The lens is a soft, transparent tissue that sits behind the iris. It helps refract incoming light onto the retina. Common disorders of the lens include those that affect its transparency (such as cataracts), and those that affect the placement of the lens.
Cataracts are a condition in which the lens becomes covered in a cloudy film that affects sight, eventually causing total blindness.
In foals, cataracts are the most common congenital defect of the eye. They are inherited in Belgian, Morgan, and Thoroughbred horses. Cataracts usually occur in both eyes. When these cataracts interfere with vision in healthy foals, surgery followed by topical therapy is successful in a majority of cases.
In adult horses, most cataracts occur as a result of the inflammation of the anterior uvea associated with equine recurrent uveitis (see Eye Disorders of Horses: Equine Recurrent Uveitis (Periodic Ophthalmia, Moon Blindness)). Horses older than 20 years of age may develop so-called senile cataracts that interfere with vision. Surgical removal of the lens is the only definitive treatment available.
Lens displacement can also occur in horses. The displacement may be due to trauma, longterm inflammation of the uvea (as occurs in equine recurrent uveitis), or glaucoma. The only effective treatment is surgical removal of the lens.