Postnatal development of central corneal thickness in chicks of Gallus gallus domesticus

Objective To investigate the changes in corneal thickness that occur during maturation of the Gallus gallus domesticus chick eye over the first 450 days of life. Animals studied Twenty‐nine chicks, of which 12 were males and 17 were females. Procedures The central corneal thickness (CCT) was measured by ultrasonic pachymetry from hatch until 450 days of age. Segmented regression was applied to capture the two phases observed in the CCT plotted against age. Eye and gender were also included in the model. Results Mean CCT values initially decreased, with the lowest point being reached at around 12 days of age. CCT then gradually increased as the chick matured. At 70 days of age the animals have completed corneal development and reached the plateau value of 0.242 ± 0.0002 mm. CCT differences between gender or between left and right eyes were not statistically significant. Prediction equations for mean CCT according to the bird's age are presented. Conclusions There is an initial decrease in corneal thickness until approximately 12 days of age, which presumably mirrors maturation of corneal endothelial cell function. The pattern of changes in corneal thickness during the first phase of development of the chick CCT was similar to the one reported for dogs and humans. However, a unique feature of the development of CCT in chicks is that after reaching a plateau at 70 days corneal thickness did not significantly change over the remainder of the study period. Additionally, unlike in humans and dogs, there is no gender difference for corneal thickness in chicks.     

Postnatal development of corneal curvature and thickness in the cat

Objective To evaluate the postnatal development of central corneal curvature and thickness in the domestic cat. Animals studied Six Domestic Short‐haired (DSH) kittens starting at 9 weeks of age and 6 adult cats. Procedures Kittens were evaluated biweekly to monthly for a 12‐month period, starting at age 9 weeks. Corneal development was monitored by hand‐held keratometry and ultrasound biomicroscopy. Standard regression analysis using a nonlinear least squares method was used to generate a formula that would predict corneal curvature as a function of age. Results Mean keratometry (K) values for the 9‐week‐old cats were 54.51 (±1.02) diopters (D) and these values steeply declined over the next 3 months to 44.95 (±0.90) D. Thereafter, K‐values gradually decreased to reach a plateau by 12–15 months of age of 39.90 (±0.42) D. Because K‐values still appeared to be slightly diminishing at this point, six other > 2‐year‐old cats were evaluated by keratometry and were found to have K‐values of 38.99 (±0.81). Two to four diopters of astigmatism was common in young kittens whereas adult cats had a low mean degree of astigmatism (< 1 D). A formula that predicted keratometry values in diopters (K) as a function of age in weeks (w) was established as follows: K = 39.83 + 26.87 exp(?0.074 w). The central cornea increased in thickness primarily during the first 4 months of life with 9 week‐old kittens having values of 0.379 (±0.012) mm; 16‐week‐old kittens, 0.548 (±0.021) mm and 67 week‐old cats, 0.567 (±0.012) mm. Conclusions The maturation process of the feline cornea proceeds over the first 1–2 years of life to attain an adult status that is characterized by a roughly spherical state of approximately 39 D corneal curvature, substantially flatter than the human cornea, and a central thickness similar to the human cornea. Research studies of the refractive or optical properties of the cornea in which cats are used as experimental animals should be conducted on animals greater than 18 months of age.     

Effect of central corneal thickness on intraocular pressure with the rebound tonometer and the applanation tonometer in normal dogs

Objective To evaluate the effect of central corneal thickness (CCT) on the measurement of intraocular pressure (IOP) with the rebound (TonoVet^®) and applanation (TonoPen XL^®) tonometers in beagle dogs. Animal studied Both eyes of 60 clinically normal dogs were used. Procedures The IOP was measured by the TonoVet^®, followed by the TonoPen XL^® in half of the dogs, while the other half was measured in the reverse order. All CCT measurements were performed 10 min after the use of the second tonometer. Results The mean IOP value measured by the TonoVet^® (16.9 ± 3.7 mmHg) was significantly higher than the TonoPen XL^® (11.6 ± 2.7 mmHg; P < 0.001). The IOP values obtained by both tonometers were correlated in the regression analysis (γ² = 0.4393, P < 0.001). Bland–Altman analysis showed that the lower and upper limits of agreement between the two devices were ?0.1 and +10.8 mmHg, respectively. The mean CCT was 549.7 ± 51.0 μm. There was a correlation between the IOP values obtained by the two tonometers and CCT readings in the regression analysis (TonoVet^® : P = 0.002, TonoPen XL^® : P = 0.035). The regression equation demonstrated that for every 100 μm increase in CCT, there was an elevation of 1 and 2 mmHg in IOP measured by the TonoPen XL^® and TonoVet^®, respectively. Conclusions The IOP obtained by the TonoVet^® and TonoPen XL^® would be affected by variations in the CCT. Therefore, the CCT should be considered when interpreting IOP values measured by tonometers in dogs.     

