Dorsal horn volume loss and pain pathway changes in Cavalier King Charles Spaniels with syringomyelia, signs of pain, and phantom scratching.

Central neuropathic pain is a core clinical sign of syringomyelia in humans and Cavalier King Charles Spaniel (CKCS) dogs. This histopathological study used spinal cords from CKCS dogs with syringomyelia to investigate the following conditions: (1) whether specific structural cervical spinal cord entities involved in nociception were affected by loss of neuroparenchyma or other pathological changes in CKCS dogs with pain-related behaviour and phantom scratching, (2) whether pain-related behaviour or phantom scratching correlated with loss of a specific anatomical entity or upregulation of glia cells, and (3) whether syringomyelia-related lesions affected specific functional spinal cord units of nociception. Spinal cord segments C1-C8 from CKCS dogs with magnetic resonance imaging-confirmed syringomyelia and clinical signs of pain and phantom scratching (n = 8) were compared with those from CKCS dogs without syringomyelia (n = 4). Dogs with unilateral scratching (n = 7) had a volume loss ( P = 0.043) of the dorsal horn laminae I-III in the ipsilateral side compared with the contralateral dorsal horn. A clear pattern of ipsilateral changes in the dorsal root entry zone characterised by deafferentation and reorganization of first-order axons into deeper laminae was found in cases with lateralised scratching. Significant changes in cell number density were not found for astrocytes or microglia, suggesting that the dogs represented cases of end-stage syringomyelia and thus could not reveal astrogliosis and microgliosis, which may be involved in the early phases of syrinx development and phantom scratching. The present relationship between clinical findings and dorsal horn and pain pathway pathology in CKCS dogs suggests that these dogs may be of interest as a supplement to experimental model pain research.