The Eye

CHAPTER 43 The Eye




The Ophthalmic Examination


Examination of the neonate must take into account age-related variations in ocular appearance and response. Generally the eyelids open at 10 to 14 days of age, revealing a cloudy cornea that begins to clear within 24 hours. The patient should demonstrate a blink in response to a bright light when the lids open. Pupillary light reflexes are present within 24 hours of eyelid separation but remain sluggish for 3 weeks. Vision and menace responses are generally poor for the first 3 weeks and may not reach adult standards until 6 to 8 weeks of age. Reflex lacrimation begins when the eyelids open. Box 43-1 summarizes additional features of the neonatal eye.




The Globe and Orbit



Congenital Abnormalities




Microphthalmia


Failure of the eye to develop to normal size is referred to as microphthalmia. The small globe is often associated with a correspondingly small palpebral fissure. Depending on the constellation of accompanying defects, vision may be normal, diminished, or absent. Microphthalmia with multiple colobomas is an autosomal recessive trait linked to merling in the Australian Shepherd (Figure 43-1).



In addition to small globes, dogs with merle ocular dysgenesis may have persistent pupillary membranes, cataract, equatorial staphylomas, choroidal hypoplasia, retinal dysplasia and detachment, and optic nerve hypoplasia. Vision is frequently impaired. Similarly affected breeds include the Great Dane, Collie, Shetland Sheepdog, Dachshund, and Catahoula Leopard Dog.


Microphthalmia is associated with inherited congenital cataracts in the Miniature Schnauzer, Old English Sheepdog, Akita, and Cavalier King Charles Spaniel, as well as with retinal dysplasia in the Bedlington and Sealyham Terriers, Beagle, and Labrador Retriever. In the Doberman Pinscher, microphthalmia, anterior segment dysgenesis, and retinal dysplasia are thought to be inherited as an autosomal recessive trait.


Microphthalmia, choroidal and optic disc colobomas, retinal dysplasia, and tapetal aplasia have been attributed to heredity and in utero viral infections in Domestic Shorthair kittens. Multiple anomalies, including colobomas, have also been described in Persian cats. Administration of griseofulvin to pregnant cats may cause microphthalmia in their offspring and has also resulted in anophthalmos, cyclopia, and optic nerve aplasia.





Acquired Abnormalities





Traumatic Proptosis


Complete displacement of the eye from the orbit is most commonly seen in brachycephalic dogs, but any dog or cat may present with proptosis with sufficient cranial trauma (Figure 43-2). A completely displaced eye is a true ocular emergency. Prognosis for vision is always poor because of optic nerve injury, but the intact globe may be salvaged cosmetically.



A soft eye, indicating rupture of the fibrous tunic, or one with extensive avulsion of extraocular muscles or optic nerve should be enucleated. Those with severe intraocular hemorrhage usually shrink over time because of irreparable damage to the ciliary body.


General anesthesia is required to reposition the globe. After flushing the eye with sterile saline solution, a blunt probe, such as a spay hook, or preplaced sutures of 5-0 nylon are used to elevate the eyelid margins. Simultaneous gentle counterpressure is applied against the cornea with a moistened cotton ball or the flat surface of a scalpel handle to push the eye back into place. The eyelids are sutured closed using horizontal mattress sutures of 5-0 nylon placed over stents. Sutures should enter the eyelid 3 to 5 mm from the margin and exit through the meibomian gland openings to prevent corneal damage.


A broad-spectrum systemic antibiotic and a tapering regimen of oral corticosteroids are recommended for 7 to 10 days. Injections should not be made into the orbital area. If the animal allows, topical antimicrobial ointment may be applied 3 or 4 times daily between the eyelid margins at the medial canthus. Warm compresses are recommended for 3 to 4 days after the replacement. One of the most frequent management errors is removing the tarsorrhaphy sutures prematurely. Sutures are left in place for 14 days or until the globe settles back into the orbit. Sequelae include lateral strabismus as a result of rupture of the medial rectus muscle, blindness, low tear production, and shrinkage of the eye (phthisis bulbi).




