Feline intraocular tumours are not common but they show interesting, even unique, aspects of biology in this species and in some cases carry real implications for the general health of the patient.

Anterior Uveal Melanoma

The anterior uveal melanoma is the most common type. These are often 'ring tumours' growing circumferentially around the iris and ciliary body and ultimately occluding the drainage angle sufficiently to cause glaucoma. The typical appearance is of nodular thickening of the iris with some degree of darkening. Some cats show a superficial darkening of the iris due to a spreading layer of melanocytes which may or may not be a pre-melanoma change. There is no difficulty in identifying a colour change in the pale yellow of the cat iris and so owners seek advice for early changes readily. Some of these remain benign but some become malignant at some stage or are malignant from the onset. Clinical distinction between benign pigmentation and melanoma can be difficult and criteria are not well defined. While the pigmentation is superficial (resembling a layer of dark brown dust or paint) then the process is less likely to be neoplastic. Where it is paler, or there is evidence of thickening indicating stromal involvement, or there is glaucoma, then neoplasia is more likely and immediate enucleation would usually be performed.

The reason for enucleation being recommended is that a higher proportion of these tumours are malignant in the cat than in the dog with a real risk of metastasis. At the malignant end of the spectrum they can be quite anaplastic with a high mitotic rate. In addition to the normal criteria for diagnosis of malignancy, invasion of the sclera is unfavourable. When this occurs there is often demonstrable intraluminal invasion of scleral veins by tumour cells increasing the risk of dissemination. Intravascular invasion is unlikely to be diagnosed clinically but episcleral pigment would be unfavourable.

Since clinical decision making can be difficult, since there is a potential risk of metastasis and since there is only one treatment available, namely enucleation, then the clinical stakes are raised in these cases. Biopsy or cytology is often suggested as a way of resolving this. It is probably not often done and so there is little statistical evidence to guide the clinician. These tumours are often very mixed in terms of cell population. There are, therefore, potential difficulties in obtaining a representative sample of the present tumour and in terms of future diagnosis since melanocytic tumours can dedifferentiate over time.

Prognostic factors are largely histological. Large size possibly and lack of pigment certainly are unfavourable factors but histology is required for a full assessment and prognosis in a uveal melanoma

Localised melanomas of the ciliary body and choroid are much less common than anterior uveal melanomas to the extent that some authors refer to them as 'atypical melanomas' in the cat. Large well defined (usually pigmented) masses may arise in the ciliary body or the choroid.

Ciliary Epithelial Tumours

Ciliary epithelial tumours arise from the non-pigmented epithelium of the ciliary body or the layers of the posterior iris. This tumour appears as a well defined pink mass protruding into the pupil from behind the iris or an intra-iris mass, usually at the base, growing into the anterior chamber. They may be classed as adenomas or carcinomas on the basis of the cellular appearance, but the metastatic potential appears to be low irrespective of this.

Medulloepithelioma

Medulloepithelioma is a rare congenital tumour of the cilioretinal junctional area usually appearing as non-pigmented tumour with predominantly neural features. These are rare but are one of the few tumours likely to be seen in a young animal.

Lymphoma

This usually affects the iris, appearing as non-pigmented thickening (not necessarily diffuse) with cells and debris shed into the anterior chamber. The eye may be the first site of tumour detected in which case thorough investigations for tumour at other sites are recommended. Sometimes no sign is found at other sites but whether truly primary or solitary lymphoma occurs in the eye is uncertain.

Metastatic Tumour

The most common type of metastatic tumour is carcinoma, those from the lung having been best described but they have been recorded from many sites. Carcinomas show a striking microscopic pattern of growth within the eye spreading as a sheet along intraocular surfaces such as the iris, ciliary body, retinal surface and subretinal space. Because of this pattern of growth three-dimensional masses are not common, the clinical appearance may be non-specific and clinical suspicion of a metastatic tumour may be low.

Post-Traumatic Sarcoma

This is a tumour apparently unique to the cat. These tumours spread widely into all ocular tissues including the optic nerve (an otherwise uncommon occurrence) and usually show a poorly differentiated sarcomatous appearance with aggressive behaviour. They may metastasise or recur locally or in the brain. The lens is usually ruptured and there may or may not be a history of trauma. Clinically the eye is often simply opaque and clinical examination unrewarding.

Key Points

 In pigmented lesions iris thickening is suggestive of a stromal and hence neoplastic process

 Histology is required for a full assessment of any uveal melanoma

 When removing an undiagnosed eye from any cat a chest radiograph plus additional medical assessment is advisable and the eye should be submitted for histology

 Any non-pigmented thickening in a cat eye requires histology for a definitive diagnosis