Ankylosing Spondylitis

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CHAPTER 35 Ankylosing Spondylitis

Serena S. Hu, MD, Dheera Ananthakrishnan, MD, MSE

Ankylosing spondylitis (AS) was first described in the late 1800s by a group of French neurologists. The hallmark of the disease is pain and stiffness of joints, mainly in the axial skeleton. The disease usually affects men in their 2nd and 3rd decades of life. There is some debate about the true male-to-female ratio. It was previously thought that the ratio was almost 10:1; however, it is now accepted that the ratio is much lower, probably about 4:1. The disease in women is usually less severe, possibly leading to what may be perceived as a decreased incidence. AS is an autoimmune condition that often results in chronic pain, disability, deformity, and fractures, much of which is of a spinal etiology. In addition, large joints, most notably the hips, knees, and shoulders, develop early arthritic changes.

The association between the major histocompatibility complex antigen HLA-B27 and AS has been well established.¹^–⁶ Approximately 90% of AS patients are positive for the HLA-B27 antigen, although less than 10% of patients who are HLA-B27 positive manifest the signs and symptoms of AS. First-degree relatives of AS patients who are HLA-B27 positive who are also positive for the antigen have a 30% risk of having AS, in contrast to the prevalence in the general population, which is 1% to 2%. The exact mechanism of the AS and HLA-B27 connection is unknown, although a bacterial association has been proposed.

AS is an inflammatory disease in which joints become arthritic and eroded, followed by autofusion (ankylosis). Microscopic evaluation of early lesions shows lymphocytic infiltrates, plasma cells, and macrophages. The first joints to be affected are usually the sacroiliac joints, followed by the vertebral apophyses, followed by the costovertebral joints. When the costovertebral joints have been fused, chest expansion is much reduced, leading to a decrease in pulmonary function. Enthesopathies are also common, leading to inflammation and erosions of the junction of the anulus and the vertebral endplate. Subchondral marrow edema is a classic finding in enthesopathies associated with AS. Erosions lead to ossification of the endplates, which is manifested by the bridging syndesmophytes seen on plain radiographs. Ankylosis of the facet joints leads to bamboo spine seen on plain radiographs. During the progression of facet ankylosis, patients tend to assume a kyphotic posture to unload the joints and relieve the pain. With time, this compensatory mechanism leads to the fixed deformities of cervicothoracic, thoracic, and lumbar kyphosis commonly seen in AS patients who present to spinal surgeons. These deformities lead to difficulty with horizontal gaze, ambulation, and activities of daily living.

When the spine has become completely ankylosed, it functions as a rigid, brittle beam, leading to an increased incidence of fracture with even minor trauma. These fractures represent the second pressing issue that spine surgeons must deal with when treating patients with AS. Osteoporosis also plays an important role in AS. The greatest decrease in bone mass occurs early in the course of the disease, although the reason for this is unknown.

AS affects peripheral joints as well. The most common joint involved is the hip joint, where protrusio acetabuli can be seen. Hip involvement is often bilateral and often occurs early in the course of the disease. In addition, the presence of thoracic and lumbar kyphosis compounds the problems seen with hip flexion contractures because these conditions contribute to an inability to stand upright. The shoulders, knees, wrists, and hands are also affected but to a much lesser degree.

Nonorthopaedic Manifestations

Although the musculoskeletal system is mainly affected by AS, there are other serious effects of the disease. The most common extraskeletal abnormality is anterior uveitis (approximately 25% of patients), which is usually treated with topical agents and rarely leads to vision loss. Ankylosis in the thoracic spine and at the costovertebral joints leads to markedly diminished chest wall expansion, resulting in restrictive lung disease with decreased lung volumes (vital capacity and total lung volume) and increased dependence on diaphragmatic excursion. Preoperative pulmonary function tests are recommended, as is smoking cessation and aggressive postoperative pulmonary toilet. Some AS patients (25% to 30%) also have a component of ileitis or colitis or both; regardless, all patients for whom surgery is being contemplated should have a preoperative nutritional assessment and be considered for perioperative nutritional supplementation. The incidence of aortic stenosis and aortic valve insufficiency is increased in these patients as well; a preoperative echocardiogram is recommended routinely.

