Two-dimensional real-time ultrasonography of the eye and orbit is ideally suited for examination of opaque eyes and soft tissues
of the retrobulbar space. It can usually be performed in awake and unsedated animals and provides good imaging detail of soft
Ultrasonograms may be acquired using an eyelid contact method, a corneal contact method or a gel offset method. The eyelid
contact and gel offset methods allow better delineation of the cornea and anterior chamber. The direct corneal method is used
when posterior segment or retrobulbar disease is suspected.
The ultrasonography may be performed with either sector scanners or linear array transducers, with 7.5 MHz or 10 MHz probes
providing the best resolution. Imaging is useful in identifying intraocular lesions such as lens luxations, retinal detachments,
hemorrhages, masses and certain foreign bodies, especially in eyes with an opaque cornea or lens. Color Doppler ultrasonography
has some clinical usefulness. Color Doppler ultrasonography identifies the major vessels or vascular regions of the orbit
and allows diseased orbits to be classified as hypo-, hyper-, or neovascularized.
The melanoma is the most common intraocular tumor. The primary melanogenic tumor in dogs is the anterior uveal melanoma. The
canine melanogenic mass is usually localized in the anterior uveal tract with low metastatic potential, whereas feline melanogenic
mass is localized in the anterior uveal tract with high metastatic potential.
The clinical appearance of melanogenic neoplasia is heavily pigmented and tan to white or diffuse iris thickening. The melanogenic
neoplasia causes an irregular pupil, blindness and ocular pain.
Glaucoma is secondary to the exfoliation or proliferation of cells within the uveal tract. Local infiltration and destruction
occur as the neoplasm grows. Enucleation is usually required although laser surgery may be used to selectively reduce intraocular
masses. In addition, a sector iridectomy is possible for uveal mass excision. Limbal and scleral melanocytic neoplasms (epibulbar
melanoma) in dogs are usually benign.
The epibulbar melanoma often arises in the superior temporal quadrant of the eye and is slower growing, less invasive and
well delineated in the sclera of older dogs. As the normal canine globe has melanocytes traversing the limbic sclera, conversion
to abnormal growth occurs when the pigmented cells proliferate and invade the sclera, cornea and conjunctiva.
Surgical correction by full thickness corneal or scleral resection, laser reduction or cryosurgery may be curative since growth
and extension of the neoplasm is slow.
Choroidal melanomas are infrequently seen in older dogs. Clinical signs include raised darkly pigmented areas of the fundus
and domed thickening of the choroid with extension to the iris.
Dr. Hoskins is owner of DocuTech Services in Baton Rouge, La. He is a diplomate of the American College of Veterinary Internal
Medicine with specialties in small animal pediatrics. He can be reached at (225) 955-3252; fax: (214) 242-2200.