The Globe And Orbit

By | November 12, 2015

Congenital Abnormalities

Microphthalmos. Failure of the eye to develop to normal size is referred to as microphthalmos. Complete absence of the eye (anophthalmos) is extremely unusual in puppies and kittens. Microphthalmos is characterized by varying degrees of enophthalmos, with or without other ocular defects. Microphthalmos with multiple colobomas is an autosomal recessive trait linked to coat color in the Australian shepherd. In addition to small globes, affected dogs may have persistent pupillary membranes, cataract, equatorial staphylomas, choroidal hypoplasia, retinal dysplasia and detachment, and optic nerve hypoplasia. Vision is frequently impaired. Other breeds in which multiple ocular defects have been associated with coat color include the Great Dane, collie, Shetland sheepdog, and dachshund. Microphthalmos is also associated with inherited congenital cataracts in the miniature schnauzer, Old English sheepdog, Akita, and King Charles Cavalier spaniel. Microphthalmos occurs with retinal dysplasia in Bedlington terriers, Sealyham terriers, beagles, Labrador retrievers, and Doberman pinschers. Administration of griseofulvin to pregnant cats may produce microphthalmos in their offspring.

Atypical Eye Position. Strabismus is outward or inward deviation of the eye. Kittens are born with an outward or divergent strabismus that is noted after eyelid opening and normally should be corrected by 2 months of age. Divergent strabismus also occurs in brachycephalic dog breeds, notably the Boston terrier. Convergent or inward strabismus (cross-eye appearance) is inherited in the Siamese cat as an autosomal recessive trait. The abnormality is caused by aberrant development of the central visual pathways, particularly the lateral geniculate nucleus. Bilateral ventrolateral strabismus may be seen in puppies and kittens with hydrocephalus because of the enlargement of the calvaria and bony orbits.

A wandering movement of the eyes in young puppies and kittens (ocular nystagmus) is generally associated with congenital blindness. Rapid, repetitive, involuntary movement of the eyes (nystagmus) commonly occurs in Siamese kittens and may be related to the neuroanatomic abnormalities that produce strabismus. Vision is functionally normal. The nystagmus may lessen as the kitten matures. Nystagmus also occurs in some cats with Chediak-Higashi syndrome.

Acquired Abnormalities

Orbital Cellulitis and Abscessation. Orbital cellulitis is usually characterized by an acute-onset unilateral exophthalmos, protrusion of the third eyelid, chemosis, pain on opening the mouth, and fever. In addition, abscessation typically causes a swelling or discoloration in the mouth behind the last maxillary molar. Foreign objects and fight wounds are likely causes in puppies and kittens. Cuterebra larvae may occasionally invade the orbit and cause the orbital cellulitis. Response to twice-daily hot compresses and a broad-spectrum systemic antimicrobial agent is generally rapid. If no response is noted within 24 hours, incising the mucous membrane behind the last maxillary molar and inserting a small hypodermic needle into the retrobulbar area establishes ventral drainage. In many cases, only a serosanguinous discharge is seen on withdrawal of the hypodermic needle. Damage to orbital structures occurs when the procedure is performed carelessly. If exophthalmos is extreme, a bland ophthalmic ointment may be applied to the corneal surface to prevent drying.

Orbital Neoplasia. Orbital neoplasia should be considered in any puppy or kitten with gradual, painless exophthalmos. Neoplasia is more common in older animals, although embryonal neoplasia can affect young animals. Orbital involvement with lymphosarcoma and fibrosarcoma may also occur in young cats.

Traumatic Displacement. Complete displacement of the eye from the orbit is most commonly seen in brachycephalic dogs, but cats may also present with proptosis, retrobulbar hemorrhage, or fracture of the bony orbit. A completely displaced eye is an emergency and should be treated as soon as possible. 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. A more favorable prognosis accompanies the intact eye with a constricted pupil. Most animals require general anesthesia to replace the eye. The eye should first be irrigated with sterile physiologic saline solution. A blunt probe, such as a strabismus or spay hook, is used to elevate the eyelid margins, and simultaneous gentle counterpressure is applied against the cornea with a moistened cotton ball to push the eye back into place. Preplaced sutures can be used in place of the blunt probe to elevate the eyelid margins. Once the eye is replaced, the eyelids are closed using horizontal mattress sutures of 4-0 nonabsorbable material placed over stents. Sutures should enter the eyelid 5 mm from the margin and exit through the meibomian gland openings to prevent corneal damage.

Broad-spectrum systemic antimicrobial agents and a taping level of corticosteroids are recommended for 7 to 10 days. If the animal allows, topical antimicrobial ointment may be applied three or four times daily between the eyelid margins at the medial can thus. Warm compresses are recommended for 3 to 4 days after the replacement. Sutures should be left in place for at least 10 to 14 days and should be replaced if the animal is unable to close the eyelids after the sutures are removed. Prognosis for vision is not known until after suture removal. Sequelae include lateral strabismus due to rupture of the medial rectus muscle, blindness, low tear production, and postinflammatory atrophy of the eye (phthisis bulbi).

Enophthalmos. Enophthalmos is recession of the eye within the orbit. Saint Bernards, Great Danes, Doberman pinschers, golden retrievers, and Irish setters often appear enophthalmic owing to their large orbits and deeply set eyes. Congenital enophthalmos is most often associated with microphthalmos. Acquired causes may include chronic orbital inflammation, loss of retrobulbar fat in debilitated or slow-growing animals, and phthisis bulbi. Causes of transient enophthalmos include Horner’s syndrome, in which loss of sympathetic tone in the orbital fascia results in the eye’s sunken appearance, or any painful eye disorder. Enophthalmos may be accompanied by mucoid to mucopurulent discharge, ptosis, and entropion.

 

Selections from the book: “Veterinary pediatrics: dogs and cats from birth to six months”. Johnny D. Hoskins. (2001)