Texas Equine Veterinary Association

www.texasequineva.com • 8 when inflammation (uveitis) causes iris atrophy or adhesions from the iris to the cornea (anterior synechiae) or lens (posterior synechiae). Synechiae often result in an immobile or misshapen pupil. In some cases posterior synechiae may result in a miotic (constricted pupil). Sedation with detomidine hydrochloride or xylazine hydrochloride is often needed when evaluating painful eyes or non-compliant horses 9 . Furthermore, an auriculopalpebral block is important for eliminating upper lid mobility to facilitate examination 9 . A complete ocular exam should be performed following sedation and nerve block 1 . A direct ophthalmoscope or PanOptic ophthalmoscope can be used to complete an exam. Veterinary ophthalmologists often utilize a slit lamp biomicroscope and indirect ophthalmoscope for a more detailed assessment of the anterior and posterior segments respectively. Common clinical signs of glaucoma include conjunctival hyperemia, corneal edema, corneal vascularization, Descemet's striae (Haab's striae), and/or optic nerve atrophy 1,3,6 . Evidence of uveitis can also be present including keratic precipitates, aqueous flare, iris hyperpigmentation, corpora nigra atrophy, cataract, and/ or lens luxation 6 . Horses with glaucoma may not exhibit classic ocular discomfort (ex: squinting or tearing), which often delays the time to diagnosis as owners and handlers are unaware of a problem 1,4 . Increased intraocular pressure is both a cardinal sign of glaucoma and the primary target for clinical therapy 10-11 . Intraocular pressure measurement is called tonometry 10-11 . Tonometry is recommended for all horses with evidence of ocular disease except those with deep corneal ulcers and descemetoceles. When performing tonometry, it is important to consider the horse's head position, disposition, and what type of tonometer is used. The horse's head should be held above the heart to avoid significant intraocular pressure elevations associated with lower head positions 12 . The same sedation, nerve block(s), tonometer, and examiner should be used each time to achieve the most consistent follow-up measurements 1,4 . Tonometers report intraocular pressure based on either rebound or applanation principals. Rebound tonometers (ex: TonoVet) use a magnet to propel a small probe towards the eye 10. Daily calibration and topical anesthesia are not required with this method 10 . A small beep is elicited while taking the required six readings. Audible sounds can be silenced in the newer, TonoVet Plus, model which improves patient compliance. The mean intraocular pressure for horses with a TonoVet is 22.1 ± 5.9 mmHg (range 10-34 mmHg) 10 . Applanation tonometers (ex: TonoPen) Photo credit to Dr. Ellison Bentley, DVM, DACVO Figure 1: 18-year-old Thoroughbred gelding with glaucoma. Buphthalmos, corneal edema, and a mydriatic pupil are most evident.

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