Central corneal thickness in koi fish: effects of age, sex, body length, and corneal diameter

OBJECTIVE: To establish the central corneal thickness (CCT) of normal koi fish by ultrasonic pachymetry, and its relationship to age, sex, body length and corneal diameter. METHODS: Age, sex and body length of 33 koi fish (17 male and 16 female fish) were recorded. Horizontal and vertical corneal diameters of each eye were obtained using Jameson calipers. Central corneal thickness of all eyes was measured by ultrasonic pachymetry. Intraocular pressure (IOP) by rebound tonometry was obtained for a subgroup of nine koi (18 eyes). RESULTS: Mean central corneal thickness was 325.9 microm. Central corneal thickness of female koi was greater than CCT of male fish (P < 0.01). Central corneal thickness increased with increasing age overall and within both sexes (P < 0.01). Central corneal thickness increased with increasing body length (P < 0.001). For male and female fish, CCT increased with increasing horizontal and vertical corneal diameters (P < 0.01). Mean horizontal corneal diameter (HCD) was 8.05 mm, mean vertical corneal diameter (VCD) was 7.38 mm, and HCD was consistently greater than VCD. Mean IOP of a subgroup of these koi was 4.9 mmHg by rebound tonometry. CONCLUSIONS: Koi CCT increases with increasing age, body length and corneal diameter.     

In vivo measurement of central corneal thickness in normal chicks (Gallus gallus domesticus) with the optical low-coherence reflectometer

Objective To determine the practicability and accuracy of central corneal thickness (CCT) measurements in living chicks utilizing a noncontact, high‐speed optical low‐coherence reflectometer (OLCR) mounted on a slit lamp. Animals studied Twelve male chicks (Gallus gallus domesticus). Procedures Measurements of CCT were obtained in triplicate in 24 eyes of twelve 1‐day‐old anaesthetized chicks using OLCR. Every single measurement taken by OLCR consisted of the average result of 20 scans obtained within seconds. Additionally, corneal thickness was determined histologically after immersion fixation in Karnovsky’s solution alone (20 eyes) or with a previous injection of the fixative into the anterior chamber before enucleation (4 eyes). Results Central corneal thickness measurements using OLCR in 1‐day‐old living chicks provide a rapid and feasible examination technique. Mean CCT measured with OLCR (189.7 ± 3.34 μm) was significantly lower than histological measurements (242.1 ± 47.27 μm) in eyes with fixation in Karnovsky’s solution (P = 0.0005). In eyes with additional injection of Karnovsky’s fixative into the anterior chamber, mean histologically determined CCT was 195.2 ± 8.25 μm vs. 191.9 ± 8.90 μm with OLCR. A trend for a lower variance was found compared to the eyes that had only been immersion fixed. Conclusion Optical low‐coherence reflectometry is an accurate examination technique to measure in vivo CCT in the eye of newborn chicks. The knowledge of the thickness of the chick cornea and the ability to obtain noninvasive, noncontact measurements of CCT in the living animal may be of interest for research and development of eye diseases in chick models.     

Keratoprosthesis with retrocorneal fixation: preliminary results in dogs with corneal blindness

Objective To evaluate the use and complications of a penetrating keratoprosthesis implantation in the management of corneal opacification in dogs. Methods A retrospective clinical study describes the indications for the surgical technique utilized and the outcomes of this procedure in 20 eyes of 19 dogs with blindness of corneal origin. A successful surgical outcome was defined as a clear keratoprosthesis optic and improvement or restoration of functional vision over a follow‐up period ranging from at least 8 months to a maximum of 7 years. Results Eyes with total corneal opacification resulting from chronic superficial keratitis (n = 11), keratoconjunctivitis sicca (n = 5), endothelial dystrophy (n = 3) and chemical burn (n = 1) were treated by unilateral (n = 18) or bilateral (n = 1) full‐thickness implantation of a keratoprosthesis. Keratoprostheses were retained in 15 eyes (75%) which regained vision to the date of reporting. Among these eyes, six had uncomplicated postoperative course, five developed retroprosthetic membranes and four developed granulation tissue over the optic of the keratoprostheses. These complications were successfully removed surgically in the nine eyes. The five remaining eyes (25%) developed serious early postoperative complications, for which enucleation had to be performed. Conclusion In keratopathies in which the corneal opacification could not be treated by standard medical or surgical procedures, this keratoprosthesis appears to be promising to restore vision in chronic superficial keratitis and deep corneal dystrophy. It appears to have a poor prognosis in keratoconjunctivitis sicca in brachycephalic dogs. The post operative complications retro‐prosthetic membranes and granulomatous overgrowth could be treated well.     

