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Recent Advances in Equine Cataract in Veterinary Surgery

Mohsin Gazi Md. Moin Ansari
Vol 2(1), 25-37
DOI-

Cataract can cause a horse to go blind and surgery treatment is the only treatments that may help the horse regain sight, although the surgery is difficult in horses owing to their large size. Equally important is the need to have frequent eye examinations after the surgery to ensure that all is healing well and that no complications are developing. If owners note any change in the eye after surgery they should not hesitate to contact the ophthalmologists. Some horses need some minimal amount of medication for several weeks after surgery. After surgery, glaucoma and endophthalmitis are potentially disastrous sequelae. In foals, immune compromise associated with Rhodococcus equi pulmonary abscesses may be a contributing factor in postoperative endophthalmitis, and chest radiographs should be taken as part of the surgical selection protocol in foals As in other species, post surgical posterior capsular opacity is a major problem in the horse and may necessitate a second surgery in an attempt to clear the visual axis by posterior capsulorrhexis. Potential complications of lens removal in the horse may include persistent or recurring intraocular inflammation, corneal ulcers, corneal cloudiness or edema and retinal detachments. Any of these complications may lead to discomfort, further treatment, visual impairment and/or blindness of the eye. This communication reviews the cataract in equines and placed records on the aetiopathogenesis, classification and management.


Keywords : Cataract horse management recent advances

Introduction

Cataracts are opacities of the lens. Technically a cataract is defined as any opacity or alteration in the optical homogenecity of lens involving one or more of the following anterior epithelium, capsule, cortex, or nucleus (Kincaid 1996). Cataracts occur in many species of animals, and horses are no exception and are the most frequent congenital ocular defect in foals. Horses manifest varying degrees of blindness as cataracts mature. The horse has a total visual field of nearly 360 degrees, meaning a horse can just about see its tail with its head pointed forward. A small frontal binocular field of 65 degrees develops post-natally. The horse’s retina is adapted for detection of movement, and the horse utilizes both eyes until an object approaches within 3-4 feet, when it is forced to turn or lower its head to continue to observe with one eye. Cones are present in the horse’s retina suggesting that they have the capacity for color vision, in the form of blues and reds. Very small incipient lens opacities are common and not associated with blindness. As cataracts mature and become more opaque, the degree of blindness increases. McMullen and Utter (2010) review to discuss the evolution of equine cataract surgery over the past 50 years to its current stage. Equine cataract surgery is performed similarly compared with the techniques used in human ophthalmology and in other veterinary species. However, enough differences exist to make surgical lens removal and intraocular lens implantation in the horse an intrinsically unique endeavor. Due to the size of the adult equine globe, the introduction of species-specific instrumentation has provided the cornerstone to many of the changes made regarding surgical technique over the last 15-20 years. The continuing development of an equine specific, foldable intraocular lens implant (IOL) has provided much needed data supporting the use of such lenses in the horse to improve upon the post operative visual outcome. Finally, the methods utilised to assess visual capacity and the effects of intraocular lens implantation on the globe (e.g. ocular ultrasonography, electroretinography and streak retinoscopy) are gradually becoming more important in preoperative patient assessment and IOL development in the horse. It is the hope of the authors that a broader group of equine veterinarians will become aware of the many changes that have taken place in equine cataract surgery over the last half-century. Although aspiration was implemented nearly 40 years ago in foals for the treatment of congenital cataracts, phaco-fragmentation (phacoemulsification) techniques have only recently become routine in mature horses undergoing lens extraction.

