Spondyloarthritis (SpA), refers to a group of heterogenous diseases that share a combination of several clinical features: chronic low back pain, heel enthesitis, dactylitis, and arthritis. There is a strong association with sacroiliitis on imaging and also with the human leukocyte antigen (HLA)-B27. Additional features include skin and genital lesions, eye and bowel inflammation, an association with preceding or ongoing infectious disorders, positive family history, elevated acute phase reactants, and a strong association with the human leukocyte antigen (HLA)-B27.

The group includes diseases such as ankylosing spondylitis (AS), psoriatic arthritis (PsA), arthritis associated with inflammatory bowel disease (IBD), and reactive arthritis.

• Pathophysiology

The pathogenesis of SpA is not entirely clear. It is the result of a complex interaction between genetic risk factors and environmental triggers that leads to the activation of autoimmunity and autoinflammation. There is a strong family inheritance and HLA B27 plays a very important role in disease pathology.

• Clinical Presentation

The major clinical features which differentiate spondyloarthritis (SpA) from other forms of arthritis are the distribution and type of musculoskeletal manifestations and certain extraarticular features.

Patients with axial SpA characteristically have chronic low back pain for greater than 3 months duration and before the age of 45. It typically improves with exercise and not rest. It is associated with a significant early morning stiffness. An additional feature of low back pain in some patients with axial SpA is a good response to treatment with a nonsteroidal anti-inflammatory drug (NSAID).

Patients with either axial or peripheral SpA can exhibit any or more than one of these peripheral musculoskeletal features, which include dactylitis (sausage digits), enthesitis (heel pain and/or swelling), and peripheral arthritis.

Certain non-articular manifestations can be associated with the musculoskeletal features seen in SpA, including uveitis, psoriasis, and features of inflammatory bowel diseases. Reactive arthritis is sometimes associated with genital lesions.

The peripheral arthritis in SpA usually has an acute onset, is asymmetrical and often predominantly involves the lower extremities, especially the knees and ankles associated with swelling.

Enthesitis (or enthesopathy) refers to inflammation around the enthesis, which is the site of insertion of ligaments, tendons, joint capsule, or fascia to bone, and is relatively specific to SpA. The most common observable clinical manifestation of enthesitis is swelling at the heels, at the insertion of the Achilles tendon, or at the insertion of the plantar fascia ligament into the calcaneus.

Other sites of enthesitis which are often not swollen but which can be painful and/or tender include those at the iliac crests, greater trochanters, epicondyles at the elbows, tibial plateaus, costochondral junctions at the sternum, humeral tuberosities, manubrial-sternal joints, occiput, and spinous processes.

Dactylitis (sausage digits) – A characteristic feature of SpA, especially psoriatic arthritis and occasionally reactive arthritis, is dactylitis, also known sausage toe or sausage finger. It causes a global inflammation of the fingers and toes in an asymmetric fashion.

Several non-articular features include inflammatory eye disease known as anterior uveitis, inflammatory bowel disease and Psoriasis.

• Articular disease related to SpA is the most common extra-intestinal feature in IBD, although the clinical course of both diseases is independent. IBD has been diagnosed in 10% patients with AS. On the other hand, AS is diagnosed in 3–10% patients with IBD, although 15–50% patients have sacroiliitis on imaging. Sixty percent of patients with AS show microscopic inflammation of bowel mucosa without IBD symptoms.

There are no laboratory findings that are absolutely specific for spondyloarthritis (SpA). Most notable are human leukocyte antigen (HLA)-B27, present in 90% of those with AS, and elevated acute phase reactants, and, in the case of reactive arthritis, those associated with the preceding infections. However, a positive HLA-B27 by itself is not diagnostic of SpA, since a significant proportion of subjects in the general population are also positive.

CRP and ESR have both low sensitivity and specificity and they do not represent the disease process or activity in AS and SpA. Elevated CRP is included in axial ASAS SpA classification; however, elevated CRP or ESR are only present in 40–50% of patients with AS.  This percentage may be greater in peripheral forms such as PsA. Thus, normal ESR or CRP levels do not rule out active disease or AS diagnosis.

The Assessment of Spondyloarthritis International Society (ASAS) criteria allows classification of SpA patients through the use of predominant symptoms in axial and peripheral SpA.

Axial SpA

Sacroiliitis on imaging plus >1 SpA feature or HLA B27 plus >2 other SpA features.

Peripheral SpA

This is usually classified according to the presence of either arthritis, dactylitis or enthesitis plus >1 of Psoriasis, IBD, Preceding infection, HLA B27, uveitis or sacro-ilitis on imaging. Or if they have either arthritis, dactylitis or enthesitis >2 remaining SpA features such as arthritis, enthesitis, dactylitis, inflammatory back pain or a family history.

• Typical US findings-

The ultrasonographic features of enthesitis include hypo echogenicity, increased thickness of the tendon insertion, calcifications, enthesophytes (bone spurs), and enhanced doppler activity.

Particular attention should be paid to the enthesis sites at the Achilles, Quadriceps, Patellar tendons as well as lateral and medial epicondyles. Plantar fascia may also be thickened.

Although ultrasound might detect the inflammatory changes of sacroiliitis, its use is presently limited to research and is not recommended in routine clinical practice.

Peripheral SpA will present with synovitis in the small joints including MCP, PIPJs, and DIPJs and MTPs as well as flexor and extensor tendons especially digital extensor tendons.

References

1. Clunie G et al. Oxford Handbook of Rheumatology. 4^th Oxford University Press 2018.