Physical Examination and Diagnosis

A patient with AS is most often a young man who gives a history of vague nonlocalizing back pain, morning stiffness, and possibly increasing difficulty with activities of daily living. Although women are affected with AS, men often present earlier or with more advanced disease. Physical examination and diminished spinal mobility especially in the sagittal plane are usually present. The Schober test is used to evaluate lumbar spinal motion: Points 10 cm above and 5 cm below the lumbosacral junction in the midline are marked on the patient in the fully upright position. With full forward flexion, there should be at least 5 cm of excursion between these two points. Chest expansion is commonly limited to less than 2.5 cm of excursion and is typically measured at the fourth intercostal space. The modified New York diagnostic criteria for AS were outlined in 1984 and are as follows:

1 Sacroiliitis confirmed by radiographs

2 Persistent low back pain greater than 3 months in duration

3 Diminished chest expansion (as just described)

4 Limited range of motion of the lumbar spine in the sagittal and coronal planes

The presence of sacroiliac inflammation and one of the other three criteria is generally considered enough to establish the diagnosis of AS.

Sacroiliitis is usually identified on an anteroposterior pelvis film (with or without a Ferguson view). It is widely accepted that the presence of sacroiliitis is crucial for the diagnosis of AS. Sacroiliac joint destruction is the earliest manifestation of AS. The earliest stages of sacroiliitis show some blurring of the cortical margins; this progresses to subcortical erosions (more commonly on the iliac side because it is less robust than the sacral side). In advanced stages, the sacroiliac joints become completely fused, and the cortical erosions disappear. Sacroiliac joint involvement usually is symmetrical and bilateral. Studies have suggested that the use of bony pelvis computed tomography (CT) or magnetic resonance imaging (MRI) in conjunction with plain radiographs may lead to earlier diagnosis of AS.⁷ It has yet to be determined whether this early diagnosis favorably affects clinical outcomes.

Management of Acute Injury

The spinal surgeon is usually not the physician making the initial diagnosis of AS but rather is called on to address spinal deformity caused by AS in the clinic and spinal trauma in an AS patient in an emergency setting. A trauma patient with AS also presents a challenge to the spinal surgeon. The spine in AS functions as a long rigid beam, acting much like a long bone. This altered biomechanical state, plus the presence of osteoporosis and the lack of ligamentous constraints, significantly decreases the fracture threshold of the ankylosed spine. The key to detecting fractures in these patients is having a high index of suspicion, especially after minor trauma. The cervical and cervicothoracic regions are the most commonly affected. Plain radiography is neither sensitive nor specific in these instances, although it should be used as an initial screen, and the standard radiographic modality used for the diagnosis of fracture in all patients is CT. Epidural hematoma, spinal cord injury, and disc injury can be visualized with MRI techniques.

A patient with AS may present with a progressive neurologic deficit without obvious bony injury or with progression of the deformity and increased pain. Many patients with missed spinal column injuries present at a later time to the clinic or the emergency department with progressive neurologic deficit or worsening of deformity or both. The evaluating clinician must also be aware of possible hyperextension through a fracture at a kyphotic segment, which may result in relatively normal sagittal alignment; attempts to determine the patient’s preexisting deformity from history and prior radiographs should always be made. There have been reports of neurologic injury in patients strapped to spine boards in a position of hyperextension when compared with their preinjury alignment.⁸^–¹⁰ Because of the stiff and osteoporotic spine, minor trauma may result in acute angulation or moderately rapid deformity progression. One should refrain from attempting acute correction through such a fracture. The patient should be initially immobilized in a halo vest in the preinjury alignment.

