Introduction

Once considered a rare disease, ankylosing spondylitis (AS) is now recognized as relatively common, affecting up to 0.5–1.0% of the population.³ The ratio of occurrence in males and females is approximately 5 : 1, although it was previously thought to be 20 : 1. Several studies now suggest that it may occur almost as frequently in females as in males, although in a milder form and with more peripheral localization.^4,5 The disease is characterized by fibrosis and ossification of ligaments and capsules rather than the joint destruction so typical of rheumatoid disease.⁶

Ankylosing spondylitis almost invariably starts at the sacroiliac joints and then extends upwards to involve the spine at increasingly higher levels. However, the sacroiliitis very often remains silent. It has been estimated that no more than 1 case in 10 ever has pain in the buttock. Most spondylitis begins as a diffuse lumbar ache, and sometimes the earlier symptoms are thoracic or cervical (Cyriax⁷: p. 366). AS frequently involves extraspinal joints, tendons and ligaments. The disorder may affect all body systems: iritis, pulmonary diseases, chronic prostatitis and cardiovascular diseases are now recognized as possible complications of the disease.^8–12

Diagnosis of AS is not always easy, particularly in the early stages when only the pelvis is affected. Clinical criteria have been developed during recent decades.13,14 These criteria are usually not appropriate. Radiologically documented sacroiliitis is obligatory for making a definite diagnosis but it may take years before the radiological abnormalities of the sacroiliac joints can be demonstrated without doubt.^15,16 Recently, magnetic resonance imaging (MRI) has proven its value in the early detection of sacroiliitis, with an estimated sensitivity and specificity of about 90%.¹⁵ Active sacroiliitis on MRI precedes the future appearance of sacroiliitis on radiographs by 8–9 years.^17,18 The clinical criteria, such as decreased chest expansion and symmetrical limitation of spinal movements, also occur relatively late in the course of AS, at a time when the disease should be obvious on other grounds (Box 43.1).

The natural history of the disease in an individual is extremely difficult to define or predict. Some patients have disease limited to the pelvis and the majority have a good outlook for a successful life pattern. Only in a small minority of patients does AS progress to the well-known total ankylosis.¹⁹

Symptoms

The patient is usually between 15 and 40 years old and complains of unilateral gluteal pain. Because the sacroiliac joints are largely derived from the first and second sacral segments, the pain commonly radiates to the back of the leg as far as the heel. The localization of the pain is thus the same as in S1 or S2 nerve root compression. However, in sacroiliac arthritis it never spreads to the foot, and paraesthesia is absent. As in discodural problems, coughing (which increases intra-abdominal pressure) may cause pain in the buttock and down the leg. The localization and extent of pain, together with the painful cough, may lead to the assumption that an ordinary discoradicular conflict is present.²⁰ Some specific characteristics then help to distinguish sacroiliac arthritis from disc diseases. The main feature is that the pain comes and goes in an irregular and unpredictable way. During a flare-up, the pain is constantly present; during remission, the patient can exercise freely without an increase in symptoms. An attack is usually unprovoked: if pain is present it is usually increased by exertion, but if it is absent it cannot be stimulated. This is the reverse of the history in disc lesions, where the pain always follows certain activities and subsides after their avoidance. Another important feature is that sacroiliac pain often alternates from one side to the other, though it is seldom bilateral except when it changes sides.²¹

Signs

Given the similarity with S1 or S2 root compression, the index of suspicion usually remains low and the diagnosis is often missed (Table 43.1).

During the examination in a standing position no suspicion arises. There may be a slight increase in gluteal pain during extension and bending towards the painful side; flexion is limited because of increasing pain in the buttock and thigh; and sometimes a slight deviation towards the painful side can be noted during flexion.²² Straight leg raising may also cause pain at the end of range.²³

It is only when the anterior part of the sacroiliac joint is tested (see Ch. 41) that the diagnosis becomes obvious. Unilateral or gluteal or posterior crural pain during the test incriminates the sacroiliac joint. This manœuvre is an extremely sensitive method of deciding whether the sacroiliac joint is affected, and a positive distraction test often precedes radiological evidence of sacroiliac arthritis by years. Although many other tests for the sacroiliac joints have been described, the distraction technique as described earlier is the most significant test of the status of the joint; it applies immediate stress to the anterior part of the joint, without using a lever – distraction forces using the patient’s femur as a lever are very non-specific and should therefore not be used as screening tests. Because of the specificity of the sacroiliac distraction test, it is an essential part of the routine clinical examination of the lumbar spine.

If the patient is examined during a flare-up, passive and resisted hip movements can also cause gluteal pain, especially passive external rotation and resisted flexion, abduction and extension.

