A 9-year-old King Charles Spaniel presented with a history of progressive forelimb weakness and paroxysmal involuntary flank scratching over a 2-year period. Neurological examination suggested a myelopathy of C1 to C4 spinal cord segments. Advanced imaging studies revealed hydrocephalus, caudal herniation of part of the caudal lobe of the cerebellum through the foramen magnum and marked syrinx formation to the level of the caudal thoracic spine, resembling Arnold-Chiari malformation with secondary hydromyelia in humans. Mechanical obstruction at the craniocervical junction, altering CSF flow dynamics, may lead to syrinx formation. Response to diuretic therapy was moderate but surgical decompression may offer better long term prognosis.