Uveal melanomas usually arise from anterior uveal tract (iris or ciliary body). Primary choroidal melanoma is extremely rare in dogs.
Uveal melanocytic tumors may be benign (melanocytoma) or malignant (melanoma), with benign forms predominating in the dog.
Most melanomas are darkly pigmented, but amelanotic (pink) melanomas are possible and are usually malignant.
Presenting signs
Iris melanoma usually presents as a discrete, dark, nodular mass arising from the iris (note contrast with iris melanoma in cats which may present as a diffuse infiltrative change or as a discrete mass) .
Ciliary body melanoma usually presents as a dark intraocular mass that protrudes through or distorts the pupil.
Often times, melanomas are not confined to either the iris or ciliary body, but in fact affect both tissues and are called anterior uveal melanomas.
Choroidal melanomas usually affect only the choroid and retina.
Unilateral presentation is the most common, although bilateral melanomas may rarely occur (if bilateral presentation occurs, especially in the posterior segment, consider metastatic spread from a distant site).
No pain, inflammation or visual deficits may occur in the early stages and the dog may be presented for only a change in the physical appearance of the eye. The tumor may also be discovered as an incidental finding on physical examination.
In later stages, the tumor may grow to cause severe space-occupying effects, local infiltration or release of local immunogenic and angiogenic factors. This statement needs to be referenced or deleted. Presenting signs in such cases may include blindness , intraocular hemorrhage , retinal detachment , uveitis , glaucoma , and/or extrascleral extension.
Metastatic spread from primary canine anterior uveal melanomas is uncommon (<5%) (Bussanich et al, 1987).
Acute presentation
Late-stage anterior uveal melanoma may be presented suddenly because of the onset ocular pain from glaucoma, the development of hemorrhage or inflammation, or a dramatic change in the appearance of the eye.
Geographic incidence
Worldwide.
Age predisposition
Young adult, middle-aged and older dogs are predisposed.
Sex predisposition
No sex predisposition.
Breed predisposition
A breed predisposition has been reported in the Labrador retriever and golden retriever (Donaldson et al, 2006) and has been suggested in the German shepherd dog (Ryan et al, 1984).
Public Health considerations
None.
Cost considerations
Costs associated with preoperative work-up prior surgical removal, diode laser photocoagulation or enucleation.
Special risks (e.g. anesthetic)
Despite the low risk of metastatic disease it is advisable to screen for metastatic disease prior to surgical intervention because it is not possible to differentiate benign from malignant melanomas solely on their clinical appearance.
Neoplastic change - predisposing genetic and environmental factors unknown in most dogs.
A heritable pattern has been demonstrated in the Labrador and golden retriever (Donaldson et al, 2006).
Predisposing factors General
Age.
Possible breed predisposition in the Labrador retriever, golden retriever, and German shepherd dog.
Pathophysiology
Melanocytoma : benign neoplasm with local extension into surrounding tissues, most commonly affecting the iris, ciliary body, and anterior chamber. In time, extrascleral extension and posterior extension along the choroid towards the optic nerve can occur.
Malignant melanoma: more aggressive neoplasm, usually with a faster growth rate and more destructive progression. Although uncommon, metastasis is possible at the time of diagnosis.
Iris freckle or nevus: benign, focal proliferations of melanocytic cells that are considered benign, nonneoplastic changes. Although the progression of a nevus to a melanocytoma is possible, such progression is poorly documented.
Iris freckle: flat, dark (brown) spot within the iris that does not affect the surface contours, thickness, or architecture of the iris; may be single or multiple, unilateral or bilateral.
Iris nevus: dark-brown, smooth, raised nodule in the iris; does not affect the size or mobility of the pupil; does not cause any other ocular changes; may be single or multiple, unilateral or bilateral
Anterior uveal melanoma: dark brown or black mass arising from the iris or visible through the pupil, possible distortion of pupil size or mobility, possible secondary inflammation and pigment dispersion. Amelanotic melanomas are not pigmented and arise as pink masses within the iris and ciliary body.
Choroidal melanoma: focal, dark-brown or black mass discovered as an incidental finding during funduscopy.
Iris freckle, nevus or anterior melanoma: color change noted within dog’s eye.
Anterior uveal melanoma: mass noted within or around the pupil.
Late presentation
Onset of pain, swelling, clouding, redness or blindness of eye. Unilateral blindness is not often recognized by the client because the dog's behavior usually remains normal.
Clinical signs
Iris freckle or nevus: discrete dark spot or small nodule on the surface of the iris.
Anterior uveal melanoma: intraocular mass arising from within the iris or possibly protruding through and distorting the pupil.
Unilateral presentation is typical, although rarely lesions may arise in both eyes (Roperto et al, 1981). If bilateral presentation, especially with posterior segment involvement, consider metastatic disease.
No pain, inflammation or visual deficits in early stage.
In later stage, secondary signs including blindness, intraocular hemorrhage, retinal detachment, uveitis, glaucoma, extrascleral extension.
Thorough general physical examination including palpation of superficial lymph nodes, auscultation of the chest and abdominal palpation.
Minimum laboratory database, chest radiography +/- abdominal ultrasonography to rule out metastatic disease.
Confirmation of diagnosis Discriminatory diagnostic features
Discrete, dark, nodular mass within the iris or protruding through the pupil that grossly distorts the architecture of the iris.
Solid, cannot be transilluminated (in contrast to iris cysts).
Attached to iris or ciliary body (never free-floating, in contrast to some iris cysts).
