Hypertrophic Osteopathy in a Cat

Background: Hypertrophic osteopathy (OH) is a syndrome characterized by progressive bilateral periosteal bone neoformation that mainly affects the thoracic and pelvic limb bones’ metaphyses and diaphyses. In most cases, it is secondary to a chronic primary lesion located in the thoracic cavity but can be associated with an abdominal injury that has already been occurred. The occurrence of this condition without being associated with a primary lesion is considered infrequent in animals. The purpose of this report was to describe a case of idiopathic hypertrophic osteopathy in a cat with a description of clinical signs and radiographic and anatomopathological findings.Case: A male adult cat was brought to the veterinarian with an initial observation by the owner of four limbs’ volume increase, apathy, and reluctance to move. Upon clinical examination, the animal presented dehydration, mucosal hyperemia, hyperthermia, and bilateral edema of the thoracic and pelvic limbs. Based on the radiographic examination, a periosteal reaction with palisade-like appearance was found in the metacarpals, radios, ulna, humerus, scapulae, tibias, and fibulae. There were no significant changes in additional exams. Due to poor prognosis, the cat was euthanized and referred for necropsy and histopathological examination. All organs were examined both macroscopically and microscopically. Fragments were fixed in 10% formalin and routinely processed for histological slides with hematoxylin–eosin and Masson's trichrome, and limb bones that were not fixed in formalin were macerated. Lesions were observed only in the thoracic and pelvic limbs. Macroscopically, there was a diffuse, regular, slightly firm volume increase, covered by abundant gelatinous whitish tissue. The periosteal bone neoformations were characterized by numerous papillary projections distributed throughout the phalanges, radius, ulna, humerus, scapula, tibia and fibula. Histologically, the original bone matrix was surrounded by lightly organized eosinophilic material in bundles. Young bone matrix-formed trabeculae perpendicular to the original bone matrix, projecting into the mature bone tissue, was attached to the cortical layer.Discussion: The hypertrophic osteopathy diagnosis was based on clinical signs, radiographic examinations, and anatomopathological findings. Although the long bones of the pelvic limbs are often the most affected, the lesions were more marked in the thoracic limbs in this cat. The presumptive diagnosis was made through radiographic examination. Radiography is considered a very useful diagnostic method in animals suffering from this condition. Histological changes are compatible with persistent phases of hypertrophic osteopathy resulting from the action of osteoclasts and osteoblasts, with osteoid matrix deposition in the existing cortical bone. Since this cat didn't present macroscopic and microscopic alterations beyond the bone lesions, it was not possible to attribute them to a primary cause. For this reason, osteopathy was considered idiopathic in this case. We concluded that lesions of hypertrophic osteopathy may be more pronounced in the thoracic limbs. In cases of animals with signs of limb volume increase even in the absence of thoracic and abdominal lesions, hypertrophic osteopathy should be included as a differential diagnosis. The radiographic and anatomopathological examinations are enough to diagnose this condition