Aetiology and Pathogenesis

There are multiple causes of equine cataract. A horse may be born with cataract, congenital which develops at early age, juvenile which develops at adult stage of life and the other one is senile which develops as a result of old age. Congenital and juvenile cataracts are probably due to either heredity or accident of development inside the mare. These are fortunately rare in horses. Consideration should be taken as to the wisdom of breeding these horses and their close relatives. Most cataracts in adult horses develop secondary to diseases that cause intraocular inflammation like Equine Recurrent Uveitis (‘moon blindness’) or ERU. Trauma is another possible cause, resulting from damage to the foal during late pregnancy or during foaling. Infection or metabolic imbalances in the mare while pregnant. Another cause of equine cataract include lack of proper nutrition. A description of various types of cataract appearing in equine eyes, each identified by colour, was published in 1737 by a Dr.Bracken (Wortley-1906). Some cataracts are very small and have little effect on vision. Others are progressive and, in time, can lead to blindness.

Physicochemical causes of loss of optical homogeneity of the lens include Accumulation of abnormal amounts of insoluble crystalline aggregates within the cytoplasm, Degeneration of the cell membranes, causing osmotic disabling and turgescence of the cells. Disruption of the limited intercellular space, causing dehiscencence of cells and creating clefts or vacuoles within the lens, Spatial distortion of the cellular framework,Gross distortion of the lens shape, for example, after dehydration and shrinkage, Anterior epithelial reactivity and capsular opacification, Mineralization. It occurs secondary to ocular diseases like diabetes mallitus, as eye and brain can’t utilize anything other than glucose. In diabetes mellitus there is higher level of glucose in blood so higher level in lens also. This increased level of glucose leads oversaturation of the normal pathway, so shifts to another metabolic pathway i.e from aldose reductase hexokinase system there is formation of sorbitol.Once sorbitol is formed cataract cannot escape from lens and opacity occurs. Specific primary (hereditary) luxation of the lenses in adult horses has not been documented. Acquired anterior or posterior luxation and subluxation of the lens occur relatively frequently in the horse, however, and are encountered as sequelae to ocular trauma, panuveitis, and glaucoma (Lavach and Brooks, 1999. Luxated lenses invariably become cataractous, and intracapsular extraction may be possible (Lavach, 1990). Because intraocular damage of sufficient severity to result in lens luxation would also be expected to cause significant visual impairment or frank blindness, evisceration and implantation of an intrascleral silicone prosthesis is a more pragmatic alternative,   however (McLaughlin et al., 1992).

Classification of Cataracts

As in other species, equine cataracts may be variously classified according to their anatomic location within the lens, their ophthalmoscopic appearance, their presumptive aetiopathogenesis, or their stage of progression or maturity (Matthews, 2000; Davidso and Helms, 1999; McLaughlin et al., 1992.). No single classification system wholly or satisfactorily encompasses the wide range of cataracts observed in the horse. Most equine cataracts are non-progressive, and classification of equine cataracts according to stage of progression (viz. incipient (implying progression), immature, mature, hypermature is appropriate only in a few cases (viz. some post uveitic cataracts). Morgagnian cataract, a hypermature cataract in which the nucleus ‘‘sinks’’ within the liquefied cortex, has been described in the horse but is probably extremely rare (Hardman et al., 2001). Early equine clinical texts (Dollar, 1895) describe the basic anatomic division of equine cataracts into capsulolenticular cataracts (viz. involving the capsule and subjacent cortex) and lenticular cataracts (viz. involving the lens cortex and nucleus). This may be further refined by more specific localization of the cataract (viz. anterior capsular, sutural, perinuclear, equatorial). A fundamental and practical division of equine cataracts by aetiogenesis is into acquired or secondary cataracts and developmental cataracts. Acquired or secondary cataracts occur as a result of influences arising from outside the lens; in the horse, these most commonly arise as a consequence of intraocular disease.

Developmental cataracts result from disrupted evolution and accretion of the crystalline lens in utero and after birth, and they have been elegantly described as aberrations rather than arrests (Mann, 1937). They include all congenital cataracts, with the exception of rare congenital uveitic cataracts in foals. Their origins lie in interrupted or abnormal differentiation and growth of embryonic ectodermal derivatives or in disruption of the normal maturation and laying down of secondary fibers in the fetal and postnatal lens.

