Nonclassified Lymphoma as a Cause of Radial Nerve Paralysis in a Horse.

A 4-year-old American Quarter Horse gelding was evaluated for acute non-weight-bearing lameness of the right thoracic limb with swelling in the right shoulder region. Physical examination revealed radial nerve paralysis of unknown etiology. The primary differential diagnosis was musculoskeletal trauma. Ultrasonography of the right shoulder region identified a heterogeneous mass that extended from the point of the shoulder to the thoracic inlet. Cytologic analysis of fluid collected by fine needle aspirate of the mass was consistent with large cell lymphoma. Based on the cytological findings, locally invasive neoplasia was diagnosed and considered the likely cause of the radial nerve paralysis. Because of the poor prognosis, the horse was euthanized, and postmortem examination confirmed the diagnosis of a nonclassified large cell lymphoma that extended from the deep tissues of the right pectoral muscle group into the thoracic inlet and pleural cavity, as well as the right brachial plexus. The mass in the region of the brachial plexus encompassed and mechanically compressed all of the nerves within the area, resulting in the clinical sign of radial nerve paralysis. Although neoplasia as a cause of radial nerve paralysis is rare, it should be considered as a differential diagnosis, regardless of age.