Page 1330 - Clinical Small Animal Internal Medicine

1268 Section 11 Oncologic Disease

intraocular pressure (IOP) allows differentiation of (a)
VetBooks.ir buphthalmia from glaucoma (large corneal diameter and
high IOP) and exophthalmia (normal corneal diameter
and IOP).
Optic nerve lesions may result in unilateral or bilateral
blindness (the latter if the optic chiasm is affected),
which may be sudden in onset, as well as optic nerve
head pallor, papilledema, or marked protrusion and con-
gestion of the optic disc on ophthalmoscopy. Relatively
mild exophthalmia with vision loss suggests optic nerve (b)
neoplasia because other orbital tumors typically cause
profound exophthalmos before visual loss. Tumors
affecting more posterior portions of the optic nerve may
not cause exophthalmia or a visible change in the optic
nerve head.

Diagnosis
Differential diagnoses include nonneoplastic orbital
inflammatory diseases such as granulomas, cellulitis,
abscesses, and myositis of the extraocular and masticatory
muscles. Inflammatory orbital disorders typically exhibit
marked pain on opening the mouth. The location of an
orbital mass can usually be determined by careful physical Figure 138.5 (a) Orbital mass in a dog with exophthalmos and
examination, including determination of the direction of dorsal deviation of the left globe. (b) Sequential postcontrast
malposition of the eye, retropulsion of the globe, palpation transverse CT images of the same dog. There is an aggressive soft
of the orbit, and oral examination caudal to the last molar. tissue orbital mass with lysis of the left palatine, lacrimal, frontal,
and zygomatic bones and extension into the nasal cavity (left) and
In addition to physical examination, cytology of regional oral cavity (right). Source: Courtesy of University of Wisconsin‐
lymph nodes, orbital imaging (computed tomography, Madison Comparative Ophthalmology Service Collection.
magnetic resonance imaging, orbital ultrasound), thoracic
radiographs, and possibly abdominal ultrasonography
should be performed. In one study of cats with orbital excision. If bony involvement is not present, orbital
neoplasia, 59% had orbital bone lesions on skull radio- exenteration by widely dissecting around the mass (strip-
graphs and 15% had evidence of metastasis on thoracic ping periorbita if necessary) is usually preferred, as the
radiographs. CT or MRI offers far superior visualization advanced stage of the tumor at the time of diagnosis typ-
of the orbit and facilitates planning of either radiation ically makes it impossible to completely excise the mass
or surgical therapy (Figure 138.5). and preserve a functional or comfortable eye. If perior-
Histologic characterization by FNA or needle core bital bones are involved, a radical orbitectomy, which
biopsies (performed via the mouth or through the perio- resects the affected orbital tissues and surrounding
cular skin), with ultrasound or CT guidance if necessary, is bones, may be considered. When treating optic nerve
essential for arriving at a definitive diagnosis. It is impor- tumors, as much of the orbital optic nerve as possi-
tant to avoid the globe, major orbital blood vessels, and ble should be removed in an attempt to obtain clear
optic nerve when collecting these samples. If less invasive margins.
methods of obtaining specimens are nondiagnostic, If preservation of a comfortable eye and vision appears
exploratory orbitotomy or exenteration may be required. possible, a variety of orbitotomy techniques have been
If the optic disc appears normal, electroretinography, described, ranging from small incisions through the eye-
MRI, and cerebrospinal fluid taps may aid in distinguish- lid or mouth to reflection of the zygomatic arch, tempo-
ing optic nerve neoplasia from sudden acquired retinal ralis muscle elevation, and zygomatic process osteotomy.
degeneration syndrome and posterior optic neuritis. Postoperative complications are common and may
include secondary enophthalmia with entropion, diplo-
pia (double vision), facial nerve palsy, and severe orbital
Therapy
hemorrhage. Surgical debulking can be palliative in some
Primary orbital and optic nerve tumors that have not patients, and some dogs may survive a year or more with
disseminated elsewhere may be amenable to surgical minimal therapy.