The heritability of developmental cataracts in horses has not been widely studied. Some, mainly nuclear cataracts, have been described as having a sporadic familial occurrence in Arabs and Thoroughbreds. These reports are vague, however, and although a dominant mode of inheritance is most commonly suggested (Lavach, 1990). Millichamp and Dziezyc.(2000) recessive inheritance has been reported (Weber, 1947). In Morgan horses, bilateral non-progressive nuclear or perinuclear cataracts inherited in an autosomal dominant fashion have been described (Beech and Aguirre, 1984; Beech and Irby, 1985). Similar non inherited nuclear cataracts also occur in this breed (Beech and Irby, 1985). Nuclear cataracts have been reported in association with complex heritable ocular dysgeneses in the horse, including aniridia (iris hypoplasia) in Belgian Horses (Eriksson, 1955) and Quarter horses (Joyce, 1983) and anterior segment dysgenesis syndrome in Rocky Mountain Spotted Horses (Ramsey, 1999). Nuclear sclerosis describes the altered refractivity of the central lens occurring in older animals, which is caused by progressive compression of the nucleus by enveloping secondary fibers as part of normal lens growth (Davidson and Helms, 1999). In ungulates, lens growth slows markedly with increasing age (Schmidt and Coulter, 1991) which may restrict the development of true nuclear sclerosis in these species. Increased definition of the nuclear-cortical junction on retro illumination is a common observation in horses older than 18 years of age, however. The nucleus remains optically clear in these animals, although in extremely old animals, central brunescence of the nucleus may be an occasional finding. The typical bluish appearance of the central lens in diffuse illumination described in nuclear sclerosis in dogs (Davidson and Helms, 1999) is an inconsistent finding in the horse. These nuclear changes in older horses are frequently accompanied by senile cataract. Slowly progressive cataracts are commonly observed in horses aged older than 18 years. These are generally accepted to be age-dependent ‘‘senile’’ cataracts. It is unclear how the aging process affects the mammalian lens, and the cataracts may derive from an age-determined susceptibility of the lens to autocrine injury or from subclinical oxidative insult associated with deterioration of the anti-oxidative defense mechanisms in the aging lens (Jacques and Chylack, 1996).

Senile cataracts are reported to be associated with visual impediment in more extensively affected horses, particularly in dim lighting conditions (Cutler, 2002). Because senile retinopathy is almost invariably present in these eyes, however, any visual disability is probably a cumulative effect. Extralenticular opacities (Matthews AG. 200) are not cataracts but are nonprogressive, congenital, retrolental opacities derived from the TVL and vascular framework of the iridopupillary membrane, and they are likely to contain vascular, pigmentary, and connective tissue elements. They include Mittendorf’s dot and the more extensive fine fibrillar opacities commonly found adherent to the axial and ventral posterior lens capsule. These may have a triradiate configuration, possibly reflecting the distribution of the major vessels in the TVL. Acquired or secondary cataractogenesis is initiated and driven by influences arising outside the lens. These include ocular or systemic disease and well-documented extrinsic factors, such as UV light, ionizing and microwave radiation, and ingested toxins (Kincaid, 1996; Davidson and Helms, 1999). In most cases, these cataracts are potentially progressive depending on the persistence or recurrence of the primary insult. The major cause of secondary cataracts in the horse is unquestionably uveitis (McLaughlin et al., 1992), encompassing traumatic and infectious uveitis and the heterogeneous group of presumptively immunogenic uveitides described as equine recurrent uveitis (ERU).

