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Ankylosing Spondylitis of the Cervical Spine
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Ankylosing spondylitis (AS) is a seronegative spondyloarthropathy of unknown cause characterized by inflammation of the axial skeleton. It typically affects the sacroiliac joints at early stages in the disease, which is followed by enthesopathy of the paravertebral joints and disk spaces of the spine. Left untreated, this condition causes early fusion of the paravertebral zygapophyseal joints and intervertebral disk spaces leading to the “bamboo spine” that characterizes the disease, hyperkyphotic posture, and compromised sagittal balance. These deformities can lead to severe functional impairment and can also predispose patients to traumatic spinal injury.
Several diagnostic and treatment strategies are available for early and late manifestations of AS, including medical therapy and operative management for late deformity correction. Similarly, diagnostic and management approaches are established for managing traumatic spinal column injuries and their complications. The ankylosed cervical spine presents a unique set of challenges, in addition to those listed earlier, because of the characteristic chin-on-chest deformity that results from the hyperkyphotic spine. The purpose of this chapter is to discuss the epidemiology, evaluation, management, and complications associated with operative and nonoperative treatment of patients with AS of the cervical spine.
Epidemiology
AS typically affects young adults, most commonly male patients (3:1) in their second to fourth decade of life. The estimated prevalence of AS in the United States is 197 in 100,000 adults, with a range of 68 to 210 in 100,000 adults reported worldwide. Adequate evidence indicates that the incidence of AS has remained stable and is estimated to be 7.3 in 100,000 person-years in the United States. Up to 20% of those patients diagnosed with AS have a positive family history of the disease, and 80% to 95% are human leukocyte antigen (HLA)-B27 positive. In the general population, however, AS is likely to develop in only 1% to 2% of HLA-B27–positive adults. No studies have specifically investigated the epidemiology of cervical spine involvement in AS.
Etiology
The true cause of AS is still undetermined, and genetic and environmental factors likely play significant roles in the etiology of the disease. Although the direct involvement of HLA-B27 in the pathogenesis of the disease is well established, not all individuals who are HLA-B27 positive develop AS, and several other theories have emerged. In addition to the well-established genetic basis of the disease, which includes HLA-B27 and numerous other genes, researchers have postulated the contribution of the immune system to the disease and have investigated the possibility of an autoimmune component to the disorder. Additionally, other theories implicate autoimmune responses to specific bacterial antigens as a potential environmental cause of the disease, by noting the elevated levels of antibodies to Klebsiella pneumoniae and Escherichia coli in patients with AS. The true etiology, however, is undoubtedly multifactorial and remains a subject of considerable research and debate.
Pathophysiology
The hallmark pathologic features of AS include inflammation of the axial joints and large peripheral joints, accompanied by bony destruction. Fibrous tissue and inflammatory cell infiltrates invade the bone adjacent to entheseal attachments. The new bone that forms in response to this process leads to ankylosis of the affected joints. In the spine, subsequent loss of motion secondary to this ankylosis leads to syndesmophyte formation and the radiographic bamboo spine characteristic of AS.
All regions of the spine can be affected by the disease process, and although inflammation typically ascends the spine, the cervical spine can be involved first, and the disease may skip vertebral segments. Two major factors inherent to the disease process are integral to understanding the effect of AS on the cervical spine: altered vertebral bone composition and altered spine biomechanics. The combination of these factors leads to deformity and results in the observed increased incidence and prevalence of vertebral fractures in AS, as well as the increased associated risks of such fractures.
Altered Vertebral Bone Composition
The prevalence of osteoporosis or clinically significant low bone mineral density (BMD) in patients with AS is reported to be up to 62% in the literature. This number surprisingly underestimates the true trabecular bone loss resulting from spurious increases in BMD caused by syndesmophyte formation and ligament ossification in AS. Furthermore, conventional assessments of BMD such as dual-energy x-ray absorption yield falsely normal results for the same reasons. Men with AS lose bone at a rate of 2.2% annually, with a 2.9% annual loss of total body calcium, compared with an annual loss of total body calcium of only 0.7% in men who are more than 50 years old who do not have AS.
Osteoporosis associated with AS leads to a higher rate of vertebral fractures, as well as a higher risk of vertebral fracture from significantly lower-energy mechanisms secondary to altered bone biology. Unfortunately, the true cause of osteoporosis in AS remains unknown. Studies suggest a multifactorial etiology, with phases of enhanced bone resorption or reduced bone deposition at inflammatory sites early in the disease, paralleled by inflammatory cytokine mediation and altered hormonal influences. With progressive AS, the patient has continued demineralization of the axial skeleton that contributes significantly to progressive deformity and an increased rate of vertebral fractures.
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