Application of Central Corneal Thickness for the Diagnosis of Canine Glaucoma

Abstract

Glaucoma is second most disease causing vision loss in dogs. Retinal ganglion cell death caused by elevated intraocular pressure (IOP) and it leading to irreversible vision loss in dogs and humans. Early detection and regularly monitoring of glaucomatous dog is essential to maintain vision for a long time. Currently, detection and monitoring of glaucoma mainly rely on measurement of IOP by tonometers in dogs. Since introduction of the ultrasonic pachymeter, correlation between central corneal thickness (CCT) and glaucoma is widely investigated in humans. CCT is known to the most potent predictor of conversion of ocular hypertension into glaucoma in the Ocular Hypertension Treatment Study. Although, central and peripheral thickness of the cornea was already investigated, there has been little attention for correlation between glaucoma and CCT in dogs. Therefore, the purpose of the present study was to evaluate the possibilities of CCT for diagnosis and monitoring of glaucoma in dogs. This study consists of two chapters. Chapter I demonstrated the effect of CCT on IOP measurement by tonometers in normal dogs. Both eyes of 60 normal beagles dogs were used in this study. After ophthalmic examinations of both eyes, IOP was measured by the TonoVet, followed by the TonoPen XL in half of the dogs and the other half were applied reverse order. All CCT measurements were performed 10 minutes after the use of the second tonometer. There was a correlation between IOP values obtained by the two tonometers and CCT readings in regression analysis (TonoVet : p = 0.002, TonoPen XL : p = 0.035). The regression equation demonstrated that for every 100 ㎛ in CCT results in 1 and 2 mmHg elevation of IOP as measured by the TonoPen XL and the TonoVet, respectively. Chapter II evaluated changes in CCT according to experimental adjustment of intraocular pressure (IOP) in canine eyes. Both eyes of 25 clinically normal beagle dogs were used in this study. To adjust and measure IOP, each eye was cannulated with two 26-gauge needles under inhalant anesthesia. One needle was connected to a pressure transducer, and the other was connected to an adjustable bag of physiologic saline. IOP was stepwise increased from 10 mmHg to 70 mmHg in 10 mmHg increments (Group T). Also, IOP was maintained at 15 mmHg (Group C15), 30 mmHg (Group C30), 45 mmHg (Group C45), 60 mmHg (Group C60) and 75 mmHg (Group C75) during the experiment. CCT was measured with an ultrasonic pachymeter every 10 minutes after cannulation. According to increased IOP, the CCT showed an initial decrease and then an increase after passing the lowest point. Based on the results of the present studies, effect of CCT should be considered in IOP measurement with tonometers in dogs. Also, CCT itself changed with an increased IOP, exhibiting an initial decrease and then a subsequent increase in dogs. Therefore, it is suggested that CCT could serve a factor for diagnosis and monitoring of canine glaucoma.