Capsulolenticular cataracts may be unilateral or bilateral and are non progressive. They are seldom, if ever, associated with compromised vision in the horse. Anterior capsular cataracts appear as small irregular focal opacities on the anterior capsule. They may be single or multiple and show no gross evidence of epithelial reactivity or fibrosis. Posterior capsular cataracts are morphologically similar to anterior capsular cataracts and may be associated with minor anomalies, such as adhering vitreal fibrils. Anterior polar cataracts are congenital cataracts and have two distinct forms in the horse. Lenticular cataracts, with the exception of embryonic nuclear cataracts, are usually bilateral but are not necessarily symmetric. They may be grouped into four types as discussed in the following sections. Zonal cataracts probably arise largely as a result of transient disruption of embryogenesis and growth. Normal development precedes and follows this disruption and is reflected in the optically clear lens adjacent to the cataract. Their geographic location in the lens is determined by the timing of the cataractogenic insult. Perinuclear or lamellar cataracts are the most commonly encountered developmental cataracts (Roberts SM. 1992). Sutural cataracts may be congenital or appear after birth. They are potentially progressive, but any progression is likely to be slow. Anterior sutural cataracts are rare in the horse and may adopt various configurations. Posterior sutural cataracts are relatively common and invariably have a Y shaped configuration. Dendritiform or staghorn extensions may be present near the abaxial extremities of the ‘‘Y”.

Axial cataracts evolve in a relatively precise geometric configuration around the axis of the lens and are identified using descriptive terminology. Floriform cataracts are nonprogressive, discrete ‘‘petal-shaped’’ cataracts within the posterior perinuclear cortex. These usually have a triradiate distribution, suggesting some developmental association with the posterior sutures. Elliptic cataract describes non-progressive, extensive, ellipsoid, encircling, perinuclear opacities (Walde, 1983.). The nucleus may be cataractous, and vision is usually compromised in affected eyes. Coralliform or crystalline cataract is a non-progressive, irregularly triradiate, and highly refractile cataract located in the posterior nucleus. Complete congenital cataracts present as dense and uniform opacifications of the nucleus and cortex. In some lenses, a narrow zone of clear periequatorial cortex may be evident. Vision is severely compromised. These cataracts are frequently associated with other ocular abnormalities, most commonly microphthalmos. Care should be taken in differentiating complete congenital cataracts from the rare cases of complete cataracts in young foals caused by congenital uveitis. In the latter case, there are usually obvious post inflammatory changes of the iris.

Management of Equine Cataracts

Most cataracts have no significant functional effect on vision in the horse; as a result, treatment is likely to be attempted in only a few selected cases. Medical treatment of senile cataracts in human beings has been a focus of clinical interest for some time, and various antioxidant, anti-inflammatory, and aldose reductase inhibitor preparations have been examined but with little success (Jacques and Chylack, 1996). In the horse, oral aspirin at a dose of 30 mg/kg/d has been advocated as a means of suppressing cataractogenesis in postuveitic eyes (Lavach, 1990), although there are no published evaluations of the efficacy of this approach. Protracted mydriasis achieved using chronic topical atropinization has been suggested as a means of improving vision in the eyes of riding horses with central cataracts (McLaughlin et al., 1992; Boydell, 1997). This is likely to be of very limited practical value, however, and carries the known risk of ileus in some animals (Brooks, 1999). As in other species, the standard management of cataract in the horse is surgical removal of the lens (Brooks, 1999; McLaughlin et al.,1992; Hardman et al.,2001; Millichamp and Dziezyc, 2000; Whitley et al.,1990; Dziezyc et al.,1989).