Although some authors find tenderness over the sacroiliac joint highly indicative of the existence of sacroiliac arthritis,²⁴ we believe that palpating for tenderness adds no further information and only confuses the examiner. First, the joint, covered as it is by the overhang of the ilium and the sacral extent of the sacrospinalis muscle, remains beyond the direct reach of the palpating finger. Second, the sacroiliac region is a common site for referred tenderness in lumbar discodural conflicts.

Further examination

Radiological evidence of sacroiliitis is accepted as being obligatory for the diagnosis of AS. However, the clinical symptoms may predate the radiological abnormalities by months or even years. In the early stages, when radiological signs are minimal and of questionable significance, it may help to use computed tomography (CT) for demonstrating joint narrowing and fusion.²⁵

The changes are classified according to the New York criteria in five grades (grades 0–IV; Table 43.2). Initially, there is patchy periarticular osteoporosis, leading to loss of definition of the subchondral bone plate. The joint thus appears to be widened. Further evolution of the process results in superficial erosion, together with focal sclerosis of subchondral bone. Further proliferative changes result in irregular bridging across the articular cavity. This causes blurring and indistinct margins on both sides of the joint. Finally, the radiograph shows complete osseous fusion.

The best way to detect active sacroiliitis is on MRI. An MRI is considered as ‘positive’ if the areas of bone marrow oedema (BME) are located at typical sites, i.e. they are periarticular to the sacroiliac joints. When only one BME lesion is visible on an MRI slice, it should be clearly visible on consecutive slices. Enthesitis, capsulitis and synovitis reflect active inflammation as well and are certainly compatible with AS; however, they are not sufficient for a ‘positive’ MRI if present without concomitant BME.²⁷

Association with HLA-B27

The association between the genetic marker HLA-B27 and AS is well known.28,29 The frequency of HLA-B27 in healthy populations is between 1% (Japanese and African) and 14% (Caucasian), whereas the marker is present in 90% of the AS population.¹⁷ However, the presence of HLA-B27 plays little or no role in diagnosis of the disease: a patient with repeatedly normal radiographs is unlikely to have the disease, regardless of HLA status; in contrast, a B27-negative individual with symptoms suggesting AS has the disease if the radiograph shows the typical changes.³⁰

Natural history

The prognosis for an individual is difficult to predict. In some patients the disorder is limited to the pelvis, whereas others quickly develop spinal and extraskeletal disease. The younger the patient is at the age of onset, the worse the outcome, and men usually fare worse than women.³¹ When sacroiliac arthritis appears after the age of 25 years, the disease is likely to follow a mild course: bilateral sacroiliitis continues flaring up and subsiding for some years until bony ankylosis is complete and the pain disappears. If the disease spreads upwards, its spread is very slow and the thoracic spine is only affected when the patient is 40 or 50 years of age. In these patients the cervical spine usually remains unaffected and the hips retain full mobility. In contrast, when sacroiliac arthritis appears before the age of 20 years, or spondylitis has reached the lumbar spine before the age of 25 years, early and severe disablement is very probable; pain and stiffness spread upwards along the spine very quickly and there is also a strong chance of hip involvement within 20 years of onset.³²

Treatment

It is vital for patients to have some knowledge of the natural history of the disease. They should be told that the concept of inevitable, progressive stiffening of the joints, ending in complete ankylosis and crippling disability, is not correct. The diagnosis of AS is usually not as serious as is generally believed. The patient should be made aware that the majority have a good prognosis for a normal social, family and professional life, and that the disease leads to incapacity in only a few cases.

No specific treatment of a curative nature presently exists. The aim of treatment is therefore preventive and symptomatic: avoidance of pain and deformity.

In order to prevent further deformity, the patient should adopt an appropriate routine. A strict daily routine of positioning and extension exercises is more valuable than physiotherapy. Sleeping on a hard mattress and avoiding lying bent on the side are basic. Lying face downwards on a rigid surface at least once a day for half an hour is also recommended. During the day, extension exercises should be performed as often as possible. Attention should also be paid to posture at work and all opportunities for mobility exploited. Swimming is the best routine sport.

Pain and inflammation are treated by non-steroidal anti-inflammatory drugs (NSAIDs). Indometacin is considered the drug of choice. The patient must be informed that therapy should be continuous and that the purpose of medication is to allow normal activities to be pursued and the daily posture and exercise routine to be carried out. Anti-tumour necrosis factor (TNF) agents are recommended in the case of NSAID failure.³³ Over the past few years, several placebo-controlled and open trials have shown a dramatic response in active AS to TNFα-blocking agents (infliximab and etanercept). In these trials, 50–70% of patients showed an improvement of 50% or more.^34,35