The Eyelids



Congenital Abnormalities





Entropion


Inversion of the eyelid margin occurs commonly in dogs but infrequently in the cat. The lower eyelid is more often affected. Accompanying clinical signs include increased tearing, squinting, corneal vascularization, and ulceration (Figure 43-4).



Several genes that define the eyelid structure, globe-orbit relationship, and facial skin are likely to influence the degree of entropion. The narrow palpebral fissure of the Chow Chow, the deeply set eye of the Golden Retriever, and the redundant facial folds of the Shar-Pei are examples of the variables that determine lid conformation. Painful ocular disorders also cause entropion secondary to spasm of the orbicularis oculi muscle. This mechanism is commonly implicated in cats with chronic herpesvirus infection.


Young animals with entropion but without corneal disease may be treated palliatively with an ophthalmic lubricant ointment to postpone surgery until the patient matures. Delaying surgery in the Shar-Pei may not be possible because of the severity of the breed’s entropion and the risk of corneal ulceration. Temporary “tacking” is used to evert the lid margins and forestall corneal damage. The procedure is most effective when performed in the 3- to 4-week-old puppy. The older the animal is when tacking is first performed, the less likely it will be to correct the entropion without additional surgery. Local anesthetic blocks can be used in the very young animal, but masking the patient with isoflurane expedites the procedure. The first bite of a 5-0 nylon vertical mattress suture enters 2 mm from the eyelid margin and engages about 4 mm of eyelid skin and subcutaneous tissue. Avoid suturing the eyelid margin directly because postsurgical notching can irritate the cornea. The needle is then reinserted into the skin and deeper tissue overlying the orbital rim, adjusting the tension to evert the eyelid margin in a slightly overcorrected position (Figure 43-5).



The number of required sutures varies with the individual patient. A topical lubricant ointment is applied twice daily. Ideally the sutures are left in place for weeks. Surgical staples may also be used, with the advantages of rapid placement and excellent retention.


As long as the cornea remains healthy, definitive surgery is ideally postponed until the patient is 6 months of age or older. The modified Hotz-Celsus technique is the classical procedure for entropion repair, everting the lid margin by removing an ellipse of eyelid skin equal to the amount of lid inversion. Preoperative assessment should be performed after application of a topical anesthetic to eliminate any spastic component that exaggerates the degree of entropion. It is also critical to establish a surgical plan before anesthesia, when enophthalmos and loss of eyelid tone alter eyelid position significantly. With the eyelid supported by a Jaeger lid plate inserted into the conjunctival cul-de-sac, the initial incision is made with a scalpel blade 2 to 3 mm from and parallel to the lid margin, along the line where the eyelid hair begins, extending at least 1 mm medially and laterally to the entropic section. The distal incision gently arcs, joining with the two ends of the primary incision and outlining the tissue that must be excised to successfully evert the margin. The incised ellipse of tissue is removed with scissors. The wound is closed with 4-0 to 6-0 simple interrupted braided nylon, silk, or polyglactin sutures, placing the first suture at the midpoint of the incision and continuing to bisect the remaining wound segments with sutures until the defect is perfectly apposed. Topical antibiotic ointment is applied to the eye twice daily until suture removal. A systemic nonsteroidal antiinflammatory drug (NSAID) provides satisfactory analgesia. The wound is protected with an Elizabethan collar. Sutures are removed in 10 days, although 6-0 polyglactin sutures are commonly left to resorb. A modification of the Hotz-Celsus procedure that excises an arrowhead-shaped area of skin around the lateral canthus can be used to correct the lateral canthal inversion in the Shar-Pei, Chow Chow, and Retriever breeds. A combination of techniques may be necessary in breeds with excessive facial folds or elongated lids, prompting referral of these patients to an ophthalmologist.







