Evaluation of equine corneal disease using spectral domain optical coherence tomography (SD‐OCT)

Effects of corneoconjunctival transposition, posterior lamellar keratoplasty and modified deep lamellar endothelial keratoplasty on streak retinoscopy in equine cadaver eyes

OBJECTIVE

Evaluate the effects of corneoconjunctival transposition (CCT), posterior lamellar keratoplasty (PLK) and modified deep lamellar endothelial keratoplasty (DLEK) on streak retinoscopy in equine cadaver eyes.

METHODS

Intraocular pressure (IOP) was maintained at 25 ± 3 mmHg in 35 equine cadaver eyes. Streak retinoscopy was performed prior to (NO VISCO) and following (VISCO) intracameral injection of 1.0 ml of viscoelastic. Following optical coherence tomography (OCT), an axial CCT [n = 11] or PLK [n = 12], or peripheral DLEK [n = 12] was performed. Streak retinoscopy and OCT were repeated postoperatively.

RESULTS

Postsurgical net meridional and spherical refraction for CCT [Horizontal (H): 3.4 (95% CI 2.4-4.4) diopters (D), p < .001, Vertical (V): 3.5 (95% CI 2.7-4.4) D, p < .001, Spherical: 3.5 (95% CI 2.6-4.3) D, p < .001] and PLK [H: 3.2 (95% CI 2.2-4.1) D, p < .001, V: 2.8 (95% CI 2.0-3.6) D, p < .001, Spherical: 3.0 (95% CI 2.2-3.8) D, p < .001] were significantly increased from presurgical (VISCO) values. No difference between presurgical (VISCO) and postsurgical values were identified for the DLEK. Postoperative OCT revealed space between anterior and posterior corneal grafts and corneal undulation associated with suture placement.

CONCLUSION

Meridional and spherical refraction had a hyperopic shift following CCT and PLK, with a significantly higher value in the vertical meridian for the CCT. Pre- and postoperative refraction was not significantly different for the DLEK. This supports that the DLEK has a lesser effect on immediate postoperative refraction compared with CCT or PLK.

Corneal thickness and anterior chamber depth of the normal adult horse as measured by ultrasound biomicroscopy

OBJECTIVE

To determine corneal thickness (CT) and axial anterior chamber depth (ACD) using ultrasound biomicroscopy (UBM) in normal adult horses. To compare corneal thickness measurements between UBM and ultrasonic pachymetry.

ANIMALS STUDIED

Sixty eyes of 30 healthy adult horses aged 8-24 years.

PROCEDURES

Ultrasonic pachymetry (velocity of 1640 m/s) was utilized to obtain measurements of the central, superior, temporal, inferior, and nasal cornea. Triplicate images of the same corneal locations were acquired using UBM (50 MHz). Images of the axial anterior chamber were used to measure ACD. Intraocular pressure (IOP) was estimated using rebound tonometry, and axial globe length was measured using ultrasonographic biometry.

RESULTS

CT (mean ± SD µm) measured by UBM was 854 ± 61 (central), 994 ± 58 (superior), 930 ± 57 (temporal), 979 ± 55 (inferior), and 898 ± 48 (nasal). CT measured by UBM was greater than that measured by ultrasonic pachymetry at all locations and was statistically significant at all locations except inferior (p = 0.0006-0.048). No sex nor age effect was detected for CT at any location. The repeatability of ultrasonic pachymetry was superior to that of UBM. Mean ± SD ACD was 5.74 ± 0.41 mm. A weak positive correlation was identified between central CT and IOP and between central CT and axial globe length.

CONCLUSIONS

Normal data for CT and ACD of the adult horse obtained using UBM are provided. CT determined by UBM was greater relative to pachymetry at all corneal locations.

The feasibility of clinical evaluation for anterior uveitis through spectral-domain optical coherence tomography in dogs

OBJECTIVE

To evaluate the clinical application of spectral-domain optical coherence tomography (SD-OCT) for anterior uveitis in dogs.

ANIMALS AND PROCEDURES

Client-owned dogs presenting with anterior uveitis and clinically healthy dogs were enrolled in this study. Included eyes were divided into 5 groups by flare grade and 3 groups by cell grade through slit-lamp biomicroscopy. Each eye was examined using SD-OCT following slit-lamp biomicroscopy. The ratio of aqueous signal intensity to air signal intensity, which is called the aqueous-to-air relative intensity (ARI) index, was used to evaluate the flare grade. Cell number, central corneal thickness (CCT), and the presence of keratic precipitates (KPs) were analyzed on SD-OCT. The OCT parameters, including ARI index, cell number, and CCT, were compared to the slit-lamp clinical flare and cell grade.

