Lumbar cerebrospinal fluid in dogs with type I intervertebral disc herniation

Background: Cerebrospinal fluid (CSF) in dogs with Hansen type I intervertebral disc herniation (IVDH) is classically described as normal or mildly inflammatory with a predominance of large mononuclear cells or neutrophils in severe acute herniations. However, we have observed a moderate to marked pleocytosis with a predominance of lymphocytes in some dogs with IVDH. Hypothesis: Moderate to marked CSF pleocytosis occurs more commonly in dogs with type I IVDH than is reported in the literature. Lymphocytic predominance is more common than nonlymphocytic pleocytosis in dogs with chronic IVDH. Animals: Four hundred twenty-three client-owned dogs with type I IVDH. Methods: Retrospective study. Lumbar CSF of dogs with surgically confirmed type I IVDH was evaluated cytologically. Information obtained from medical records included signalment, prior clinical history, time from onset of signs to presentation, neurologic status, and intraoperative findings. Dogs with prior history and/or intraoperative evidence consistent with chronic IVDH before an acute herniation were termed acute-on-chronic (AOC). Results: Pleocytosis (> 5 cells/uL) was present in 51 % of dogs, including 23% with cervical IVDH and 61 % with thoracolumbar IVDH. Moderate or marked inflammation (> 20 cells/uL) was identified in the CSF of 51 % of dogs with thoracolumbar IVDH and pleocytosis. A predominance of lymphocytes was significantly more common in dogs examined > 7 days from onset of signs (P = .032) and in dogs with AOC IVDH (P = .0013). Conclusions and Clinical Importance: Moderate to marked CSF pleocytosis in dogs with type I IVDH is more common than previously reported. Lymphocytic pleocytosis is most common in dogs with chronic progression or AOC IVDH. Lymphocytic inflammation in the CSF of some dogs might suggest an immune-mediated response to chronically herniated disc material.