For a patient with a neurologic deficit, MRI is imperative. MRI may reveal an epidural hematoma. Hematomas can occur in these patients owing to bleeding from minor trauma from the osteoporotic bone or from scarred epidural vessels adjacent to a fracture. Evacuation of a hematoma is essential in the presence of progressive neurologic deficit. The decompression required may significantly destabilize the AS patient, and so the surgeon should be prepared to stabilize the spine at the same setting. Usually rigid instrumentation is required, although rarely halo immobilization may be sufficient for some cervical cases. As with instrumentation for elective cases, the screw-bone interface is compromised because of osteoporosis; these constructs should generally be supplemented by external support such as with a halo vest. Laminar hooks may be more rigid in many patients, but external bracing should still be considered.

It is generally accepted that AS patients sustain more spinal fractures and dislocations than individuals without AS.¹¹^–¹⁵ Cooper and colleagues¹⁶ retrospectively looked at 158 patients in Rochester, Minnesota, with AS and found a sevenfold increase in the incidence of spinal fractures over that of a cohort of patients without AS. They found no such increase in extremity fractures. The patients with spinal fractures tended to be older and had a greater preinjury involvement of the spine than patients without fractures. Cooper and colleagues¹⁶ also noted that this higher incidence was mainly during the first 5 years after diagnosis and suggested that this was due to a greater percentage of bone density loss during this period, resulting in a decreased fracture threshold. In addition, the dampening structures present in a normal spine have lost their load-absorbing qualities in the ankylosed spine. The intervertebral discs are stiff, as are the ligamentous structures, and the facet joints are ankylosed.

The incidence of neurologic injuries in these patients is quite high, owing to excessive bleeding at the fracture site leaking into the confined epidural space and translation (displacement) at the fracture site. This translation causes direct injury to the spinal cord and persistent bleeding owing to motion, resulting in an enlarging compressive hematoma. Most spinal injuries in AS patients are three-column injuries (owing to stiffening of the load-absorbing structures). These injuries are highly unstable because there are two long lever arms hinging at the fracture site. In addition, the presence of preinjury kyphosis increases the likelihood of translation at the level of the injury, which subsequently increases the likelihood of neurologic injury. Lastly, poor bone stock and difficult radiographic evaluation can lead to a delay in diagnosis. Most of these injuries (60% to 75%) are at the cervicothoracic junction, which is notoriously difficult to evaluate with plain radiographs.

Whang and colleagues ¹⁷compared a cohort of 12 patients with AS who sustained spinal injuries with 18 patients with diffuse idiopathic skeletal hyperostosis (DISH) who sustained spinal injuries. The DISH group represents a group of patients of similar age whose spinal condition results in stiff segments above and below any spinal fracture. Falls from a standing position were the most common mechanism of injury. There was a greater likelihood that the DISH patients did not incur any neurologic deficit (44.4%) compared with AS patients (25% of whom did not have a neurologic deficit). Complication rates were higher in the AS group (42% vs. 33% in the DISH group). There were two deaths in each group related to the injury or its treatment, all of which were considered to be related to the use of the halo vest (aspiration [two deaths], respiratory failure, and multisystem organ failure). Several patients died of unrelated causes during the follow-up period; however, all surviving patients were contacted and were classified as having excellent or good outcomes.

Finkelstein and colleagues ¹⁸ looked retrospectively at 21 AS patients with a diagnosis of spinal trauma. One third of these patients had a delay in diagnosis; three had complete spinal cord injuries on presentation, and three experienced neurologic deterioration to complete spinal cord injuries after admission. Finkelstein and colleagues¹⁸ recommended quick screening cervical and thoracic MRI (one film) and screening lumbosacral spine MRI (one film) for diagnosis, in addition to minimal transfers and immediate stabilization. They did not comment on their definitive protocol for treatment of these patients (operative vs. nonoperative).

Hitchon and colleagues ¹⁹ retrospectively reviewed 11 patients with AS and thoracic and lumbar fractures. They found 10 of these patients had sustained three-column injuries; 9 patients had extension-type injuries. More than half of these patients had a neurologic deficit (the specifics of which the study authors did not mention); half of these neurologically injured patients had some improvement in function. Hitchon and colleagues¹⁹ recommended surgical intervention for stabilization of thoracic and lumbar three-column injuries because of their inherent instability.

