Equine veterinary practitioners are often presented with an eye that has developed an opaque light-blue color, not unlike that of a robin’s egg. Whether the blue color covers part or all of the horse’s eye, astute clients are sure to note the change. While clients may not know the reason for the change, they certainly recognize that the eye doesn’t look normal.
Many conditions can cause a blue eye. It behooves anyone caring for horses to recognize a few of the common ones.
Why Did It Happen?
The corneal stroma does not contain as much water as it can hold. The mechanism by which the cornea remains relatively dehydrated is called deturgescence, which is critical to normal corneal function and clarity. The process prevents excessive corneal hydration, and without it the eye would be functionally blind.
Deturgesence is a collaborative process between the impermeable corneal epithelium on the surface of the cornea and a metabolic transport system that exists in the corneal endothelium on the inside. Excessive hydration of the cornea—due to any process that causes corneal edema—results in an opaque, cloudy cornea.
A bluish color to the eye almost always means corneal edema, accordingly. Corneal edema can occur subsequent to conditions such as uveitis, glaucoma or endothelial dystrophy, to name a few.
How Much of the Eye?
An eye that turns only partially blue is indication of a focal problem. A variety of conditions can cause a focal color change, including corneal scarring, corneal dystrophy, neoplasia or focal inflammation, such as is seen, for example, with corneal ulceration.
On the other hand, an eye that turns completely blue is an indication of a problem affecting the entire globe or anterior segment. Of particular importance in horses are uveitis (intraocular inflammation) and glaucoma, two completely different conditions that can begin with confusingly similar presentations.
Extensive neoplasia can cause eyes to turn blue. The presence of mature cataracts, while not affecting the cornea, can give a bluish reflection that may be noticed by observant clients.
Is the Eye Painful?
Painful or partially closed eyes usually indicate an ocular emergency, and such cases should be examined on the same day a call is placed. It’s always a good idea to examine both eyes as the normal eye will be a good point of comparison for the abnormal eye.
Both eyes should be examined thoroughly, paying particular attention to the size and responsiveness of the pupil. A constricted, or miotic, pupil suggests that inflammation is occurring either in the eye or on the surface of the eye, whereas a normal pupil suggests the source of pain is extraocular.
Corneal ulcers are one of the most common ocular problems seen by equine practitioners. The disruption of the corneal epithelium accompanying corneal ulceration can be readily seen when fluorescein stain is applied to the eye. The size of the stain retention reflects the extent of the corneal ulceration. A bluish appearance to the eye usually indicates the ulcer is deeper or is chronic, and clients should be advised accordingly.
The treatment of uncomplicated corneal ulcers is usually rather straightforward, and most superficial ulcers heal rapidly with topical treatment. Deeper ulcers are associated with significant pain and may be harder to manage, usually requiring several weeks of frequent treatment for a successful resolution.
Equine Recurrent Uveitis
ERU is probably the most common cause of blindness in the horse, with Leptospira infection and genetic predisposition being leading risk factors. Uveitis is an immune-mediated process involving both humoral and cellular mechanisms. Affected horses are usually in pain, but the eyes do not retain fluorescein stain.
The pupil of the affected eye is miotic, and the anterior chamber may contain fibrin or pus in severe cases. The posterior chamber may appear to be a dark yellow or orange color due to the inflammation; this can usually be appreciated even when viewed through a bluish cornea.
Intraocular pressure is below normal. Chronic cases may include changes such as a cataract, lens luxation or retinal detachment.
ERU has a guarded prognosis. Initial treatment involves mydriatic agents (e.g, atropine ointment), systemic and topical anti-inflammatory agents. Treating chronic cases can be very difficult, and some may be resolved only by enucleation.
Glaucoma encompasses several ophthalmic disorders that impair equine vision. The common thread is an increase in intraocular pressure that makes a healthy eye impossible.
Glaucoma is rarely congenital. More commonly, it is seen as a secondary problem subsequent to chronic uveitis. Spontaneous primary glaucoma is also recognized, especially in older horses.
A number of medications are available for medical management of glaucoma, and they are not cost-prohibitive. Some cases may be candidates for surgery, and treatment with surgical lasers may be helpful. Nevertheless, glaucoma often results in blindness in the affected eye.
Dr. David W. Ramey is an author, lecturer and Southern California equine practitioner.
Originally published in the July 2016 issue of Veterinary Practice News. Did you enjoy this article? Then subscribe today!