Cataracts are cloudy areas in the lens inside the eye – which is normally clear – causing blurry or obscured vision that may result in spooking or impaired performance. If your horse is afflicted, what are your options?
When you hear the term cataract, you likely think of it as a geriatric human complaint. In fact, cataracts can affect horses (and humans) of all ages, robbing the sufferer of vision.
A cataract is any area in the lens of the eye which has become clouded or opaque. Cataracts can be large or small, and obscure vision to varying degrees. In a healthy eye, the transparent lens works with the cornea to refract light and focus images onto the retina. Able to change shape to accommodate this task, the lens is made up of a jelly-like substance and is suspended by ligaments, which are attached to ciliary muscles. When cataracts are present, the lens’ function can be disrupted and vision altered.
What Causes Cataracts?
Horses can be born with cataracts or develop them over time. Congenital cataracts (present at birth) are common in foals, while juvenile cataracts are less common. Cataracts may develop in mature horses and true senile cataracts are most often present in horses over the age of 20.
There can also be a hereditary component to the development of cataracts, with some breeds being more prone than others. Additionally, cataracts can form as the result of injury to the eye or other imbalances in the body. Acquired cataracts are much more common than inherited, and they can develop for a number of reasons, according to Dr. Marnie Ford, of Pacific Animal Eye Speciality Services, a board-certified veterinary ophthalmologist based in British Columbia. She noted that several traumas, including contusion or penetrating injury, lens luxation or subluxation (dislocation or partial dislocation of the lens), nutrition, radiation, toxicity or inflammation can all cause acquired cataracts.
Alternatively, a cataract may be present as one component in a collection of abnormalities associated with ocular malformation, such as microphthalmia, or ‘small eyes.’
Dr. Ford cautions against breeding horses with inherited cataracts. “Dominant inherited cataracts have been reported [in] Belgian and Thoroughbred horses, and moon blindness has been documented most commonly in the Appaloosa breed. Morgans are also reported to have smaller and non-progressive cataracts that do not interfere with vision.
“This breed list is not inclusive, as other breeds can develop inherited cataracts and it is prudent to assume that all cataracts that develop without a known underlying cause to be inherited.”
Identification & Diagnosis
Behavioural changes can be a sign of vision impairment. A horse that begins to spook or shy uncharacteristically or show reluctance to perform tasks should have an eye exam performed, which may reveal a visible white spot within the normally black pupillary space. The presence of a cataract can also cause the horse to squint in bright light.
Dr. Ford said diagnosis can be helped immensely if the owner is able to provide background regarding any prior history of eye conditions or injuries and information regarding onset and rate of formation.
For acquired cataracts, Dr. Ford stressed that early treatment is the key. “Early treatment is aimed at reducing ocular inflammation that may result in cataract formation,” she said. “Early veterinary assessment is required for an eye that has been contused or perforated.” She added, “If an animal has been systemically ill, it may also require anti-inflammatory therapy to minimize the development of cataract formation.”
Dr. Ford explained that three general aspects are used to classify the severity of cataracts: the degree of lens involvement, the location within the lens and the age at onset.
A progressive classification is used to identify the degree of lens involvement. Incipient denotes the smallest degree, followed progressively by immature, mature and hypermature. The rate of progression can vary depending on whether the cataract is inherited or acquired.
“Incipient cataracts are very small dots in the lens, like salt and pepper flecks on a dinner plate,” said Dr. Ford. “Unless numerous, these uncommonly affect vision or performance of the animal, and rarely require removal.
“An immature cataract ranges in size from a very small focal opacity to a large focal opacity, or even to a thin diffuse density. If the fundus, which is the back of the eye and includes the retina, retinal blood vessels and optic nerve head, can be seen through or around the cataract, the cataract is classified as immature. It is like looking at a thin sheet of ice covering a clear shallow lake from a helicopter. The pilot would still be able to see the lake bottom – or fundus – through the thin ice.
“A mature cataract [is characterized by] complete lens opacity, or, to again use the lake analogy, the whole lake is frozen over and the pilot cannot see the bottom.
“A hypermature cataract has previously been mature, but, with aging of the lens, begins to break down, or, the edges of the frozen lake start to melt. [This type] is very hard and the clear lens capsule is wrinkled. Vision is not typically restored by breakdown.”
