Chapter 13 Orbit
The conditions involving the eye and orbit essentially can be divided into those in which the eye itself is abnormally sized (e.g., microphthalmos, phthisis bulbi, glaucoma) and those in which the position of the eye in the orbit is abnormal because of either an increase or decrease in other orbital contents or a traumatic displacement of the eye from the orbit (proptosis). Microphthalmos is a developmental condition often associated with other congenital ocular lesions that may be inherited or may result from some physical or inflammatory insult to the developing eye. The globe looks smaller and more recessed into the orbit. Phthisis bulbi is a shrinking of the globe resulting from reduced aqueous humor production secondary to chronic inflammation or glaucoma.
Enophthalmos is recession of the normally sized eye within the orbit. The condition may result from loss of other ocular contents (fat) or from scarring caused by inflammation. Enophthalmos is also seen as a component of Horner’s syndrome, because of sympathetic denervation of the circumferentially arranged muscle fibers in the orbital cone that help maintain the normal position of the globe.
Exophthalmos is a protrusion of the globe resulting from the presence of a space-occupying lesion in the orbit (inflammation or infection, neoplasia, zygomatic salivary gland enlargement, vascular malformation, or myositis).
Proptosis is usually traumatic and results in forward displacement of the globe from the orbit. The proptosed globe may undergo varying degrees of injury—most notably, avulsion of extraocular muscles or the optic nerve.
Figure 13-3 Two views of the same eye of the dog shown in Figure 13-2. This Australian shepherd has hereditary multiple ocular anomalies. A large staphyloma (arrows) is present in the peripheral fundus, with choroidal dysplasia (choroidal vessels not formed). The purple streak is the long posterior ciliary artery.