Bleb characteristics post-AGV surgery in dogs

Study Overview

• The study assesses bleb characteristics after Ahmed glaucoma valve (AGV) surgery in dogs
• Ultrasound biomicroscopy (UBM) is used to evaluate blebs and analyze their correlation with postoperative intraocular pressure (IOP)
• The study was conducted on sixteen eyes of thirteen canine subjects

Methods

• UBM examinations assessed bleb characteristics, including wall thickness and reflectivity
• IOP was recorded at the time of UBM imaging
• The duration from AGV surgery to UBM imaging was recorded
• Histological examination was conducted on an enucleated eye removed due to uncontrolled IOP

Results

• A significant correlation was observed between IOP and relative reflectivity (r = 0.60, P = .01)
• A negative correlation was observed between bleb wall thickness and relative reflectivity (r = -0.72, P = .002)
• No significant correlation was observed between the duration from AGV surgery to UBM imaging and either bleb wall thickness or relative reflectivity
• Histological examination revealed collagen-rich fibrous encapsulation of the bleb wall, including myofibroblasts with positive α-smooth muscle actin immunostaining

Clinical Relevance

• Less dense, thick-walled blebs on UBM tended to maintain IOP within the normal range in dogs that underwent AGV surgery
• Denser, thinner-walled blebs showed IOP levels above the normal range despite the use of antiglaucoma medications
• UBM is a useful tool for evaluating bleb characteristics and their influence on IOP regulation after AGV surgery in dogs

Canine Glaucoma and Treatment

• Canine glaucoma is a group of diseases characterized by retinal ganglion cell and optic nerve degeneration with elevated IOP
• There are limited methods for lowering IOP, such as reducing aqueous humor production, increasing outflow, or creating a bypass
• Surgical procedures that divert aqueous humor from the anterior chamber are considered when IOP becomes unresponsive to medical treatment
• Shunting surgery has been practiced in veterinary medicine since the 1980s, but prognostic research regarding postoperative blebs is limited

UBM

• UBM uses a 50-MHz transducer, providing higher resolution compared to standard ultrasonography
• UBM enables visualization of the ocular anterior segment with penetrability up to 5 mm
• In veterinary ophthalmology, UBM has various applications, including analyzing corneal and scleral thickness, measuring anterior chamber depth, and visualizing cystic or neoplastic changes
• UBM is extensively used in human medicine to evaluate blebs formed after filtering or shunting surgery

Enrollment specifics

• All procedures and animal care were conducted in accordance with guidelines approved by the Institutional Animal Care and Use Committee of Seoul National University
• Informed consent was obtained from all animal owners
• Patients diagnosed with primary angle-closure glaucoma and under follow-up after AGV surgery were enrolled in a prospective study conducted from June 2021 to September 2023
• All AGV surgeries were conducted in the superior temporal quadrant of the globe by a single board-certified veterinary ophthalmologist

Pre-operative and Post-operative Care

• Pre-operative individualized treatment plans included medications like topical carbonic anhydrase inhibitors and prostaglandin analogs
• Some patients were administered β-blockers
• Both topical and systemic NSAIDs were administered to all patients before surgery
• Postoperative care comprised topical medications, including antibiotics, mydriatics, anti-inflammatory drugs, lubricants, and antiglaucoma medications
• Intracameral tissue plasminogen activator injections were administered if postoperative complications were observed

Exclusion criteria

• Patients presenting with aqueous flare or complications related to the AGV implant were excluded
• Those with simultaneous AGV and cataract surgeries, AGV implant removed, or underwent salvage procedures were also excluded
• Patients who lost vision for reasons other than increased IOP were excluded

UBM Examination Protocol

• UBM examinations were conducted on patients who had undergone AGV surgery at least 6 weeks prior
• UBM scanning was performed under fixed lighting conditions with constant brightness, contrast, gain, and zoom settings
• After topical anesthesia, each dog was gently restrained, and the upper eyelid raised to avoid compression
• The probe was positioned at the margin of the bleb closest to the limbus, conducting scanning perpendicularly
• Sagittal sections of the bleb along the full length of the AGV body were captured, avoiding the simultaneous capture of the sclera and tube

UBM Image Analysis

• Bleb wall thickness, bleb wall reflectivity, and scleral reflectivity were measured using image processing software
• The bleb wall thickness was defined as the distance from the first reflective signal of the bleb fluid cavity to the top of the conjunctiva
• Pixel intensity quantified reflectivity, measurements taken within a 0.5 X 0.5-mm area, with background noise correction
• Relative reflectivity calculated as the ratio of bleb reflectivity to scleral reflectivity after background noise correction

Histological Examination Protocol

• The enucleated globe was preserved in 10% neutral buffered formalin, sagittally trimmed along the midline of the bleb and the AGV body, and processed for paraffin embedding
• H&E staining discerned general tissue structures, picro-sirius red staining identified collagen deposition
• Immunostaining for α-smooth muscle actin (α-SMA) performed to detect myofibroblasts, visualizing using 3,3'-diaminobenzidine, and Mayer hematoxylin

Statistical analysis

• Breeds, age, sex, the operated eye, duration from AGV surgery to UBM imaging, and IOP were collected
• Patient data and UBM measurements were expressed as the median and IQR
• Pearson product-moment correlation was used to assess relationships between variables
• Statistical significance was set at P < .05

Results demographics

• A total of 16 eyes of 13 dogs that underwent AGV surgery were included in the study
• Dog breeds were varied (American Cocker Spaniel, Shiba Inu, Dachshund, Poodle, Coton de Tulear, mixed breed)
• None underwent additional glaucoma surgeries
• Twelve eyes were receiving antiglaucoma medications at time of UBM imaging
• Median duration from AGV surgery to UBM imaging was 333 days
• Median IOP at the time of UBM imaging was 19 mm Hg

Bleb UBM parameters

• Some patients showed broad range values by a median of 93.92 for Bleb wall reflectivity
• Scleral reflectivity demonstrated less variability (median of 96.87)
• Relative reflectivity had a median of 1.06, and bleb wall thickness, 1.02 mm

Correlation Analysis findings

• No significant association between duration from AGV surgery to UBM imaging and bleb characteristics
• A significant correlation was observed with relative reflectivity (Pearson r = 0.60, P = .01) when assessing the correlation between IOP at the time of UBM imaging and bleb characteristics
• When assessing the correlation between bleb wall thickness and relative reflectivity, a significant negative correlation was observed

Discussion

• Prevent irreversible changes, and monitor the condition of the bleb, which are important for the long-term success of surgery
• Evaluate the blebs by knowing the variations in bleb characteristics and their impacts on postoperative outcomes
• Demonstrated a positive correlation with IOP and a significant negative correlation between bleb wall thickness and reflectivity in comparing devices

Findings in Human Studies

• Various methods have been introduced to assess the prognosis of surgery by evaluating blebs (Yamamoto et al and Pavlin et al)

Fibrosis

• Fibroblastic transformation is critically involved in bleb scarring and the impairment of aqueous humor drainage

Thickness

• The negative correlation identified between relative reflectivity and wall thickness suggested that an increase in bleb wall reflectivity, potentially indicative of fibrosis, was associated with a tendency for the wall to become thinner