopy to structurally characterize this atypical aqueous outflow pathway and to emphasize the clinical importance of identifying subtle postoperative outflow routes.
Case: A 13-year-old spayed bitch Pomeranian underwent phacoemulsification for an intumescent cataract in the right eye. Preoperative evaluations, including ultrasound biomicroscopy, ocular ultrasonography, specular microscopy, and electroretinography, revealed no abnormalities that would contraindicate surgery, aside from a narrowed iridocorneal angle. The procedure proceeded without complications and included placement of a capsular tension ring due to an intraoperatively identified zonular fiber suggestive of zonular instability, followed by in-the-bag intraocular lens implantation. In the immediate postoperative period, a slight leak from the main port caused transient hypotony, accompanied by a small corneal ulcer and anterior chamber inflammation. All of these complications responded to medical treatment. On postoperative day 14, a single zonular fiber was observed protruding anteriorly and contacting the corneal endothelium at the 5 o’clock position. At postoperative month 1, intracameral tissue plasminogen activator was administered; however, no change in the protruding zonular fiber was observed. By postoperative month 2, slit-lamp examination revealed nearly 360° posterior synechiae, yet the intraocular pressure remained within the low-normal range, and the iris maintained a flat contour. Ultrasound biomicroscopy demonstrated a small residual gap between the iris and anterior lens capsule at the 5 o’clock region, maintained by the anteriorly displaced zonular fiber. Because the eye remained comfortable and the zonular fiber appeared to preserve aqueous outflow, zonulectomy was not performed. Between postoperative months 4 and 6, anterior chamber inflammation gradually improved, intraocular pressure remained stable at 7 mmHg, and no glaucoma or retinal complications developed. However, focal corneal lipid deposition appeared at postoperative month 5, likely associated with long-term corticosteroid use, and was managed by adjusting medical therapy.

Discussion: This case demonstrates, based on ultrasound biomicroscopy findings, that a single zonular fiber can function as a mechanical spacer, preventing complete posterior synechial closure and maintaining aqueous humor flow in situations where pupillary block would otherwise be expected. While vitreous wick syndrome in humans also involves anteriorly displaced intraocular structures that affect aqueous dynamics, the mechanism in this case differed, as the thin zonular fiber appeared to preserve rather than obstruct aqueous passage. This case emphasizes the diagnostic value of ultrasound biomicroscopy in identifying subtle postoperative structural variations that may critically influence aqueous humor outflow. It also illustrates how imaging can clarify discrepancies between clinical expectations and physiological behavior, potentially preventing unnecessary interventions such as synechiolysis, iridectomy, zonulectomy, or early glaucoma surgery. Although limited to a single case, this report highlights the potential diagnostic value of ultrasound biomicroscopy in understanding atypical postoperative aqueous humor dynamics. It may inform imaging-guided clinical decision-making in similar cases in the future.

Keywords: anterior segment imaging, aqueous humor dynamics, cataract surgery, posterior synechiae, ultrasound biomicroscopy, zonular fiber.