Glaucoma in the horse – part I

Glaukom beim Pfrd - 1. Teil

Kellner S J

DOI: 10.21836/PEM19940205
Year: 1994
Volume: 10
Issue: 2
Pages: 107-113

Glaucoma in the horse has rarely been established up till now which may be attributed to an additional supraciliary meshwork and its resulting uveoscleral outflow of aqueous humour and to difficulties in determining the normal, let alone the elevated, intraocular pressure in the horse. The anatomy of the ciliary body and the iridocorneal angle are discussed as well as the aqueous drainage pathways, especially the uveoscleral flow. Electronic applanation tonometry with the Tono-Pen gives accurate readings of 14 and 23 mm Hg in normal eyes anaesthetized with 0.5% proxymetacaine hydrochloride. Glaucomatous eyes usually showed pressure readings of 35 to 45 mm Hg, however readings above 25 mm Hg should be considered abnormal. Acute glaucoma in the horse presents with elevated intraocular pressure, blepharospasm, epiphora, congested episcleral vessels and ocular pain. In the early stages the pupil is responsive to light and only slight corneal edema is present. Later the pupil is fixed and delated, corneal edema is increased and the optic nerve may become cupped. Corneal erosions are seen quite often. In the chronic state blindness may develop, corneal edema is severe and a superficial vascular keratitis develops. Intraocular pressure is still elevated but the eye is not all that painful any more. In many causes glaucoma is secondary to equine recurrent uveitis and intraocular pressure normalizes as soon as intraocular inflammation has been controlled. Other cases may lead to buphthalmic globes or phthisis bulbi with loss of vision. It is not clear if parasympathicomimetics and atropin are a good therapeutic choice. Atropin might enhance uveoscleral outflow but it can also produce an angle closure glaucoma and pilocarpin may exacerbate iridocyclitis. Both drugs should therefore not be considered first choice. The application of beta-adrenergic antagonists like timolol maleate 0.25% and metipranolol 0.3% twice a day is very effective in reducing intraocular pressure. Approximately half the glaucoma cases seen are secondary to intraocular inflammation. The etiology of the other cases could not be established sufficiently. There seems to be no predilection to age, sex or a certain breed.