RESULTS

Thirty-six eyes with anterior uveitis and 27 healthy eyes were enrolled. The ARI index showed a significant correlation with clinical flare grade (r_s  = 0.811, p < .001). In multiple regression analysis, the ARI index and CCT showed a significant negative correlation (r = -0.258, p = .044). The number of cells on SD-OCT significantly increased with cell grade on slit-lamp biomicroscopy (r_s  = 0.653, p < .001). The clinical flare grade and CCT were significantly correlated in the partial correlation analysis after controlling for age (partial correlation coefficient = 0.471, p = .002). KPs were observed in 61% of the eyes with flare using SD-OCT (22/36 eyes).

CONCLUSIONS

Spectral-domain optical coherence tomography could provide quantitative information, including the ARI index, cell counts, and CCT in dogs. SD-OCT is an auxiliary modality for slit-lamp biomicroscopy when evaluating anterior uveitis in dogs.

Comparison of image quality of corneal and retinal optical coherence tomography using sedation and general anesthesia protocols with or without retrobulbar anesthesia in horses

OBJECTIVE

To compare image quality and acquisition time of corneal and retinal spectral domain optical coherence tomography (SD-OCT) under 3 different sedation-anesthesia conditions in horses.

ANIMALS

6 middle-aged geldings free of ocular disease.

PROCEDURES

1 randomly selected eye of each horse was evaluated via SD-OCT under the following 3 conditions: standing sedation without retrobulbar anesthetic block (RB), standing sedation with RB, and general anesthesia with RB. Five regions of interest were evaluated in the cornea (axial and 12, 3, 6, and 9 o'clock positions) and fundus (optic nerve head). Three diagnostic scans of predetermined quality were obtained per anatomical region. Image acquisition times and total scans per site were recorded. Corneal and retinal SD-OCT image quality was graded on a subjective scale from 0 (nondiagnostic) to 4 (excellent).

RESULTS

Mean values for the standing sedation without RB, standing sedation with RB, and general anesthesia conditions were 24, 23, and 17, respectively, for total cornea scan attempts; 23, 19, and 19 for total retina-scan attempts; 14.6, 13.2, and 9.2 minutes for total cornea scan time; 19.1, 9.2, and 13.0 for total retina scan time; 2.0, 2.3, and 2.5 for cornea grade; and 2.7, 2.9, and 2.5 for retina grade.

CONCLUSIONS AND CLINICAL RELEVANCE

The RB facilitated globe akinesia and improved the percentage of scans in frame and region of interest accuracy for retinal imaging via OCT in horses. Retrobulbar blocks improved clinical image acquisition while minimizing motion artifact.

Evaluation of equine corneal disease using ultrasound biomicroscopy

OBJECTIVE

To describe the clinical use of ultrasound biomicroscopy (UBM) to evaluate equine corneal disease.

METHODS

Images were obtained using a 50-MHz probe ultrasound biomicroscopy system (Quantel Aviso) and Clear Scan® probe cover. Six horses with corneal disease were evaluated via UBM for lesion size, lesion depth, and continuity of Descemet's membrane. Horses were sedated and received auriculopalpebral nerve blocks and application of topical anesthetic prior to UBM.

RESULTS

Ultrasound biomicroscopy was easily performed in all cases. UBM evaluation of three cases of corneo-limbal squamous cell carcinoma yielded information regarding lesion depth for planning of keratectomies using fixed-depth keratomes and subsequent β-radiation therapy. Corneal depth and continuity of Descemet's membrane were determined in two horses with stromal abscesses and allowed for planning of therapeutic options. In one horse with a corneal foreign body, UBM contributed to accurate assessment of the foreign body's stromal depth, which could not be assessed during ophthalmic examination due to extensive corneal cellular infiltrate. The information regarding corneal depth allowed for more accurate pre-surgical planning in patients with opaque corneal lesions.

CONCLUSIONS

Ultrasound biomicroscopy was easily performed and provided useful information regarding lesion depth and continuity of Descemet's membrane for patients with corneo-limbal squamous cell carcinoma, stromal abscesses, and a corneal foreign body, allowing for increased precision in pre-surgical planning and development of therapeutic protocols.