BASICS
- The glaucomas are a group of diseases resulting from alterations of aqueous humor dynamics that cause an IOP increase above that which is compatible with normal function of the retinal ganglion cells and optic nerve
- Glaucoma in horses is being recognized with increased frequency, although the prevalence of glaucoma in the horse is surprisingly low given the horse's propensity for ocular injury and marked intraocular inflammatory responses
- All glaucomas consist of 5 stages: (1) an initial event or series of events that influence the aqueous humor outflow system; (2) morphologic alterations of the aqueous outflow system that eventually lead to aqueous outflow obstruction and IOP elevation; (3) elevated IOP or ocular hypertension that severely reduces retinal ganglion cell sensitivity and function; (4) subsequent retinal ganglion cell and optic nerve axon degeneration; and (5) progressive visual deterioration that eventually leads to blindness
- The glaucomas are frequently categorized into primary, secondary, and congenital types. While all types of glaucoma have a causative mechanism, primary glaucomas possess no overt ocular abnormality to account for the increase in IOP, whereas secondary glaucomas have an identifiable cause, such as intraocular inflammation, neoplasia, or lens luxation
- Primary bilateral glaucoma has been rarely reported in the horse
- Secondary glaucomas due to anterior uveitis and intraocular neoplasia are most commonly recognized in the horse
- Congenital glaucoma is reported in foals and associated with developmental anomalies of the iridocorneal angle
- Equine glaucoma may not be easily recognized in the early stages of the disease due to the subtle nature of the clinical signs
- Initially, the pupils of affected eyes are often only slightly mydriatic, and overt discomfort is uncommon
- Afferent pupillary light reflex deficits, corneal striae, fibrosis of the drainage angle, decreased vision, lens subluxations and luxations, mild iridocyclitis, and optic nerve atrophy/cupping may also be found in eyes of horses with glaucoma
- The presence of corneal striae, or corneal endothelial “band opacities,” in non-buphthalmic horse eyes warrants a high degree of suspicion for the finding of elevated IOP, which may also be found in eyes that are normotensive at the time of examination. Corneal striae are linear, often interconnecting or branching, white opacities found deep in the cornea, caused by stretching or rupture of Descemet's membrane, and may be associated with increased IOP
- Aqueous humor is produced in the ciliary body by energy-dependent and -independent mechanisms. The ciliary enzyme carbonic anhydrase plays an important role in aqueous production. Aqueous humor passes into the posterior chamber, through the pupil into the anterior chamber, and then exits through the iridocorneal angle (conventional) outflow pathway or through the uveovortex and uveoscleral (unconventional) outflow pathways
- Perfusion and morphologic studies indicate potentially extensive unconventional aqueous humor outflow pathway involvement in the horse. The extensive low-resistance equine conventional aqueous humor outflow pathway and the prominent unconventional outflow pathways in the horse may minimize development of glaucoma in many cases of anterior uveitis
- However, anterior uveitis can lead to formation of preiridal fibrovascular membranes that limit aqueous absorption by the iris and to physical and functional obstruction of the iridocorneal angles with inflammatory cells and debris
- Iridal and ciliary body neoplasms and endophthalmitis can cause secondary glaucoma by infiltration of the outflow pathways
DIAGNOSIS
TREATMENT
MEDICATIONS
FOLLOW-UP
MISCELLANEOUS