Cataracts and Lens Problems

Everyone has heard of cataracts. We think of them as affecting older humans, and sometimes they are found in children and young adults. Yet, cataracts also are seen in horses. A cataract is an opacity or clouding of the lens (a large transparent structure found midway between the cornea and the retina that is used to focus images close to the horse's head. (See "Normal Eye Anatomy" on page 42.)

The lens has an outer cortex region and an inner nuclear (center) portion. Made of a multitude of individual fibers, the lens is suspended in place by zonules (ligaments holding the lens in position) attached to the ciliary body. When lens fibers grow, they form lens suture lines that can be seen in many foals. The anterior suture has a "Y" shape with the posterior suture varying in shape from a "Y" to a sawhorse or star pattern. The anterior sutures are difficult to visualize in Thoroughbreds, but easier to see in Standardbred and Saddlebred foals.

Cataracts: What are They?

Cataracts block the visual image as they increase in size, but do not block the light. Congenital (present at birth) cataracts are frequent defects in foals; most affected foals have them in both eyes. In adult horses, cataracts might be caused by heredity, traumatic injury, nutritional deficiencies or toxicities, or be secondary to other inflammatory eye conditions such as equine recurrent uveitis.

Careful examination is needed to determine if eyes with cataracts also have recurrent uveitis. This is especially important when the eye is being considered for cataract extraction, since there is an increased risk of complications and a poorer prognosis for vision when uveitis is the cause of the cataract.

A dominant mode of inheritance has been reported in Belgian and Thoroughbred horses, but the heredity of most equine cataracts is not known. Morgan horses are known to have nonprogressive, nuclear cataracts that do not seriously interfere with vision. Rocky Mountain Horses are known to have an eye syndrome that results in cataracts and lens problems. Horses older than 20 years can develop old age or senile cataracts. These must be differentiated from the normal cloudiness of the horse lens that is seen with aging.

Cataracts are categorized as to their level of maturity. Incipient or early cataracts (see Figure 3 on page 43) involve small areas of the lens and do not affect vision. Immature cataracts involve more of the lens with increasing effects on vision. Mature cataracts (see Figure 4) involve the entire lens and cause blindness.

Diagnosis of Cataracts

Cataracts are associated with varying degrees of blindness. Brisk pupillary light reflexes and a strong blink reflex in response to bright light usually indicate a functional retina. If the retina is not visible preoperatively, ocular ultrasound can be used to insure that the retina has not detached. Because of hereditary congenital stationary night blindness in the Appaloosa, preoperative electroretinography should be considered to accurately predict the vision that can be obtained after surgery.

Medical and Surgical Therapy

Therapy for cataracts is surgical. Cataract surgery is indicated if the loss of vision is such that the horse is unable to perform his regular activities. Most veterinary ophthalmologists recommend surgical removal of cataracts in foals less than six months of age if the foal is healthy, has no uveitis or additional ocular problems, and the foal will tolerate aggressive topical therapy. Adults with visual impairment due to cataracts are also candidates for cataract surgery. Foals are easiest to operate on because the globe (eyeball) is small enough that standard human cataract surgical equipment can be used, general anesthesia is less of a risk in foals, and foals heal very quickly following cataract surgery. Early return of vision is paramount in foals for development of the higher visual centers.

Foals should be carefully evaluated for subclinical infectious systemic diseases, such as Rhodococcus pneumonia, as sight-threatening postoperative endophthalmitis (inflammation of the internal tissues of the eye) can result seven to 10 days following surgery if the condition is not detected and treated prior to cataract surgery.

Cataract surgery requires general anesthesia. The most common technique for removing congenital cataracts in foals and mature horses is phacoemulsification (see Figure 5 on page 43). In this procedure, a specially designed vibrating needle is inserted into the anterior chamber, the cataract is broken up with ultrasound, and the fragments of the cataract are aspirated out of the eye through the dilated pupil. Recent advances in this technique have increased the success rate to nearly 80%. Surgical success is highest in foals less than six months of age, as postoperative inflammation in foals is generally minimal. The size of the phacoemulsification needles becomes a limiting factor to successful cataract surgery as the lens and globe increase in size up to a mature age. Corneal ulcers, hyphema (hemorrhage within the anterior chamber of the eye), and infectious endophthalmitis are serious potential postoperative complications in both foals and adults.

The foal or adult horse should be quite farsighted postoperatively, but the amount of vision following successful cataract surgery in adult horses and in foals is "functionally normal" (see Figure 6).

Lens Luxation

Due to a congenital defect in zonule formation in foals or severe eye trauma in adult horses, the lens can luxate or dislocate forward or backwards from its normal position. Movement of the iris from lens contact, shallow or deep anterior chambers, and aphakic (no lens) crescents (the edge of the lens is seen) might be present (see Figure 2). Cataract formation also might be noticed. Dislocation of the lens into the vitreous body might not necessitate surgical intervention; however, movement into the anterior chamber usually requires lens removal to prevent secondary glaucoma (disease characterized by increased intraocular pressure). Lens extraction surgery for displaced lenses in the horse has extremely poor results due to severe postoperative inflammation. Another option is to not do surgery at all if the lens is dislocated but not causing a problem.

Editor's note: This is the ninth in a series of eye articles by Dr. Brooks. See the first article, "Eye Anatomy and Physiology," article Quick Find #2797 at , for more information on normal eye anatomy.

5 TIPS: Equine Cataracts

  1. A cataract is an opacity (clouding) of the lens.
  2. Cataracts can affect both foals and adult horses.
  3. Cataracts block image, but not light, and can cause blindness as they progress.
  4. They can be caused by heredity, trau- matic injury, nutritional deficiencies or toxicities, or be secondary to other inflammatory eye conditions.
  5. Cataract surgery is indicated if the loss of vision is such that the horse is unable to perform his regular activities.

About the Author

Dennis E. Brooks, DVM, PhD, Dipl. ACVO

Dennis E. Brooks, DVM, PhD, Dipl. ACVO, is a professor of ophthalmology at the University of Florida. He has lectured extensively, nationally and internationally, in comparative ophthalmology and glaucoma, and has more than 140 refereed publications. He is a recognized authority on canine glaucoma, and infectious keratitis, corneal transplantation, and glaucoma of horses.

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