Graham and van Peteghem ²⁰ looked retrospectively at 15 patients over 6 years (1978-1984) comparing types of injuries and treatments. Of patients, 12 had cervical spine injuries; 9 of these had spinal cord injuries. The two patients with thoracic injuries had anterior cord syndromes. There were no compression-type injuries; most injuries resulted from a flexion-extension type of mechanism. The only patient treated with operative intervention was the patient with the lumbar injury, who had hardware failure and had to undergo revision. Two patients died, and three patients had pulmonary complications.

Apple and Anson ²¹ looked at AS patients with spinal fracture and spinal cord injury, comparing operative versus nonoperative treatments. This study was a retrospective, multicenter study of 59 patients. In the operative group, 37 patients were treated with a variety of procedures. Patients in the nonoperative group were placed in halo traction and then halo vests and placed on bed rest. There were no significant differences between the two groups with regard to motor recovery, fusion complications, or mortality rate (22% in both groups). The nonoperative group did have significantly shorter hospital stays. No analysis of the patients according to type of injury or treatment was done, and no discussion of the deaths was presented.

Hunter and Dubo ¹² reviewed the cases of 19 AS patients who had sustained cervical spine fractures. Five of these patients had a complete spinal cord injury, and all of these patients died after their injury. All of these patients were treated nonoperatively. No patient developed neurologic deterioration, and all of the patients with incomplete cord injury regained some function. Hunter and Dubo¹² concluded that nonoperative treatment worked well in these patients, although they suggested that surgery be considered in patients with grossly unstable injuries.

Bohlman retrospectively reviewed 300 patients with cervical spine injuries.^21a He found only eight patients who carried a preinjury diagnosis of AS. Five of these patients died of pulmonary or gastrointestinal causes. Clinically significant epidural hematomas were found only in the AS patients. Bohlman recommended decompression for patients with progressive neurologic deficit. There was a delay in diagnosis in four patients, all of whom developed spinal cord injuries.

The generally accepted protocol with respect to the management of spine trauma in AS patients is as follows: If the clinician has even the slightest suspicion of spinal injury, the patient should be immobilized in the preinjury position. Plain radiography and fine-cut CT with reconstructions should be obtained. If the patient has a neurologic injury, MRI should be considered, looking for an epidural hematoma. If a fracture is detected and displacement or gross instability is noted, low-weight in-line traction should be used in an attempt to facilitate a reduction. If a reduction is obtained, the patient should be placed in a halo vest for definitive treatment. If a reduction cannot be obtained, internal fixation is recommended, with or without decompression as indicated by the patient’s neurologic status. Postoperative immobilization in the form of a halo vest is then recommended. In a patient with a progressive neurologic deficit, MRI is likely to reveal the presence of a hematoma. In a patient with a stable deficit and no hematoma, the cord injury likely occurred at the time of injury; as long as the spine is stable, management of the neurologic injury should be expectant. If the spine is unstable, reduction and stabilization either with traction and a halo vest or with surgery is recommended.

Complications can arise at the time of injury and from treatment of the injury. Deformity and neurologic injury can occur as a result of the injury; treatment with decompression and internal fixation carries risks of nonunion, hardware failure, failure of the bone-screw interface resulting in loss of fixation, and infection. Even halo management has complications. Skull fractures, pin tract infections, intracerebral hemorrhage, and intracranial air all have been reported with halo immobilization in these patients.^9,²² Taggard and Traynelis²⁴ described a posterior cervical fusion (lateral mass plating) they used in seven AS patients who had sustained fractures. The fusions were supplemented with autologous rib grafts. Postoperatively, the patients were immobilized in collars only, with the exception of one, who was placed in a sternal-occipital-mandibular immobilizer. Fusion occurred in all patients; there were two deaths in quadriplegic patients. Taggard and Traynelis²⁴ recommended operative intervention as a means of avoiding postoperative halo immobilization.