Nasal bone osteochondroma in a horse.

This short communication describes the case of a colt affected by an osteochondroma of the nasal bone, which was removed surgically, and describes the follow-up two years after treatment. An 18-month-old warmblood colt was examined because of the presence of a single median mass gradually increasing in size, deforming the nasal profile. There was no history of trauma, and no other bony growths were evident on physical examination. The lesion was approximately 6 x 4 x 3 cm in size, smooth, bony, hard, cold, indolent and fixed to underlying bone. Radiography revealed an osseous proliferation arising from the nasal bones, showing areas of irregular radiolucency. Cytological examination of a fine-needle biopsy from the tumor showed a few scattered, large to medium-sized mesenchymal cells, with moderate anisocytosis and anisokaryosis. Individual cells appeared round to spindle-shaped or stellate and often had wispy trails of cytoplasm. The cytological interpretation of these findings was a benign mesenchymal tumour. The horse underwent surgery under general anaesthesia in lateral recumbency. Histologically, a diagnosis of osteochondroma was made.One month later, the horse’s nasal profile presented an irregular, fibroelastic mass, protruding by 1•5 cm, in the area of the previous surgery. Radiography showed an area of irregular radiodensity with a peripheral opaque outer rim of soft tissue, resembling a periosteal reaction. Two years later, the change in the nasal profile persisted, with the same clinical and radiological features, and a cytological evaluation did not reveal any atypical cells. In this case it was decided to remove the mass without excising a bone flap around its base, to avoid causing interference with the breathing process and leaving the possibility to remove a large bone flap in a second surgical procedure, depending on the histological findings. The follow-up two years after surgery confirmed that, in horses, the behaviour of solitary osetochondromas involving flat bone is benign, unlike in other species such as the cat. The prognosis after surgical excision appears to be good except for the aesthetic results, which may be relevant in show horses. The cause of osteochondroma has still not been defined. The clinical presentation, radiological and histological features of osteochondroma of the nasal bone in this horse appeared to be characteristic, but the differential diagnosis should include granuloma, ossifying fibroma, osteoma, chondroma, osteosarcoma and chondrosarcoma. A histological evaluationis needed for a definitive diagnosis.