Orbital disease can be a challenge to diagnose and treat appropriately. Only the more common acquired and pathologic orbital
conditions will be discussed within the scope of this lecture.
Changes in globe position characterize orbital disease, and are readily identified in most cases as facial asymmetries. Clinical
examination is the most important single diagnostic technique. Direct orbital palpation and globe retropulsion can also be
performed for diagnostic purposes. Since dogs and cats lack bony ventral orbital structures, an oral exam, usually under sedation,
is considered part of a complete orbital exam. Signs of pain on orbital and/or oral examination are clinically relevant and
should be noted. Although radiography is occasionally helpful, more advanced orbital imaging techniques like ultrasound, computed
tomography (CT), and Magnetic Resonance Imaging (MRI) are more useful in most cases. The information obtained from imaging
helps guide sample collection, such as for cytology, culture and sensitivity, and histopathology.
Exophthalmia, or anterior displacement of the globe, is the most common orbital condition and must be differentiated from
buphthalmia, or globe enlargement. Enophthalmia, or inward sinking in of the globe, occurs less commonly as a pathologic
entity, and must be differentiated from a phthisical or shrunken globe, as well as from exophthalmia of the contralateral
globe. Strabismus (globe deviation) and globe displacement represent related, but slightly different, abnormalities of globe
position, and may be associated with exophthalmia.
Exophthalmic globes may grossly bulge or simply be relatively resistant to manual retropulsion when compared to the contralateral
eye. Indentation of the caudal globe may be identified by ophthalmoscopy. Intraocular pressure elevation may or may not occur
due to compression of the globe by surrounding tissues. Secondary keratitis due to increased exposure is common. Fortunately
for diagnostic reasons, exophthalmia is almost always unilateral in nature. The more common differentials for acquired exophthalmia
in dogs include orbital cellulitis/abscess, orbital neoplasia, salivary cyst/mucocele, hematoma, and extraocular muscle myositis.
The latter condition is a rare condition seen only in dogs, while the former conditions apply to cats as well. Extraocular
muscle (EOM) myositis is also the exception in that it is bilateral in nature, unlike most cases of exophthalmia. It is an
autoimmune condition in which both eyes grossly protrude, and is typically seen in Golden Retrievers. Orbital imaging will
reveal dramatic enlargement of the major extraocular muscles. There is some risk associated with the biopsy of these small
muscles, but results may be diagnostic. Treatment via systemic immunosupression is usually at least initially successful,
if the disease is caught in its acute stages, before muscle fibrosis and atrophy occur. The combination of good history-taking
and general physical exam with indications of trauma, should provide an index of suspicion for orbital hematoma, which can
be supported by ultrasound, treated presumptively, and further diagnostics be pursued only if nonresponsive to time and treatment.
Diagnostic efforts in most cases of unilateral exophthalmia in the dog and cat should be aimed at differentiating orbital
inflammatory disease (infectious cellulitis, abscess, cellulitis secondary to salivary mucocele) from orbital neoplasia.
Orbital inflammatory disease most commonly occurs in young febrile animals, for which jaw excursion and globe retropulsion
elicits pain; while the opposite is true of small animal patients with orbital neoplasia- they are usually older, afebrile,
and painless. Inflammatory disease is typically acute in nature, whereas neoplasia tends to be gradually progressive. These
are not hard and fast rules, and should be interpreted with caution. Following an initial clinical exam, including an oral
exam, imaging is usually indicated. Orbital ultrasound can often help to differentiate inflammatory from neoplastic disease.
Cystic structures, such as seen with true orbital abscess, are readily identified in this manner. Unfortunately a more generalized
inflammatory process, cellulitis, is a more common manifestation of septic retrobulbar infection than organized abscess.
Sometimes orbital foreign bodies may be identified within the retrobulbar space as well, in association with the inflamed
soft tissues. Advantages of ultrasound include relatively low expense compared to CT and MRI, lack of need for general anesthesia,
and its usefulness in guiding biopsy and aspiration. Its major disadvantages are lack of specificity and inability to provide
a clear idea of extent and origin of neoplastic lesions. MR imaging is ideal for most orbital lesions, although CT may be
preferred if bony involvement is suspected. In a study of 25 patients with orbital disease, 22/25 were correctly diagnosed
with MRI alone, in the absence of confirmatory diagnostic tests, whereas radiography was useful only for advanced neoplasms
extending out of the orbit, and ultrasound gave both false positive and negative results (Dennis R).