A retrospective study of 30 cases of frozen lamellar corneal graft in dogs and cats

Frozen lamellar corneal grafts and nictitating membrane flaps were used in 18 dogs and 12 cats to repair deep corneal defects. In all dogs either melting corneal ulcers or descemetoceles were present. In the 12 cats, nine had either a melting corneal ulcer or descemetocele, two animals had acute bullous keratopathy, and one cat had corneal sequestrum. Initial vascularization with gradual clearing of the graft occurred during the first 45 days postoperatively. At 60 days postoperatively, all eyes were visual. Frequent postoperative complications included: focal dehiscence of the wound ( n  = 9); melting of part of the graft ( n  = 7); and pigmentation of the graft ( n  = 4). The frozen lamellar corneal graft was a very safe technique, and restored the tectonic and the optical function of the cornea. It provided the best results in corneas with nonperforating corneal defects. This technique provides poorer results when the cornea was perforated prior to surgery or during the surgical procedure.     

The use of preserved equine renal capsule to repair lamellar corneal lesions in normal dogs

The purpose of this study was to evaluate the use of equine renal capsule preserved in 98% glycerine to repair lamellar corneal lesions in normal dogs. For this purpose, 12 dogs, divided into six groups ( n  = 2), were used to evaluate the 1st to 7th day, 15th day and 30th to 60th postoperative day. In order to perform the histologic study, the clinical procedures were analyzed, while the recipient's corneas were collected. The photophobia and blepharospasm also were more intense in the 1st to 7th postoperative day, and regressed in the 15th postoperative day. Therefore, the edema and the vascular events were both more frequent in the intermediary phases and regressed in the late periods. On the other hand, the morphological evaluation demonstrated an inflamatory exudate, also in the intermediary and late periods. These results suggested that the equine renal preserved capsule could be a useful alternative tissue to repair lamellar corneal lesions in dogs.     

In vitro measurement of rabbit corneal epithelial thickness using ultrahigh resolution optical coherence tomography

The objective of this study was to reproducibly measure corneal epithelial thickness centrally and at the limbus in the rabbit cornea using ultrahigh resolution optical coherence tomography (OCT). Twelve freshly enucleated New Zealand white rabbit eyes were kept in a moist chamber at 4 degrees C. An ultrahigh resolution OCT system with a spatial resolution of 1.3 microm was used to image the cornea and its component layers. The central and peripheral (limbal) regions of all the samples were scanned within 6 h of harvest in order to minimize the post-mortem degradation of the corneal epithelium. The thickness of the corneal epithelium was determined by measuring the pixel equivalents of the obtained image. Unpaired Student's t-test was used to evaluate differences. The epithelial thickness centrally was found to be 45.8 +/- 2.2 microm, and 37.6 +/- 1.4 microm at the limbus (P < 0.001). Rabbit corneal epithelium is thicker centrally than at the limbus when measured by ultrahigh resolution OCT. This technique will aid in delineating the pathophysiology of diseases of the anterior cornea.     

The use of porcine small intestinal submucosa for the repair of full-thickness corneal defects in dogs, cats and horses

OBJECTIVE: To evaluate the efficacy of using a porcine small intestinal submucosa (SIS) graft covered by a conjunctival flap for the surgical repair of full-thickness corneal wounds in dogs, cats and horses. PROCEDURE: All records dating from August 1999 to February 2003 from Purdue University Veterinary Teaching Hospital of patients that had undergone ophthalmic surgical procedures and received a SIS corneal graft for a full-thickness lesion were reviewed. Fifteen cases were identified including six dogs, two cats and seven horses. Requirements for inclusion in this study were that SIS was used as a corneal graft in a full-thickness corneal defect and that the graft was completely covered with a conjunctival flap. RESULTS: Of the 15 cases, one canine patient had received SIS following removal of an epibulbar melanocytoma. The remaining five canine patients had undergone this surgical procedure for the repair of corneal perforation. The two feline patients had been presented for corneal perforation following chronic ulceration. One equine patient had been presented for a deep melting ulcer, three for stromal corneal abscesses, and three for corneal perforations. Complications encountered postoperatively included aqueous leakage, conjunctival flap dehiscence, synechia, cataract and fibrin in the anterior chamber. Fourteen out of 15 patients were visual at the final re-evaluation. CONCLUSION: SIS is an inexpensive, easy-to-handle biomaterial that appears to be suitable for the repair of full-thickness corneal wounds in dogs, cats and horses. Results of our study support the conclusion that this relatively new product is an effective alternative to traditional implantation materials utilized in veterinary ophthalmology.