Ocular neoplasms can be divided into adnexal and ocular surface tumors, intraocular tumors, and orbital tumors (8). While primary ocular neoplasms are more common than secondary neoplasms, secondary neoplasms are more common among intraocular neoplasms in dogs (4,14). In a previous study, uveal melanocytic tumors were found to be the most common tumors, accounting for approximately 27% of ocular tumors (14). Most uveal melanocytomas are located in the anterior uvea, and only approximately 6% are located in the choroid (8).

Ocular ultrasonography is a rapid and useful method for detecting intraocular or retrobulbar tumors (6). After detecting the mass, cytological examinations or histopathology may be required for diagnosis (6). In a canine retrospective study of choroidal melanocytic tumors, specifying the location of the mass by fundoscopy was possible for nearly 60% of eyes with the tumor (2). However, fundoscopic location was not obtainable when the mass showed excessive expansion posteriorly or when concurrent disease was present in the anterior segment (2).

The prognosis of malignant melanocytic tumors is poor even if they do not frequently metastasize (1,3,5,7,9-12). Macroscopically, benign tumors tend to be more darkly pigmented than malignant tumors (2), but clinical appearance cannot solely distinguish benign from malignant tumors (8). Moreover, evidence of bulbus destruction is also insufficient to assess malignancy (13). Instead, mitotic figures are the most important yet simple criterion for differentiating uveal melanocytomas from malignancies (14). This report presents a case involving a choroidal melanocytoma with optic nerve involvement that was diagnosed after enucleation and histopathological examinations.

A 10-year-old spayed female beagle presented with a history of corneal opacity for a year and third-eyelid elevation in the left eye (OS) for 2 months. The Schirmer tear test-1 (STT-1; Merck Animal Health, Madison, NJ, USA) value was 24 mm/min in the right eye (OD) and 25 mm/min in the OS. Rebound tonometry (Icare_® Tonovet; Icare, Helsinki, Finland) revealed an intraocular pressure (IOP) of 11 mmHg in both eyes. The menace response, dazzle, and pupillary light reflexes were normal in both eyes. However, lagophthalmos and decreased retropulsion of the OS were observed. The results of slit-lamp biomicroscopy (SL-D7; Topcon, Tokyo, Japan) were essentially normal in the OD and corneal degeneration and third-eyelid protrusion were found in the OS. A superficial focal corneal ulcer was also observed on the OS. Indirect ophthalmoscopy (Vantage Plus; Keeler Ltd., Windsor, Berkshire, UK) revealed tapetal hyper-reflectivity and a prominent black mass in the OS, making it impossible to identify the optic disc (Fig. 1). Ocular ultrasonography was performed and showed a 0.74 × 0.67 cm echogenic posterior segment mass around the optic nerve protruding into the retrobulbar space and a liquefied vitreous body (Fig. 2). Physical examination, blood analysis, thoracic radiography and computed tomography (CT) were performed to evaluate metastasis. No lymph node enlargement was noted on physical examination and no remarkable findings were detected on thoracic radiography. Complete blood count and serum chemistry profile findings were within the normal ranges. CT revealed a contrast-enhanced soft tissue mass in the posteromedial aspect of the OS protruding into the retrobulbar region, and the optic nerve was suspected to be involved (Fig. 3). CT also revealed no evidence of osteolytic changes in the adjacent bone. Additionally the thoracic CT scan did not show any signs of distant metastasis.

Figure 1. Fundoscopy of the affected left eye. Note the prominent black mass (asterisk) covering the optic nerve in the OS.

Figure 2. Ocular ultrasonography showing a liquefied vitreous body and a 0.74 × 0.67 cm echogenic posterior segment mass around the optic nerve protruding into the retrobulbar space.

Figure 3. Computed tomography demonstrating a contrast-enhanced soft tissue lesions in the posteriomedial aspect of the left eyeball (circle) protruding into the retrobulbar region (arrow).

Although the eye was visible, enucleation was planned to diagnose the mass histologically and prevent further invasion of adjacent tissues. Enucleation was performed using a subconjunctival approach. Macroscopic examination of the enucleated globe revealed a black mass extending from the globe around the optic nerve. The optic nerve appeared unpigmented, but it was surrounded by the black mass. Therefore, orbital tissues were excised as much as possible during the operation to exclude the potential risk of local invasion (Fig. 4). The periorbital fascial tissues were closed first in purse string pattern with polydioxanone 4-0 (Ethicon, NJ, USA) to minimize the dead space. Closure of the subcutis followed with same suture material. The skin suture was performed in simple interrupted pattern with blue nylon 3-0 (Ailee, Busan, Korea).

Figure 4. Gross images of the enucleated left eye. Black mass (asterisk) extending into retrobulbar region is observed. The black mass was surrounding the optic nerve (arrow) and the optic nerve remained unpigmented macroscopically.

