Diagnostic Ophthalmology

History and clinical signs

A 10-year-old spayed female mixed-breed dog, was referred to the ophthalmology service at the Western College of Veterinary Medicine because of a small black mass. The mass was reported to be within the anterior chamber of the left eye. It was noticed by the owner approximately 2 mo earlier and confirmed by the referring veterinarian; both felt that the mass was becoming larger. The menace responses, and the direct and consensual pupillary light reflexes, palpebral, and oculocephalic reflexes were present in both eyes. Schirmer tear tests (Schirmer Tear Test Strips; Alcon Canada, Mississauga, Ontario) were 18 mm/min bilaterally. The intraocular pressures were estimated with a rebound tonometer (Tonovet; Tiolat Oy, Helsinki, Finland), and were 20 mmHg bilaterally. No anterior chamber mass could be detected in either eye by direct examination, so gonioscopy (Keoppe lens) was completed bilaterally and a black structure was identified in the left iridocorneal angle (Figure 1). The pupils were dilated with tropicamide (Mydriacyl; Alcon Canada, Mississauga, Ontario), and biomicroscopic (Osram 64222; Carl Zeiss Canada, Don Mills, Ontario), and indirect ophthalmoscopic (Heine Omega 200; Heine Instruments Canada, Kitchener, Ontario) examinations were completed. Additional abnormalities in the anterior and posterior segments were not detected in either eye. Fluorescein stain (Fluorets, Bausch & Lomb Canada, Markham, Ontario) was applied and rinsed from the corneal surfaces and the eyes were examined under cobalt-filtered light; staining was negative.

Figure 1.

A photograph through a gonioscopy lens of the left iridocorneal angle of a 10-year-old mixed-breed dog.

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Discussion

Our clinical diagnosis was a partially deflated uveal cyst that had been swept into the filtration angle of the left eye. The owners were advised that the diagnosis was an incidental anterior uveal cyst as no other cysts were detected during the gonioscopic, biomicroscopic, and indirect ophthalmoscopic examinations. They were also told that treatment was not required and that uveal cysts are usually a benign incidental finding, although multiple anterior uveal cysts are commonly diagnosed. Re-evaluations were advised in 1 year to monitor for any additional cyst development.

The etiology of uveal cysts is not well-documented and may include previous or ongoing uveitis, or they may be idiopathic, or be inherited as certain dog breeds are overrepresented (1). The development and pathogenesis of uveal cysts is unknown; however, we speculate that they most likely develop secondary to focal neuroectodermal misdirection of aqueous humor secretion by the non-pigmented ciliary epithelium. This may create a sub-epithelial fluid bleb between the 2 layers of iris or ciliary epithelium. With continued aqueous humor secretion, these blebs enlarge and become cystic. They eventually break free from their site of origin, float in the posterior chamber, and eventually enter the anterior chamber through the pupil. When they arrive in the anterior chamber they are commonly noticed by the owner who then seeks veterinary advice.

The clinical manifestations include pigmented round to oval structures that are either free-floating or attached to the lens capsule, corneal endothelium, or iris, or rarely in degenerate vitreous (2). The diagnosis is confirmed by retro-illumination which confirms the fluid contents and excludes the diagnosis of melanoma, melanocytoma, or ciliary adenomas which are solid tissue neoplasms (1). Occasionally, heavily pigmented cysts preclude retro-illumination and ultrasonography is required to confirm the fluid-filled cyst. These cysts may be single or multiple, unilateral or bilateral. Most are incidental, and they slowly enlarge until they spontaneously rupture and their remnants become attached to the structures within the anterior chamber. Most cysts do not require therapy unless they are occluding the pupil or visual axis. These are either laser-perforated or removed by aspiration techniques. The prognosis for vision is excellent although new cysts commonly develop after treatment. The histologic manifestations of uveal cysts include a neuroepithelial enclosed fluid-filled structure within the globe (3).

The difficulty in the case presented here was locating the pigmented mass and gonioscopy was required for confirmation as the partially deflated cyst was lodged on the pectinate ligaments. No additional cysts were noted on the ciliary processes, around the filtration angles in either eye, or within the posterior or anterior chambers, and clinical manifestations of concurrent or previous uveitis were not detected. Our diagnosis was idiopathic anterior uveal cyst.