Iris melanoma is raised from surface of iris in contrast to benign iris pigmentation (freckling) and alters the architecture of the iris and/or surrounding tissues (in contrast to an iris nevus).
Definitive diagnostic features
Definitive diagnosis can only be made by histopathological examination of tissue, which is usually performed following enucleation.
Rarely, a focal iris melanoma may be resected via intraocular surgery (iridectomy) and submitted for histopathology.
Gross autopsy findings
Darkly pigmented mass arising within iris and/or ciliary body of enucleated globe.
Metastasis to regional lymph nodes, lungs and liver or direct extension to the orbit and brain with malignant melanomas.
Histopathology findings
Variably pigmented cells containing melanin granules and a single prominent nucleolus.
Cell types vary between tumors and have been classified as epithelioid, spindle, plump or balloon cells (Diters et al, 1983; Bussanich et al, 1987; Ryan and Diters,1984).
Melanocytomas have a low mitotic rate. Malignant melanomas have a high mitotic index (>4/hpf) (Wilcock and Peiffer, 1986).
Consider referral for specialist assessment to ascertain the best course of therapy, or for possible immediate surgical removal of the iris mass or diode laser photocoagulation.
Consider immediate enucleation if the mass distorts the shape of the iris, the shape and size of the pupil, if it is amelanotic, if it arises from a point behind the pupil, or if it has caused any secondary intraocular complications.
Alternatively, regular monitoring (eg every 2-3 months) may be done of small confined lesions that do not distort the shape of the iris or the pupil. Consider enucleation if the lesion is progressive, if there is concern about its metastatic potential or serious secondary complications (eg glaucoma, intraocular hemorrhage, retinal detachment, etc.).
Monitoring
Regular monitoring of iris freckes and nevi, eg every 3-6 months.
Frequent monitoring of small iris lesions that have a clinically benign appearance (every 2-3 months).
Subsequent management
Treatment
If iridal pigmentation changes are progressive, consider referral for specialist assessment.
If enucleation is considered, perform a complete medical work-up prior to surgery to rule out metastatic disease.
Following surgical excision, if the lesion was benign, no further treatment is required.
If the lesion was malignant, confer with a veterinary oncologist. Although clinical studies are lacking in the use of the canine malignant melanoma vaccine for ocular melanomas, there is reason to hope that these tumors will respond to the vaccine because it targets surface antigens common to most melanomas (Milner et al, 2006).
Monitoring
If iridal freckle or nevus lesion is nonprogressive, regular monitoring every 3-6 months.
If ocular melanoma was benign on histopathology and excised in its entirety, no further monitoring is required.
If ocular melanoma was malignant on histopathology, monitor and perform systemic diagnostic tests every 3 months. Most malignant lesions metastasize within 3-12 months (Bussanich et al, 1987; Ryan & Diters, 1984, Wilcock & Peiffer, 1986).
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Donaldson D, Sansom J, Scase T et al(2006) Canine limbal melanoma: 30 cases (1992-2004). Part 1. Signalment, clinical and histological features and pedigree analysis.Vet Ophthalmol9 (2), 115-119 PubMed.
Milner R J, Salute M, Crawford C et al(2006) The immune response to disialoganglioside GD3 vaccination in normal dogs: a melanoma surface antigen vaccine.Vet Immunol Immunopathol114 (3-4), 273-84.
Schub T (2004) New vaccine approach targets melanoma.Lab Anim 33 (8), 9.
Giuliano E A et al(1999) A matched observational study of canine survival with primary intraocular melanocytic neoplasia.Vet Ophthalmol2 , 185-190.
Bussanich N M, Dolman P J, Rootman J et al(1987) Canine uveal melanomas: series and literature review.J Am Anim Hosp Assoc23, 415-424.
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Wilcock B P & Peiffer R L (1986) Morphology and Behavior of Primary Ocular Melanomas in 91 Dogs.Vet Pathol23 , 418-424 PubMed.
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Roperto F, Restucci B, Crovace A (1981) Bilateral ciliary body melanomas in a dog.Prog Vet Ophthalmol3 (4), 149-152.
Other sources of information
Grahn B H & Peiffer R L (2007) Fundamentals of Veterinary Ophthalmic Pathology. In: Veterinary Ophthalmology4th edn. Ed. KN Gelatt, Blackwell Publishing, Iowa, USA, pp3 55-437.
Hendrix D V H (2007) Diseases and Surgery of the Canine Anterior Segment. In: Veterinary Ophthalmology4th edn. Ed. KN Gelatt, Blackwell Publishing, Iowa, USA, pp812-858.
Gould D (2003) Ocular Tumours. In: BSAVA Manual of Canine and Feline Oncology2nd Edition. Eds JM Dobson and BDX Lascelles, BSAVA Publications, UK, pp329-337.
Crispin S M (2002) The uveal tract. In: BSAVA Manual of Small Animal Ophthalmology2nd edition. Eds S Petersen-Jones and S Crispin, BSAVA Publications UK, pp162-184.
Vetstream contributor(s)
Dr David Gould BSc(Hons) BVM&S PhD DVOphthal DipECVO MRCVS, RCVS and European Specialist in Veterinary Ophthalmology , Davies Veterinary Specialists, Manor Farm Business Park, Higham Gobion, Herts SG5 3HR, UK.
Dr Rhea Morgan DVM DACVI DACVO , Smoky Mountain Veterinary Services, Walland, TN, USA.
Unilateral blindness is not often recognized by the client because the dog's behavior usually remains normal.