Postoperative care included oral administration of carprofen (Rimadyl_®; Pfizer, New York, USA) 2.2 mg/kg, misoprostol (Alsoben_®; Unimed, Seoul, Korea) 5 µg/kg, omeprazole (Ome-Q_®, Ildong, Seoul, Korea) 0.5 mg/kg, and amoxicillin and clavulanic acid (Augmentin_®, Il Sung Pharmaceutical, Seoul, Korea) 12.5 mg/kg twice daily (BID) for 10 days and tramadol (Tridol_®; Yuhan, Seoul, Korea) 2 mg/kg BID for 3 days. The surgical site was disinfected with 10% povidone-iodine for 10 days. The skin sutures at the enucleation site were removed 10 days later.

The enucleated globe and additionally excised orbital tissues were fixed with 10% neutral buffered formalin and submitted to the Comparative Ocular Pathology Laboratory of Wisconsin (COPLOW) for histopathological evaluation. Histologically, an infiltrative and densely cellular pigmented neoplasm expanded into the central choroid and peripapillary orbital tissues. Neoplastic cells were arranged in sheets of round cells with distinct borders and a heavily pigmented cytoplasm. No mitotic figures were observed in the 400X fields. Therefore, the tumor was diagnosed as a central choroidal melanocytoma, a benign melanocytic tumor. In addition, the retinal area uplifted by the neoplasm was atrophied (Fig. 5A, B), and the anterior uvea was intact without invasion of the neoplastic cells. The neoplastic cells multifocally extended into the optic nerve (Fig. 5C), however, the additionally excised optic nerve showed no histopathological evidence of neoplastic cells. At six-month intervals, follow-up evaluations were performed, which included local recurrence assessments such as palpation of the surgical site and lymph node evaluation. There was no evidence of recurrence until 16 months after enucleation.

Figure 5. Histopathologic findings of the affected OS (hematoxylin and eosin). (A), (B) Longitudinal section of the enucleated eye. The retinal area uplifted by the neoplasm was atrophied. The neoplastic cells were extending into the optic nerve and peripapillary tissue. (C) Round cells with an abundant amount of heavily pigmented cytoplasm can be observed.

This report presents a case of choroidal melanocytoma that invaded the optic nerve without causing vision loss. In this case, the tumor was identified through routine ophthalmic examinations, including indirect ophthalmoscopy. Most choroidal melanocytic tumors are found incidentally on fundoscopic examination (4). In this case, a black mass was first identified through fundoscopy, and ocular ultrasonography was performed to specify its location and size. Thus, thorough routine ocular examination may facilitate the detection of intraocular tumors.

Choroidal melanocytic neoplasms can cause chronic uveitis, secondary glaucoma, and blindness as a result of retinal detachment or intraocular hemorrhage (8,13). However, at the time of diagnosis, most patients show no discomfort or difficulties in vision, making it difficult for veterinarians to recommend enucleation to the owner (8). Nevertheless, it is necessary to notify risk of local invasion or metastasis due to clinical appearance of malignant and benign tumor showing no difference. For intraocular tumors, fine-needle aspiration is associated with a high risk of inflammation, infection, and hemorrhage (14). Therefore, enucleation or exenteration can be both diagnostic and curative methods, especially for posterior intraocular tumors. The tendency of intraocular expansion to cause secondary inflammation can be a persuasive indication for surgery for owners (2).

In this case, the decreased retropulsion and lagophthalmos in the affected OS could have been caused by the mass effect of the intraocular mass protruding into the retrobulbar space, which subsequently caused corneal ulcers and third-eyelid protrusions. The nictitating membrane was thought to be protruding not because of the retrobulbar mass, but because of corneal ulcer considering the mass was limited within intraconal region. The patient still had vision and appeared normal; however, the possibility of a chronic recurrent corneal lesion and optic nerve involvement made enucleation a reasonable option. Although the mass protruded into the retrobulbar region, enucleation was performed because the mass was suspected to have a clear border based on CT images. The orbital tissues were excised as much as possible during enucleation to reduce the potential risk of local invasion.

In this case, enucleation was chosen as a surgical approach. As the mass could be observed in intraocular space and was protruding through the optic nerve with clear border, it was more appropriately an extension of the intraocular mass rather than an orbital mass. Therefore, considering the structural limitation of the orbit, careful macroscopic observation of the mass and subsequent excision of the optic nerve region following enucleation were concluded to be beneficial than planning exenteration from the start.

Bandanes et al. (2) reported enucleation of two of the four globes because melanocytic tumors showed evidence of tumor invasion of the optic nerve. In their report, one eye was diagnosed with melanocytoma and the other eye showed a malignant melanoma (2). In the present case, although the optic nerve was macroscopically intact, involvement of the optic nerve was observed histopathologically. However, complete excision of the mass was thought to be made due to the absence of neoplastic cells in the resected optic nerve. No local recurrence was observed until the 16-month re-examination.

Choroidal melanocytoma with optic nerve involvement should be treated with enucleation, even in patients with vision, to avoid local invasion of the optic nerve. As in this case, the optic nerve should be removed as much as possible by performing enucleation to prevent metastasis and local recurrence. After surgery, regular follow-up is essential to detect signs of recurrence.

The authors have no conflicting interests.