Spinal Abscesses

Lisle W. George

■ Definition and Etiology Most abscesses of the spinal cord originate from a preexisting osteomyelitis in a vertebral body. The bone is usually infected hematogenously.

If the infectious agent remains localized in the vertebral body, the patient usually shows signs consistent with a transverse myelopathy. If the infection erodes through the dura mater, the animal develops signs of septic meningitis. If the bone infection is extensive, the vertebrae may fracture suddenly, which results in signs characteristic of spinal trauma.

■ Pathogenesis Hematogenously derived abscesses arise because of embolization of septic thrombi into the metaphyseal arteries.

■ Clinical Signs The neurologic deficits of animals with vertebral body abscesses without pachymeningitis (inflammation of the dura mater) are similar to those described previously for spinal fractures.⁵ Animals with mildly compressive cervical abscesses show a characteristic “weathervane posture,” are stiff, and are reluctant to eat food from the ground.^5,7,8 Ruminants with this lesion hold the neck in extension and attempt to prehend food from the ground by using their tongue.⁵ Additional signs of spinal abscess include heat, pain, swelling, or crepitus over the affected areas and associated signs of bacteremia. Abscesses in the thoracolumbar spine cause hindlimb weakness and ataxia of variable severity, from mild gait abnormalities to complete recumbency. Stud males may be unable to breed because of weakness and pain.¹⁴ The differential diagnosis includes trauma, aberrant parasite migration, tumor-related and pathologic fractures, hemorrhage into or around the spinal cord (e.g., postanesthetic myelopathy), myopathies, caprine arthritis-encephalitis, and fibrocartilaginous embolism.¹⁵ Spinal abscesses can be mistaken for other painful conditions such as orthopedic disorders or traumatic reticuloperitonitis.¹⁶

■ Clinical Pathology and Radiographic Findings Radiographs are the best method for obtaining a definitive diagnosis of spinal abscess. Affected vertebrae exhibit a random pattern of lucency and increased bone density characteristic of osteomyelitis, with or without adjacent soft tissue mass lesions indicative of abscess formation.¹⁷ Diskospondylitis usually results in detectable osteolysis in the intervertebral joints.⁷ Nuclear scintigraphy can be used when the bone lesions are not well defined on plain radiographs.¹³ In addition, myelography may be used to detect the specific site of the spinal cord compression.

A complete blood cell count may indicate the presence of a chronic inflammatory focus, including hyperfibrinogenemia, neutrophilia, monocytosis, nonresponsive anemia, and left shift. The plasma globulin levels are often increased in affected adults but may be increased or decreased in neonates, depending on the adequacy of colostral immunoglobulin transfer.

Changes in CSF depend on the location of the abscess in the CNS tissue and the meninges. In most cases the abscess does not infiltrate through the meninges, and the CSF is normal or shows xanthochromia and mild increases in the protein concentration (60 to 120 mg/dL), with mild to no increase in nucleated cell count.¹⁸ If the infection infiltrates into the meninges, pachymeningitis or septic myelitis will result; in affected animals, the CSF contains high numbers of WBCs (>100 neutrophils∕μL) and a greatly increased protein concentra­tion (>200 mg/dL). The CSF may clot after collection because of high concentrations of fibrinogen. Bacteria may be observed in a Gram-stained smear of CSF sediment. Spinal abscesses have been associated with Brucella abortus and Mycobacterium bovis infections in horses, and so affected horses should be tested for exposure to these organisms.^19,20

■ Pathology The most common sites of involvement are the costovertebral and intervertebral articulations and the vertebral body epiphyses.²¹ Lumbar vertebrae are frequently involved. The bone is uneven, deformed, and softened. The abscessed area is interspersed with calcified trabeculae and pockets of necrotic debris. Sequestration of necrotic bone may occur in some cases. The meninges may be adherent to the abscessed site, and occasionally a fistulous tract may extend from the center of the abscess pocket to the subarachnoid space. In other cases the abscess is compartmentalized away from the CSF, but the proliferating bone impinges on the spinal cord.

■ Treatment and Prevention If spinal abscessation is recognized early, prolonged antimicrobial therapy is generally effective. The results of cultures from the patient's blood, urine, feces, and CSF should guide selection of appropriate antimi­crobial treatment. When bacteriologic culturing yields inconclusive results, a broad-spectrum antimicrobial should be chosen. NSAIDs may be administered for pain relief. Immobilization of the head and neck in a fiberglass cast extend­ing from the thorax to the nose may provide support to smaller patients with a cervical abscess. Surgical drainage of the abscess and curettage of the necrotic bone may be feasible in animals with sufficient economic value to justify the procedure.^10,22 Surgical intervention in adult cattle and horses is usually difficult because of the size of the epaxial musculature and inaccessibility of the spine in large animals.