IN-DEPTH: DERMATOLOGY AND LUMPOLOGY


Fig. 3. Melanoma masses in the left (A) and right (B) guttural pouches of a 12-year-old grey mare. There was significant swell- ing of the parotid region. CT and ultrasound examination con- firmed that these melanomas both involved the parotid salivary glands, as well as the pharyngeal and parotid lymph nodes.


Fig. 4. Intraocular melanoma can be alarming but the clinical effects are variable. A, The lesion shown in the cremello horse remained static for many years and the horse showed no evidence of any ocular compromise. Notice that it also developed two small eyelid lesions (arrows). B, In contrast the grey showed persistent ocular pain and corneal endothelial compromise as a result of impact between the tumor and the corneal endothelium. This horse ulti- mately developed a massive intraocular melanoma which suddenly bled heavily and thus required exenteration.


within the pouch. Interestingly, repeated endos- copy will often confirm that these do not remain static but may expand slowly or may regress signif- icantly over time; most develop more lesions here with time. The clinical appearances of these early lesions are like “splashes of black paint.” There may be larger or smaller areas involved. In any case, they have a benign appearance and a singu- larly benign nature for the most part. The more serious aggressive and sometimes malignant forms are much more obvious as spherical or unevenly spherical shaped coalescent tumors. They usually have a shiny surface and again are most often found on the walls of blood vessels. Despite their size and apparent aggression, they seldom ulcerate but when they do they may bleed considerably leading to a misleading diagnosis of guttural pouch mycosis. The author commonly uses these lesions as an


index of the rate of progression of the condition because they are relatively protected and unlikely to be significantly affected by any other factors such as external or self trauma. Prior to any surgical in- tervention it is wise to examine guttural pouches so that the presence of internal melanoma lesions can be established prior to the surgery. Primary melanoma can also develop within the


eye itself (Figs. 4 and 5). Tumors developing in the eye can be serious in that the expanding tumor (usually within the ciliary body or the iris) has ef- fects on vision and ultimately cause contact corneal edema and non-ulcerative keratopathy. The most common forms encountered in the


horse are ciliary or iridal melanomas. They are often located in the nasal quadrants at the base of the iris; they often have a blue-black appearance rather than pure black (this may distinguish them from iris cysts and [cystic] granulae iridicae which characteristically have a very black or dark brown appearance) and a fluid filled center identified on ultrasonography. The clinical effects are related to their space occu-


pying nature rather than any malignancy. Indeed, primary iridal melanomas do not have an increased tendency to malignancy as far as the author’s expe- rience will allow such a statement! Retinal lesions


are very rare but there is no information of the effects these small aggregations might have on vi- sual acuity.


3. Malignant Melanoma


The malignant forms are usually highly aggres- sive with rapid widespread dissemination of mili- ary tumors in all major organs and body cavities. Malignant abdominal melanoma (Fig. 6) has, in the author’s experience, resulted in strangulating and non-strangulating surgical colic. Massive peri- toneal and pleural effusions (often tainted with blood and containing obvious melanocytic cells) may be encountered. Usually the abdominal dissemina- tion results in involvement of the major organs in- cluding the spleen, kidney, and liver in particular. Each of these may cause individual clinical signs. For example, renal involvement may cause hematu- ria (usually melanocytic cells are obviously present in the cellular deposit from a urine sample). In- volvement of the spleen may cause peritoneal bleed- ing (but again the abnormal melanin producing cells will be present). Within the chest cavity, malig- nant melanoma can have both space occupying and functionally limiting effects. In the author’s expe- rience, massive accumulations can cause bilateral Horner’s Syndrome if the tumors constrict or alter the function of the cranial sympathetic trunk at the thoracic inlet. Recognition of the rare malignant form of mela-


noma is important because of the implications of interference or attempts to surgically remove indi- vidual tumors. Ulcerated malignant tumors can sometimes be differentiated from their benign coun- terparts by the presence of yellow, pink, or grey areas of tissue. These probably represent areas of amelanotic melanoma and these are, in the author’s experience, always malignant. Otherwise, diagno- sis of malignancy relies on the help of a skilled pathologist. The reality is that little is known about the disease and so much of the advice that is given is based simply on anecdote and experience.


AAEP PROCEEDINGS  Vol. 65  2019 31


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