Acquired Abnormalities








The Conjunctiva




Acquired Abnormalities




Bacterial Conjunctivitis


Bacterial conjunctivitis is characterized by conjunctival hyperemia, swelling, and mucopurulent discharge. In the dog, it is not a primary ocular disease but rather a consequence of a predisposing adnexal or lacrimal abnormality. Culture may reveal one or more types of bacteria, typically susceptible to a wide range of antibacterial drugs. Response to a topical antibacterial medication is usually rapid, but clinical signs recur once treatment is discontinued. A careful examination should be performed to rule out keratoconjunctivitis sicca (KCS), ectopic cilia, foreign bodies, entropion, ectropion, and nasolacrimal disease.


In contrast, the cat may exhibit a primary conjunctival infection by C. felis. Initial infection is characterized by a mild rhinitis and unilateral purulent conjunctivitis. Second eye involvement occurs 5 to 7 days later. Lack of corneal involvement may help differentiate the conjunctivitis from that caused by herpesvirus. Basophilic cytoplasmic inclusions in conjunctival scrapings performed during the first 2 to 9 days after onset of clinical signs are diagnostic, as are indirect fluorescent antibody (IFA) and polymerase chain reaction (PCR) testing in more chronic cases. Treatment consists of topical tetracycline or erythromycin ointment 4 times daily for 3 weeks or a 3-week course of oral doxycycline (5 mg/kg twice daily) or azithromycin (5 mg/kg daily).


Mycoplasma spp. are historically implicated in feline conjunctivitis. Controversy remains with respect to the organism’s importance because infection cannot be established in the absence of other pathogens. Diagnosis is confirmed by demonstrating coccoid basophilic organisms in clusters on the epithelial cell membrane in the initial stages of infection or by PCR analysis of conjunctival swabs. Topical tetracycline is the medication of choice.



Viral Conjunctivitis


In the naive neonatal or adolescent cat, feline herpesvirus-1 (FHV-1) causes an acute conjunctival and respiratory infection. Clinical signs are directly related to the viral cytopathic effect on epithelial cells. The initial conjunctivitis is bilateral with pronounced hyperemia, accompanied by ocular discharge that changes from serous to mucopurulent. Pathognomonic dendritic corneal erosions may be overlooked if rose bengal is not used to stain the corneal surface. Kittens with viral conjunctivitis before eyelid separation will often have such severe conjunctival and corneal erosions that adjacent raw surfaces adhere together, creating symblepharon. These conjunctival adhesions can cause permanent prominence of the third eyelid, epiphora from obstruction of the nasolacrimal puncta, and corneal opacity.


Following recovery in 10 to 14 days from the initial herpesvirus infection, 80% of cats become latently infected carriers, and an estimated 45% of these will experience spontaneous viral shedding and/or recrudescence of clinical disease in the future. Subsequent episodes of conjunctivitis commonly occur without respiratory signs and affect only one eye. Diagnosis of herpetic conjunctivitis is more often based on history and clinical signs than on specific testing. Serology is predictably positive because of the widespread exposure of cats to herpesvirus. Viral detection using IFA or PCR testing fails to differentiate between wild and vaccinal virus.


Feline conjunctivitis should be considered infectious until proven otherwise. Topical corticosteroids may initially decrease hyperemia and swelling but are not recommended because they also prolong virus shedding and increase the risk of herpetic ulceration. Specific antiviral therapy is seldom used in the initial ocular-respiratory syndrome because of its self-limiting nature. If corneal ulceration is present, topical 0.1% idoxuridine may be compounded for topical use; its virostatic nature requires application 4 to 6 times daily. Topical 0.5% cidofovir requires only twice-daily application for 14 days but is much more expensive. Ocular signs improve in patients treated with oral famciclovir at a dose of 15 mg/kg every 12 hours for 14 days. Other oral antiviral medications are either ineffective or toxic to the cat. Lifelong oral L-lysine supplementation limits viral replication and may reduce the severity or frequency of relapses. The recommended dosage for kittens is 250 mg twice daily and for adult cats is 500 mg twice daily, usually supplied as a powder and mixed in food.


Dogs infected with canine distemper virus may exhibit conjunctivitis, ocular discharge, and reduced tear production. Conjunctival scrapings in the acutely infected patient may contain intracytoplasmic inclusion bodies.

Sep 11, 2016 | Posted by in SMALL ANIMAL | Comments Off on The Eye
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