The location of a cataract is important to note for the purpose of comparison at a later date, but may also present clues as to what type of cataract is present. “The location of cataract formation can sometimes be associated with the nature of formation, such as whether it is congenital or acquired,” explained Dr. Ford. “Notation of a bilateral [affecting both eyes] versus unilateral [affecting one eye] presentation is important, since most acquired cataracts will be unilateral, whereas most inherited cataracts are bilateral.”
Not all inherited cataracts develop in youth. Some inherited cataracts are known to start developing in mature animals. “Information regarding age of onset may help with understanding prognosis,” said Dr. Ford. “Unlike foals, older animals are often owned by several people so their histories – medical and physical – are often unclear leaving the potential for prior trauma or illness less certain. Knowing the age of cataract onset, especially if bilateral and symmetric, may help to round out a diagnosis of inherited vs acquired.
Surgery is the Only Option
Dr. Lynne Sandmeyer is a board-certified veterinary ophthalmologist and a professor of veterinary ophthalmology at the Western College of Veterinary Medicine at the University of Saskatchewan, where she contributes to clinical services, teaching and research. She explained that there is no medical solution for cataracts, making surgical removal the only treatment option for horses with vision impairment due to cataracts.
Several factors make a horse suitable for the surgery and can help predict the outcome or determine the level of rehabilitation that can be expected. “A complete history and assessment of the overall health and visual potential of the eye are paramount,” said Dr. Sandmeyer. “This is done through an evaluation of the ocular structure with a complete ocular examination, ocular ultrasound, and evaluation of retinal function via electroretinography [ERG].
“The general health of the animal and ability to undergo general anesthesia safely is also important. All potential candidates should have a thorough physical examination and pre-anesthetic complete blood work done.
“Finally, it is vital that the animal’s personality and temperament be assessed. Frequent application of multiple ocular mediations is required for several weeks after surgery and frequent re-evaluation of the animal is necessary. Unmanageable horses are poor candidates for surgery. If therapy cannot be delivered as directed, the animal is more likely to develop serious complications. Also, poorly manageable horses may be prone to post-operative ocular injury or injury to those providing therapy.”
Cataract surgery is most often performed on foals with congenital cataracts or horses with cataracts large enough to affect regular activities. “If the eye has a normal anatomy and a functioning retina, there is reasonable potential for vision after cataract surgery,” explained Dr. Sandmeyer.
“In horses with concurrent ocular disease such as glaucoma, retinal detachment or retinal degeneration, cataract surgery is not recommended. If a painful and blinding condition exists, then surgical therapy such as enucleation [removal of the eye], or a similar cosmetic salvage procedure should be considered.”
The main purpose of cataract surgery is to provide a clear axis for light to pass from the cornea to the retina, said Dr. Sandmeyer. “To explain the details of surgery, one requires a basic understanding of the anatomy of the lens. The lens is a biconvex structure consisting of an outer elastic capsule, which is suspended within the eye by multiple ligaments that attach at the lens equator to the ciliary processes that are positioned behind the iris.
“The goal of cataract surgery is to remove the material of the lens while leaving behind an intact lens capsule, which acts like an ‘empty bag’ into which a prosthetic lens can be placed and maintained. The function of the prosthetic lens is to focus the light on the retina such that the image seen is clear and sharp.”
Once the horse is under general anesthesia, the eye itself is prepped for surgery. Antibiotics and several systemic and topical anti-inflammatories are given in addition to drops that act to dilate the pupil. The eyelid hairs and eyelashes are removed and the surrounding area is cleaned and surrounded with sterile drapes to reduce bacterial contamination. Once the horse is placed under the operating microscope, Dr. Sandmeyer begins with a corneal incision.
“The eye is kept inflated with a clear viscoelastic fluid,” she explained. “A circular opening in the front of the lens capsule is made, called the capsulotomy, by incising, grasping and tearing the capsule in a precise manner. This allows access to the lens material and the cataract.
“The phacoemulsification instrument then enters the eye and the surgeon uses this to systematically remove the cataract by breaking it into smaller pieces. Phacoemulsification uses ultrasound energy to break up the lens material and simultaneously aspirate this material from the eye. When the lens material has been completely removed from the capsular bag, the prosthetic lens is inserted through the capsulotomy and is held in place by the remaining capsule.
