Chronic Myelomonocytic Leukemia with Central Nervous System Involvement in a Dog

An 8 yr old, male, mixed-breed dog was presented with a 2 mo history of progressive weakness, worsened in the last 2 days before examination. Neurological examination revealed ambulatory tetraparesis, ataxia, and proprioceptive deficits in all four limbs. Menace response was reduced in the right eye and discomfort was detected on neck manipulation.  Hematologic abnormalities included marked monocytosis. Magnetic resonance imaging of the brain and cervical spinal cord revealed multifocal lesions with heterogeneous contrast enhancement. Cerebrospinal fluid analysis showed atypical monocytoid cells with similar cells to those observed in peripheral blood and in bone marrow preparations. The dog was diagnosed with chronic myelomonocytic leukemia. Prednisolone and toceranib were administered with improvement of the neurological signs. After 6 wk, the dog presented for worsening of neurological signs and with the onset of epileptic seizures. The results of flow cytometry were compatible with the possible transformation to acute myeloid leukemia. Although rare, myeloid leukemia may be included in differential diagnosis of patients with central nervous system signs, especially in dogs with relevant hematological abnormalities. In this case, cerebrospinal fluid examination played a key role identifying atypical cells.