Spondylosis

Sarah M. Reuss

■ Definition Spondylosis is an ankylosing or degenerative condition of the intervertebral joints. Partial bridging occurs when osteophytes extend across the intervertebral space toward osteophytes on the adjacent vertebrae but a lucent line persists between the two.

■ Etiology and Pathogenesis In middle-aged and older animals, the thoracic and lumbar vertebrae are susceptible to chronic, progressive development of enthesophytes at the ventrolateral vertebral body margins. The exact cause is unknown but is apparently related to altered biomechanics and inflammatory mediators. One hypothesis is that spondylosis is a result of excessive dorsiflexion when the back muscles fatigue, as might be seen with exercise or breeding. There is then abnormal joint loading, mechanical stress, and degeneration of the annulus fibrosis and periarticular tissues.¹ Chemical mediators act on the periarticular ligaments and joint capsule, leading to metaplastic change, and ossification occurs within the anulus fibrosis and ventral longitudinal ligament, leading to partial bridging. Nerve roots may be compressed as they exit the intervertebral foramen, and spinal cord compression may occur if the bony proliferation extends dorsally into the vertebral canal. The cycle may continue until complete ankylosis and a continuous medullary cavity exist.² Partial or complete ankylosis at one site puts more stress on adjacent segments, so there is more active bone remodeling elsewhere in the vertebral column.³ Sixty percent (14 of 23) of horses with radiographic evidence of spondylosis had more than one site involved.⁴ The enthesophytes and then the ankylosed vertebral bodies are both at higher risk for fracture due to their inability to absorb normal forces.

A syndrome of posterior spinal paresis has been recognized in the bull artificial insemination industry, with calcification of the thoracolumbar vertebrae and ligaments. Over time there is fusion of the lumbar vertebrae with bilateral rear limb lameness and paresis. Spondylosis in bulls is not related to force of ejaculatory thrust at service, frequency of semen collection, or high-calcium diets.⁶ Despite previous thought, limited dietary calcium does not reduce the prevalence of spondylosis in bulls.⁷ In one study, a highly significant serologic association was found between posterior spinal paresis and the bovine major histocompatibility complex class I antigen BoLA-A8 (P = 0.0015).⁸

■ Clinical Signs and Differential Diagnoses Spondy­losis can be an incidental finding in animals with no history or clinical signs of back pain. Spondylosis can, however, account for acute, recurrent, and chronic back pain in some animals. Pain is due to inflammation, impingement, and fracture. Palpa­tion of the spine may be resented. Horses may guard their vertebral column by using muscle contraction to not allow ventroflexion. Resentment of girthing and mounting may be noticed. Riders may report poor performance, vague lameness, or altered gait.¹ Spondylosis is rarely the cause of back pain, however, as only 3.4% (23 of 670)⁴ and 2.7% (8 of 443)⁹ of horses presenting for back pain were found to have spondylosis in two separate studies.

Bulls with spondylosis will have a good appetite and be afebrile, but they may have difficulty rising, be reluctant to move, and have difficulty mounting or ejaculating.² In both horses and cattle, once clinical signs are present, they are slowly progressive.

Differential diagnoses for spinal pain include muscle strain, vertebral fracture, diskospondylitis, dorsal spinous process impingement, vertebral infarct, and aberrant parasite migration. Additional differentials for cattle with hindlimb neurologic deficits include lymphosarcoma, encephalopathy, progressive degenerative myeloencephalopathy, and downer cow syndrome.²

■ Epidemiology Spondylosis in horses is most common in mature animals and reportedly occurs in a higher proportion of mares.⁹ The most common location in horses is from the tenth to the fourteenth thoracic vertebrae, and most horses with spondylosis have lesions at more than one intervertebral space. The osteophytes are usually ventrolateral when found in the thoracic region, whereas they are more lateral when found in the lumbar region.³ Transrectal lumbosacral ultraso­nography has detected ankyloses of the caudal lumbar vertebrae in 10% to 20% of horses.¹¹

Bulls have a higher prevalence of vertebral enthesophytes than other cattle. Middle-aged bulls can have significant pathology, with 21 of 21 dairy bulls ages 65 to 90 months having some degree of lumbar vertebral osteophytes. Thirteen of those 21 bulls had complete ankylosis of at least one intervertebral space.¹⁰ Bulls used for artificial insemination are especially at risk, with more than 49% of those 8 years and older having spondylosis.¹²

■ Diagnosis Diagnosis is suggested by clinical signs. Palpa­tion of the dorsum may elicit signs of pain. Rectal palpation may reveal ventral osteophytes if they are located on the caudal lumbar vertebrae.

Standing lateral radiographs are the mainstay of diagnosis. However, diagnostic radiographs can be difficult to obtain given the depth of the vertebrae in the thoracolumbar region. If one suspects spondylosis, targeting radiographs to areas of predilection is helpful. Spondylosis in horses is typically in the midcaudal thoracic vertebrae, commonly T10 to T14, but may also affect the lumbar vertebrae.⁹ In bulls, the most common locations are C3 to C5, T2 to T6, and T11 to L5. The largest osteophytes occur in older bulls at the thoracolumbar junc­tion.^6,12 Radiographic changes include smooth bone that blends with the vertebral body and bridges apparently normal intervertebral discs. A grading system has been developed, with grade 0 being normal, grade 1 having an osteophyte from one vertebral body, and up to grade 5 having complete bridging of the intervertebral space.⁴ The clinical significance of radiographic change must also be determined based on history and clinical signs, as spondylosis can be an incidental finding. Nuclear scintigraphy can be useful for determining the activity and potential clinical significance of a lesion based on the radiopharmaceutical uptake. Scintigraphy may also be helpful in localizing the lesion for targeted radiography. Subjective assessment of scintigraphic images had a 21.7% sensitivity but a 95.0% specificity in a recent study using radiographic change as the standard.⁴ Infrared thermographic imaging was used in one bucking bull to show surface hypothermia, thus confirming spinal nerve root irritation from spondylosis seen radiographi­cally between L2-L3 and L3-L4.¹³

■ Treatment and Prognosis Treatment for clinical spon­dylosis consists of palliative care. Systemic antiinflammatories, cryotherapy, and stall rest are recommended during the acute phase. Extended turnout is then advocated in hopes of forming an osseous union because once complete fusion occurs, clinical signs should improve. Administration of local analgesics and corticosteroid drugs may provide relief and control of localized inflammation. Epidural administration of corticosteroids with concurrent administration of systemic polysulfated glycos­aminoglycans has been reported in a bucking bull, allowing clinical signs to improve enough to permit breeding.¹³ However, if vertebral fractures or neurologic deficits are present, the prognosis is poor.