The Ophthalmic Examination
A complete ophthalmic history is an essential part of every puppy’s or kitten’s examination. Owners may be asked questions regarding the animal’s signalment, history of the presenting complaint(s), and any pertinent medical or ophthalmic diseases in the animal’s family histories. Other historical information that may be included is the animal’s vaccination status, diet, environment, and exposure to other animals. Previous therapy should be identified to prevent repetition of an unsuccessful regimen.
Ophthalmic examination should be performed in a quiet area. Puppies usually require only gentle but firm restraint of the head. Very young puppies cooperate nicely when held in an assistant’s arms. Kittens can also be gently restrained and are less likely to demonstrate the constant ocular motion typical of puppies. Uncooperative puppies or kittens may be placed in a towel or restraint bag. Assessment of ocular abnormalities such as orbital swelling, squinting, or ocular discharge can be done in a well-lighted room, but actual ophthalmoscopic examination should be done with the lights dimmed. A bright source of focal illumination is required; the Finoff transilluminator on a fully charged direct ophthalmoscope handle is ideal.
Vision is assessed by observing the animal’s response to a threatening gesture toward the eye, throwing cotton balls across the animal’s visual field, or evaluating maneuverability through an obstacle course. Initial animal responses are most reliable, especially in kittens. The menace reflex is present but poorly developed in puppies and kittens when the eyelids separate at 5 to 14 days; it may not appear to be present until the third or fourth week of life. The cilia of the eyelids are situated on the upper eyelid of the dog and are normally absent in the cat. Active blinking movements of the eyelids are protective of the cornea and disperse the tear film over the corneal surface. Pupillary light responses are typically slow until the retina matures.
Corneal or conjunctival cultures and evaluation of tear production using the Schirmer tear test should be completed before any drops are applied to the eyes. For culturing, the lower eyelid is everted and a saline-moistened, sterile, cotton-tipped swab is rolled across the conjunctiva, being careful to avoid the eyelid margin and its contaminants. Conjunctival scrapings are obtained with a heat-sterilized spatula following topical anesthesia, and the sample is collected from the same region as the culture. The sample is gently applied to a clean glass slide and prepared with Wright’s or Gram’s stain for cytologic examination. Fluorescein dye is used to demonstrate corneal ulcers. To avoid errors in interpretation, excess dye should be flushed from the eye with a sterile saline solution before the corneal surface is viewed.
Examination of the lens and fundus requires that the pupil be dilated with one drop of 1% tropicamide, followed in 5 minutes by a second application. Maximum mydriasis occurs within 15 to 20 minutes. Examination of the anterior segment (eyelids, conjunctiva, cornea, anterior chamber, and iris) is best done with a bright-light source and a magnifying loupe. After the pupil is dilated, the penlight or direct ophthalmoscope (set at +8 to +12 diopters) can be used to examine the lens. The retina and optic disc may be evaluated by 3 weeks of age and older at a direct ophthalmoscope setting of – 2 to + 2 diopters. If the puppy retracts the eye or protrudes the third eyelid, it may help to reduce the intensity of light.
The Nictitating Membrane
Prolapse of the Nictitans Gland
Incomplete development of soft tissue attachments will cause the nictitans gland to protrude beyond the leading edge of the third eyelid. Beagles, Boston terriers, cocker spaniels, Lhasa apsos, and English bulldogs and Burmese cats are commonly affected. Exposure of the nictitans gland’s surface often contributes to conjunctivitis. A 30% to 57% decrease in tear production can be expected when the nictitans gland is removed. Partial gland resection, leaving a portion of the secretory gland tissue, is effective, but repositioning of the nictitans gland to preserve its tear-producing capabilities is preferred. The prolapsed nictitans gland may be sutured to adjacent tissue such as orbital fascia, extraocular muscle, or sclera, or its overlying conjunctiva may be imbricated to return the nictitans gland to a more normal position.
Eversion of the Cartilage
Eversion of the cartilage is characterized by a scroll-like appearance of the nictitating membrane’s leading edge. The junction of the horizontal and vertical arms of the T-shaped cartilage appears to be the weakest segment of the cartilage. Eversion occurs in several dog breeds, including the Great Dane, Saint Bernard, German shepherd, Weimaraner, German short-haired pointer, golden and Labrador retrievers, and English bulldog, as well as the Burmese cat. Clinical signs in affected animals include watery or mucoid ocular discharges and lymphoid hyperplasia in long-standing cases. Removal of the malformed section of cartilage through an incision on the bulbar surface is recommended. The gland of the nictitating membrane may prolapse after the cartilage resection.
Congenital glaucoma is usually secondary to traumatic insult or inflammation. Normal intraocular pressure in the young dog and cat ranges between 15 and 30 mmHg. Acute glaucoma is painful and accompanied by conjunctival and episcleral hyperemia, generalized corneal edema, pupillary dilation, and vision loss (). With prolonged pressure elevations, degeneration of the retina and optic nerve results in permanent blindness. Determination of intraocular pressure confirms the glaucoma diagnosis.
The aim of immediate therapy for glaucoma is to save or return vision through immediate control of intraocular pressure. Decisive treatment should be administered with minimal delay, as marked increases in intraocular pressure result in irreversible damage to the retina and optic nerve in 24 to 48 hours. A combination of agents yields the greatest reduction in intraocular pressure (). A 20% mannitol is administered intravenously at 1 to 2 g/kg early in acute glaucoma management to reduce intraocular pressure quickly. Pressure begins to decrease within 10 to 15 minutes; the effect lasts for 5 to 6 hours. Reduction in aqueous production is achieved with carbonic anhydrase inhibitors, such as dichlorphenamide administered twice daily at 2.5 to 5 mg/kg orally. Aqueous outflow may be improved by the use of a miotic, such as 2% pilocarpine applied topically three or four times daily. Miotics are not effective at intraocular pressures above 50 mmHg, so it is important to combine them with the other drugs listed previously.
Surgical therapy is indicated if medical therapy cannot lower intraocular pressure sufficiently within 24 to 48 hours, if wide variations in diurnal pressures exist, or if the owner is unwilling to dedicate time to daily therapy. There is no universal acceptance of any of the surgical procedures for glaucoma. Cyclocryotherapy is often used because it is noninvasive and an 80% success rate has been reported by various investigators. By applying a cryoprobe to the sclera overlying the ciliary body, one can damage portions of the secretory epithelium, and aqueous production is reduced.
Selections from the book: “Veterinary pediatrics: dogs and cats from birth to six months”. Johnny D. Hoskins. (2001)