Currently, the surgical technique of choice is phacoemulsification and aspiration after anterior capsulorrhexis. Most veterinary ophthalmologists would agree that cataract surgery in the horse is, at present, more challenging than in the dog. This is, in part, a reflection of the relative inexperience of most veterinary ophthalmologists with cataract surgery in the horse, along with the technical problems caused by ocular size and the specific anesthetic requirements for intraocular procedures in this species. Considerable care in the selection of horses suitable for surgery is essential. There should be overt vision loss attributable to the cataract (Millichamp and Dziezyc, 2000) and horses must be tractable and likely to be compliant with the aggressive topical medication protocols necessary throughout the pre- and postoperative periods. The only type of equine cataract that is consistently suitable for surgery is a complete congenital cataract in an otherwise normal eye. Complete cataracts have been successfully removed from microphthalmic eyes in which the cataract is the only other identifiable abnormality, however (Millichamp and Dziezyc, 2000). Nevertheless, it is imperative that horses presented for surgery be examined closely for other ocular abnormalities, particularly retinal detachment or uveitis, that would jeopardize the successful outcome of surgery. A standard pre-surgical examination protocol should include ocular ultrasonography and, when possible, electroretinography (ERG). The presence of uveitis, particularly ERU, significantly reduces the likelihood of a successful surgical outcome. Some surgeons are willing to attempt the procedure in mildly uveitic eyes in which the intraocular inflammation is quiescent or is under medical control (Brooks, 1999; Millichamp and Dziezyc, 2000), although the prognosis in these cases is inevitably more guarded, because the procedure is likely to provoke further inflammatory injury. When possible, congenital cataracts should be operated on before the animal is 6 months of age. At this age, the cataract is likely to be relatively immature and soft, facilitating removal by phacoemulsification and aspiration. After cataract surgery, aphakic horses seem to cope well with any visual impairment arising from uncorrected hypermetropia (Farrall, 1990). This may be a result of the existence of relatively large retinal receptor fields in this species, which limits the effect on visual acuity of the magnified and blurred aphakic image (Farrall and Handscombe, 1990). Deprivation amblyopia is not recognized as being a particular problem in operated foals (Roberts, 1992). Nonfoldable intraocular prosthetic lenses have been implanted in equine eyes after lensectomy, but the procedure seems to be associated with significant postsurgical complications (Millichamp and Dziezyc, 2000).The techniques of surgical treatment of equine cataracts are comprehensively discussed elsewhere (Brooks, 1999; McLaughlin et al., 1992; Millichamp and Dziezyc, 2000). The procedure is not without potential complications, however. These include intraoperative problems, such as corneal edema and posterior capsule tearing with vitreous loss. Intraoperative hemorrhage, probably arising from preiridial fibrovascular membranes, seems to be a particular problem in uveitic eyes and is difficult to control (Brooks, 2002.). After surgery, glaucoma and endophthalmitis are potentially disastrous sequelae. In foals, immune compromise associated with Rhodococcus equi pulmonary abscesses may be a contributing factor in postoperative endophthalmitis, and chest radiographs should be taken as part of the surgical selection protocol in foals As in other species, post surgical posterior capsular opacity is a major problem in the horse and may necessitate a second surgery in an attempt to clear the visual axis by posterior capsulorrhexis. The presence of anterior capsular opacities before surgery may indicate the concurrence of posterior capsular cataracts, which may intensify after surgery and contribute to posterior capsular opacification (Whitley et al.,1990). In summary, although postsurgical visual assessment is of necessity subjective, most recent authors consider that the potential for improvement in visual function after surgery makes the procedure worthwhile in carefully selected patients (Millichamp and Dziezyc, 2000). Surgical removal of the lens requires intensive medical treatment with topical and oral medications for a month or longer following the procedure to maximize chances of success. Placement of a subpalpebral lavage system at the time of surgery is usually performed for ease of treatment. A protective mask may be placed to protect the eye and lavage system from damage. During the treatment period, the horse should be kept out of bright light and have severely limited exercise.It should be noted that even after successful lens removal, vision will never be normal. Without a lens, your horse will be substantially “far-sighted,” meaning “close up” vision is poor. Fortunately, most horses seem to adapt to this and are capable of functioning adequately. Additionally, some veterinary ophthalmologists are beginning to implant artificial lenses to reduce this effect. Expected scarring of ocular structures can reduce vision somewhat, as can pre-existing ocular disease and surgical complications. A horse that has had cataract removal is never considered “sound,” because vision is diminished from that of a normal eye, and he may not be