—ORIGINAL—

Changes of Intraocular Pressure in Uveitic Horses Shinya WADA* Racehorse Hospital, Ritto Training Center, The Japan Racing Association (JRA), 1028 Misono, Ritto, Shiga 520-3085, Japan

Objective: To describe changes of intraocular pressure (IOP) in horses affected by uveitis. Procedure: Study of medical records of 9 racehorses with unilateral uveitis (4 acute and 5 chronic) where IOPs were measured using a handheld applanation tonometer. Results: At first tonometry of the 9 horses, IOPs of the affected eyes (16.5 ± 5.1 mmHg) were significantly lower than those of the contralateral eyes (23.9 ± 4.7 mmHg). Two of the acute cases had lower IOPs in the affected eyes compared to the normal eyes, and changes in the IOP were monitored during treatment. Although both cases improved clinically, IOP recovered only in 1 case. In 4 of the chronically affected cases, uveitic eyes had developed phthisis bulbi and 3 of these eyes showed lowered IOP whereas the rest had IOP equivalent to the contralateral eye. The other chronic case, which had been affected by nonulcerative keratouveitis, had lowered IOP in the affected eye for 6 months, however, IOP difference between the both eyes got smaller. Conclusions: IOP can decrease with variable extent and duration in equine uveitis patients as well as in dogs [7]. However, IOP values of the affected equine eyes in this study were often higher than those of uveitic canine eyes [7]. Minor changes of IOP in uveitic eyes could be detected more accurately by tonometry than by observing or palpating the globes. IOP difference greater than 5 mmHg in each individual was considered to be a useful indicator in diagnosis of equine uveitis. Although further study is required, tonometry is a useful method to understand equine uveitis in detail, and should be included in complete ophthalmic examination especially in cases where the problem is longstanding and/or clinical progression is minor. Key words: horse, intraocular pressure, uveitis

Intraocular pressure (IOP) is dependent on equilibrium between the rate of formation and outflow of aqueous humor [18]. IOP in veterinary medicine has become readily measured since the introduction of applanation tonometers [6]. IOP is reported to be affected in diseases such as glaucoma and uveitis in dogs [7, 11]. Tonometry has been also performed for diagnosis and prognosis of glaucoma in horses [3, 4, 8, 26, 27]. Glaucoma in the horse is being recognized with increased frequency by increased availability of applanation tonometers [3]. In dogs, IOP is described to decrease usually in eyes affected by uveitis [7]. In equine practice, incidence of uveitis is considerably This article was accepted May 2, 2006 *Corresponding author. Present address: Equine Clinic, The Horse Racing School, JRA 835–1 Ne, Shiroi, Chiba 270-1431, Japan. e-mail: [email protected]

J. Equine Sci. Vol. 17, No. 3 pp. 67–73, 2006

higher than that of glaucoma [8]. To the author’s knowledge, however, changes of IOP in equine uveitis are not documented except for limited description on digital tonometry [17]. The purpose of this report is to describe IOP changes of thoroughbred racehorses affected by uveitis in accordance with clinical progression.

Materials and Methods Medical records of the Racehorse Hospitals of Ritto and Miho Training Centers, The Japan Racing Association, were reviewed. Nine uveitic thoroughbred racehorses whose IOP had been measured were involved in this study. Ophthalmic examinations performed at admission varied among horses, including fluorescein dye stain,

68

S. WADA

direct and/or indirect ophthalmoscopy, and slit lamp biomicroscopy. Diagnosis of uveitis was based upon the presence of at least one of the following findings; miosis, aqueous flare, hypopyon, hyphaema, organized fibrin in the anterior chamber, anterior or posterior synechia, and phthisis bulbi. The IOP was measured in both eyes of each horse using a handheld applanation tonometer (Tonopen XL ; Medtronic Solan, FL). The time of day tonometry was performed varied among horses. Three readings of which deviation was less than 5% were obtained by touching the cornea several times with the tip of Tonopen XL, and then the readings were averaged. Auriculopalpebral nerve was blocked with 2% lidocaine, and topical 0.4% oxybuprocaine (Santen, Osaka) was instilled to facilitate the procedure in all cases. Xylazine was intravenously administered in intractable animals. Measurements were repeated on different days when necessary and trainers’ agreement was obtained. IOP values of both eyes were compared in each individual. IOPs between 16 and 30 mmHg were considered normal, and variations greater than 5 mmHg were considered abnormal [6]. Results of first tonometry in the 9 horses were compared between uveitic and normal contralateral eyes by the use of Wilcoxon t-test. Table 1 reports the details of the 9 horses.

Results Of the 9 horses involved in the study, 4 horses had unilateral acute uveitis and 5 had unilateral chronic uveitis. Duration from onset of ocular problems to IOP measurement was 1 (case 2) to 14 (case 4) days in acute uveitis cases. Case 5 had been affected by nonulcerative keratouveitis (NKU) for 1 month, whereas information on the duration of problem was unavailable in cases 6 through 9. At the first measurement, IOPs of affected and normal eyes in the 9 horses were 16.5 ± 5.1 and 23.9 ± 4.7 mmHg, respectively, and the difference was statistically significant (p