“The corneal incision is then sutured with tiny, absorbable sutures and a small flap of conjunctiva may be sutured over the incision as extra protection.
“If bilateral surgery is being performed the animal is repositioned for access to the other eye and the procedure is repeated.”
Potential Complications & Recovery
Several complications may arise during or after surgery that pose a threat to successful recovery or fully repaired vision. “During surgery, intraocular hemorrhage can occur, which complicates visualization,” said Dr. Sandmeyer. “Tears in the posterior aspect of the lens capsule may occur, resulting in dropping lens fragments into the back of the eye. These tears may preclude the placement of a prosthetic lens and dropped lens fragments may increase the chances of post-operative inflammation and retinal detachment.
“Complications following cataract surgery include incisional breakdown, corneal ulceration, elevated intraocular pressure [glaucoma], retinal detachment and intraocular infection. Complications such as incisional breakdown and corneal ulceration may require additional surgery to repair. Others, such as retinal detachment, glaucoma and intraocular infection often result in loss of vision and possibly the need to remove the globe or perform a cosmetic salvage procedure.”
If the recovery from general anesthesia goes as planned, owners can expect the horse to remain hospitalized under close watch for two or more days following cataract surgery. It is generally recommended that following discharge the horse remains on stall rest and that exercise is limited to hand-walking for three weeks post-surgery.
In addition to oral antibiotics and anti-inflammatory medications, Dr. Sandmeyer said that while treatments will vary depending on the surgeon, a combination of eye drops will be prescribed for several weeks following the surgery and can include topical antibiotics, anti-inflammatory drops, drops to dilate the pupil and drops to reduce intraocular pressure.
A sub-palpebral lavage system is usually placed to aid therapy and assists in providing post-operative treatments, helping to avoid a horse’s resentment of frequent administration of eye drops. “The system consists of a long tube, one end of which contains a foot-plate or flange that is secured under the eyelid in the conjunctival sac,” explained Dr. Sandmeyer. “The tube is passed through the upper or lower eyelid, like a piercing, and extended down the neck where it can be attached to the mane. It allows injection of the eye medications at the distal end of the tube and this means the eye does not need to be touched to administer the medications.”
Dr. Sandmeyer said the incision requires about three weeks to fully heal and initially is held together by corneal sutures alone. “These will gradually dissolve as the incision heals and during this healing period there may be some vascular ingrowth into the incision site, she said.” If a conjunctival graft is placed over the incision this process may not be visible. There will be a scar at the incision or graft site long-term but this tends to be in the peripheral cornea and does not affect vision.”
Dr. Sandmeyer cautioned owners to expect inflammation within the eye, referred to as uveitis as well as corneal edema, or fluid in the cornea, both of which will diminish over time and are treated with topical and systemic anti-inflammatory medications.
Follow up exams are recommended one week, three weeks, six weeks and six months after the surgery, and annually thereafter.
Effect on Performance
Dr. Ford said a horse’s ability to perform may be limited after a cataract diagnosis. “If a cataract is mature and bilateral, the performance career of that animal is over,” she said. “This is based on the assumption that the animal’s ‘performance career’ was of an athletic standard that required sharp visual reflexes.
“An incipient or immature cataract may not mean the end of a performance career, as these classifications vary in the amount of lens involvement and, as such, in the amount of useful vision remaining.
“Without cataract removal, again, depending on how much vision remains, the horse’s future may be limited to trail rides or to that of a pasture pet. Any visually impaired horse is considered unsafe for riding and must only be done so by an informed and capable rider.”
Dr. Sandmeyer added, “There are many anecdotal reports suggesting that horses’ vision after cataract surgery is functionally normal. Owners report animals being used regularly for pleasure riding and performing such jobs as roping, barrel racing and polo. However, it remains an ethical debate among veterinarians as to whether these horses can be considered safe to ride and work with.
“Evaluation of the literature shows that visual outcome is initially good following cataract surgery, but reduces over time. Cataracts themselves are visually impairing and cataract surgery remains the only opportunity for visual recovery. As technology and surgical techniques evolve, visual acuity and long-term outcome after surgery may improve.”