suitable for some occupations. Equine cataract surgery is performed similarly compared with the techniques used in human ophthalmology and in other veterinary species. However, enough differences exist to make surgical lens removal and intraocular lens implantation in the horse an intrinsically unique endeavour. Due to the size of the adult equine globe, the introduction of species-specific instrumentation has provided the cornerstone to many of the changes made regarding surgical technique over the last 15–20 years. The continuing development of an equine specific, foldable intraocular lens implant (IOL) has provided much needed data supporting the use of such lenses in the horse to improve upon the post operative visual outcome. Finally, the methods utilised to assess visual capacity and the effects of intraocular lens implantation on the globe (e.g. ocular ultrasonography, electroretinography and streak retinoscopy) are gradually becoming more important in preoperative patient assessment and IOL development in the horse. It is the hope of the authors that a broader group of equine veterinarians will become aware of the many changes that have taken place in equine cataract surgery over the last half-century. Although aspiration was implemented nearly 40 years ago in foals for the treatment of congenital cataracts, phacofragmentation (phacoemulsification) techniques have only recently become routine in mature horses undergoing lens extraction (McMULLEN and UTTER 2010). The most current technology for cataract removal (in people or animals) involves the use of phakoemulsification to remove the cataract. Phacoemulsification cataract surgery is the most useful technique for the horse. This extracapsular procedure through a 3.2mm corneal incision utilizes a piezoelectric handpiece with an ultrasonic titanium needle in a silicone sleeve to fragment and emulsify the lens nucleus and cortex following removal of the anterior capsule. The emulsified lens is then aspirated from the eye while intraocular pressure is maintained. The thin posterior capsule is left intact. There is little inflammation postoperatively in most horses following phacoemulsification cataract surgery, and there is a quicker return to normal activity with phacoemulsification than other surgical techniques. The results of cataract surgery in foals by experienced veterinary ophthalmologists are generally very good, but the cataract surgical results in adult horses with cataracts caused by ERU are often poor. The problem is that new blood vessels form on the iris and anterior lens capsule in the eyes with ERU, and they can bleed during the surgeries. The surgeon often cannot stop the hemorrhage and severe hyphema results. Using phakoemulsification we are able to restore useful vision in most horses. Just as with cataract surgery in people, a small percentage of horses may have complications associated with the surgery. The main problems encountered are inflammation (uveitis), which tends to be more severe in horses than in people and may result in scarring, which can affect vision. Some horses may develop increased intraocular pressure (glaucoma), which can cause blindness by damaging the optic nerve. Retinal detachment is occasionally seen as a complication of cataract surgery (just as in people). Infection in the eye, which fortunately occurs infrequently, can be a major complication – even resulting in loss of the eye. Obviously as with any surgery there are risks associated with general anesthesia. At EEV we try to minimize these risks by performing blood tests and sometimes chest radiographs and umbilical ultrasonography (in foals) before surgery to detect any other systemic conditions which might affect the response to anesthetics or success of the surgery. If problems are detected we will recommend further workup before surgery. All patients are closely monitored during surgery and recovery. Because we remove all of the contents of the lens it is not possible to again form a dense cataract. However some horses may have growth of a thin layer of lens cells over the remaining lens capsule after surgery which gives a hazy, gray appearance to the eye (posterior capsular opacification or PCO). In most cases this does not significantly interfere with vision.

Prevention of Cataract

A horse’s eyes are critical to the animal’s usefulness. Blind horses are usually not safe to ride and are unable to maneuver well. In addition, they may shy suddenly and may develop phobias that make them untrustworthy around people. Any time a horse has an observable eye problem, a veterinarian should make a diagnosis. Many eye problems have similar symptoms, and a treatment that works well for one eye problem may cause further damage if attempted in the presence of other eye disorders. Make sure that any time a horse gets a foreign object in the eye it is removed and treated promptly. Any time trauma, bacterial or viral eye infections, occular parasites, or systemic infections are present; they can lead to Equine Recurrent Uveitis and the development of cataracts and blindness. The best prevention is to call a veterinarian any time infection, tearing, squinting, or refusal to open an eye occurs. Most veterinary ophthalmologists recommend surgical removal of cataracts in foals less than 6 months of age if the foal is healthy, no uveitis or other ocular problems are present and the foal’s personality will tolerate aggressive topical medical therapy.

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