CHAPTER 144 Intraocular Surgery
>Intraocular surgery is performed on horses with growing frequency as advances in surgical equipment and operative technique improve outcomes. However, the expense of instrumentation and the limited availability of technical expertise may limit the accessibility of equine intraocular surgery in some geographic areas. The most common indications for equine intraocular surgery are cataract, lens luxation, and intraocular foreign body. Clinical signs, patient selection, diagnostic testing, therapeutic regimens, and complications are discussed in this chapter.
A cataract is defined as any opacity of the lens. Dense cataracts may impair vision such that the horse cannot be safely handled or ridden. Depending on their demeanor, visually impaired horses may also be prone to severe self-injury.
Cataracts are the most common congenital ocular lesion of horses. Cataracts may be seen in association with other ocular abnormalities, or the remainder of the globe may be normal. In most cases, the cause of congenital cataracts remains unknown. Metabolic, toxic, inflammatory, and nutritional causes have been proposed. Because of the possible heritable nature of cataracts, owners should be counseled that breeding of affected horses is not recommended. A dominantly inherited, congenital, nuclear cataract is seen in Morgan horses. Cataracts are inherited as a dominant trait in certain Belgian, Quarter Horse, and Thoroughbred bloodlines. Rocky Mountain horses affected with anterior segment dysgenesis (ASD) often develop cataract as well as lens luxation. ASD is a recessive trait with incomplete penetrance that develops in association with coat color, particularly the chocolate coat color.
Cataract formation in adult horses is most commonly caused by equine recurrent uveitis. Blunt or penetrating traumatic injury may also incite cataract formation. Older horses and ponies may develop senile cataracts.
To facilitate complete examination of the lens, a rapid-onset mydriatic agent such as 1% tropicamide ophthalmic solution should be applied. Cataracts may be unilateral or bilateral, and focal or diffuse. The position within the lens may be axial (along the visual axis), equatorial, capsular, subcapsular, cortical, or nuclear. Cataract stages categorized on the basis of percentage of the lens affected include incipient (less than 15%), immature (15% to 99%), mature (100%), and hypermature (100%, with resorption of cortical material). The degree of visual impairment depends on the size, location, and density of the opacity. For example, a dense focal lesion along the visual axis causes more visual impairment than a lesion of the same size and density located equatorially.
There is no effective medical treatment for cataracts. Pharmacologic mydriasis after application of 1% atropine ophthalmic solution or ointment may improve vision for horses with focal opacities. The only effective therapy for cataracts causing clinically significant visual impairment is surgical removal. Appropriate patient selection is critical to a successful outcome.
Horses with congenital or traumatic cataracts typically have the best prognosis for vision after cataract removal. However, foals with congenital cataracts in association with other congenital defects are not suitable candidates. Horses with cataracts resulting from ASD have a poorer prognosis because of the concomitant abnormalities of lens shape and position and the frequent presence of retinal dysplasia, which may predispose to postoperative retinal detachment. Horses with cataracts secondary to recurrent uveitis have a guarded long-term prognosis for vision because complications from repeated bouts of uveitis often result in keratitis, retinal detachment, synechiae, or phthisis bulbi and blindness.
The temperament of the individual horse must also be considered. The horse must be amenable to handling to prevent postoperative trauma to the globe and to make it feasible to apply the topical medications that are typically required for 4 to 6 weeks after surgery. Foals should be halter broken.
Foals with congenital cataracts should be referred early if the cataract completely obscures vision. Human infants with complete cataracts have a better success rate for proper development of the visual neural network if surgery is performed in the first 6 weeks of life. Although the definitive age by which cataract surgery must be performed in foals is not known, current recommendations favor surgical intervention before 6 months of age.
A complete physical examination is performed before surgery. Special emphasis is given to the respiratory system because occult respiratory tract infection, particularly rhodococcal infection, can lead to postoperative endophthalmitis. A complete blood count with fibrinogen concentration and a serum biochemistry profile are obtained. A complete ocular examination is performed. The foal should have brisk pupillary light reflexes (PLRs) and a dazzle reflex. Particularly if the dazzle reflex or PLRs are diminished and the cataract obscures viewing of the fundus, ocular ultrasonograsphy and electroretinography (ERG) should be performed. Ocular ultrasonography assists in identification of retinal detachment, which would preclude cataract surgery. ERGs evaluate retinal, particularly photoreceptor, function. The ERG can be useful to determine whether a horse has diffuse retinal degeneration, particularly when there is a history of recurrent uveitis or traumatic injury. An ERG is also useful to rule out congenital stationary night blindness, which is seen predominantly in Appaloosas but has also been reported in Thoroughbreds, Paso Finos, and Standardbreds.
Preoperative Therapeutic Protocol
The following section describes a typical preoperative therapeutic protocol. Blepharitis, conjunctivitis, keratitis, uveitis, and any systemic disease must be controlled before cataract removal can be performed. If uveitis is present, topical corticosteroids (prednisolone acetate 1% ophthalmic suspension) or an antimicrobial-corticosteroid preparation (neomycin-polymyxin-dexamethasone ophthalmic solution or ointment) and topical atropine (1% ophthalmic ointment or solution) are administered to control the inflammation. If uveitis is detected, systemic nonsteroidal anti-inflammatory drugs (NSAIDs) such as flunixin meglumine (1.1 mg/kg, intravenous [IV] or oral [PO], every 12 hours) or phenylbutazone (2 mg/kg, PO, every 12 hours) are administered.
The day before surgery, topical corticosteroids and antimicrobials are applied to the ocular surface every 6 hours. Atropine 1% ophthalmic solution is applied once to achieve mydriasis. Flunixin meglumine is administered. Gastric protectants such as omeprazole (2 mg/kg PO daily) are indicated, particularly for foals. Tetanus toxoid is administered if vaccinations are not up to date.
The day of surgery, three applications each of topical ophthalmic corticosteroid, antimicrobial, and NSAID (0.03% flurbiprofen ophthalmic solution) solutions are applied to the ocular surface. Atropine 1% ophthalmic solution or tropicamide 1% ophthalmic solution are administered, if needed, to achieve maximal pupillary dilation. Flunixin meglumine administration is continued. Prophylactic systemic antimicrobials (potassium penicillin G, 20,000 international units/kg, IV, every 6 hours and gentamicin, 6.6 mg/kg, IV, every 24 hours) are administered perioperatively.