Late stages and axial skeleton involvement
One of the most characteristic late signs of psoriatic arthritis is bony ankylosis (bony ankylosis). It is important to understand that this pathological process does not occur instantaneously. It manifests with the formation of fibrous tissue directly within the joint cavity. In the initial stages of imaging, this fibrous process can create a deceptive picture: radiologically, it looks like a false widening of the joint space, which requires careful interpretation of the images.
Spinal involvement, known as psoriatic spondylitis (psoriatic spondylitis), develops in approximately 20% of patients who already have the peripheral form of PsA. The clinical and radiological picture of this condition has much in common with ankylosing spondylitis (AS). Inflammatory changes are predominantly localized in the sacroiliac joints, as well as in the attachment zones of spinal entheses.
Differential diagnosis with ankylosing spondylitis
Despite the clinical similarity, diagnostic imaging allows for a clear distinction between psoriatic spondylitis and classic ankylosing spondylitis based on a number of specific patterns:
- Nature of sacroiliitis: In PsA, marked asymmetric involvement of the sacroiliac joints is observed (often accompanied by joint space narrowing), whereas symmetry is typical for AS.
- Formation of syndesmophytes: Spinal syndesmophytes (syndesmophytes) and enthesophytes (enthesophytes) in psoriatic involvement appear significantly coarser. Furthermore, they have a characteristically more horizontal orientation compared to the thin, vertical "bridges" seen in Bechterew's disease.
- Involvement of costovertebral joints: Involvement of the costovertebral joints (costovertebral disease) is extremely rare in PsA, which is an important distinguishing feature from the picture of AS.
Large joint involvement and arthritis mutilans
Pathology of large joints in PsA is frequently represented by entheseal disease (entheseal disease). The peculiarity lies in the fact that these changes are visualized quite poorly on standard radiographs. The most typical clinical manifestation is mild oligoarthritis of the large joints. It proceeds as a low-grade diffuse synovitis accompanied by recurrent joint effusions (recurrent effusions).
Particular attention should be paid to a severe form of the disease — arthritis mutilans (arthritis mutilans). Radiologically, this pathology is characterized by pronounced destruction: subluxation of the phalanges is observed (in particular, at the level of the metatarsophalangeal joints), as well as deep erosion of the middle and distal phalanges of the fingers.
Reactive arthritis: imaging findings and associations
Reactive arthritis (reactive arthritis, formerly known as Reiter's syndrome) is a sterile inflammatory arthritis. Its etiopathogenesis is based on a combination of genetic predisposition and an environmental trigger. Most commonly, the precipitating factor is a preceding gastrointestinal or genitourinary infection, after which joint syndrome manifests 2–4 weeks later.
The disease classically affects the joints of the lower extremities (both small and large) and, similarly to PsA, has a distinctly asymmetric character. Involvement of the upper extremities is considered atypical.
Radiological changes (using the feet as an example):
- The picture is largely similar to psoriatic arthritis: soft tissue swelling, joint space narrowing, and proliferative marginal erosions are revealed.
- New bone formation is actively occurring at the entheseal attachment sites (entheseal new bone formation), with calcaneal enthesis involvement being particularly characteristic.
- Enlargement of the sesamoid bones (sesamoid enlargement) may be observed as a consequence of concomitant periostitis.
Important note: new bone formation in the area of the foot entheses is a pronounced, but not strictly specific sign. Such a picture can occur as a normal variant in asymptomatic patients, as well as in osteoarthritis and other inflammatory arthropathies.
Briefly: answers to topical questions
What clinical and laboratory criteria are used for the diagnosis of psoriatic arthritis according to the CASPAR classification?
To establish a diagnosis of psoriatic arthritis according to the CASPAR classification, a patient must simultaneously present with evidence of inflammatory articular disease, spondylitis, tenosynovitis, or enthesitis, along with a score of three or more out of five possible diagnostic categories. Categories evaluated include current psoriasis, a personal or family history of psoriasis, psoriatic nail dystrophy, a negative rheumatoid factor, current or past dactylitis, and radiological evidence of juxta-articular new bone formation as peripheral bone outgrowths on hand and foot radiographs.
Which first-line drugs are used for the medical treatment of psoriatic arthritis?
In psoriatic arthritis combined with moderate-to-severe psoriatic skin lesions, drugs indicated for both conditions are preferred: methotrexate, apremilast, tofacitinib, upadacitinib, infliximab, adalimumab, etanercept, ustekinumab, secukinumab, ixekizumab, netakimab, guselkumab, and risankizumab.
For the differential diagnosis of spondylitis in PsA, remember the rule "A-H-R": Asymmetric sacroiliitis, Horizontal coarse syndesmophytes, Rarely involves costovertebral joints.
Class and exam questions
Why does the joint space appear widened in the early stages of bony ankylosis?
The ankylosing process begins with the formation of fibrous tissue within the joint, which can radiologically mimic a false widening of the joint space prior to the onset of true bony fusion.
How to differentiate spinal involvement in psoriatic arthritis from Bechterew's disease?
The key features distinguishing PsA are the asymmetric nature of sacroiliitis, coarser and horizontally oriented spinal syndesmophytes (enthesophytes), and extremely rare involvement of the costovertebral joints.
Which infections most frequently trigger reactive arthritis?
The most common triggers of reactive arthritis are preceding extra-articular gastrointestinal and genitourinary infections.
Is new bone formation in the area of the calcaneal entheses specific solely to reactive arthritis?
No, this sign is not strictly specific. It can be observed in osteoarthritis, other inflammatory arthropathies, and sometimes occurs as a normal variant in completely asymptomatic patients.
What else to study on this topic
- Radiological patterns of destruction in arthritis mutilans
- Imaging modalities for large joint enthesopathies in PsA
- Pathogenesis of spinal syndesmophyte and enthesophyte formation
- Differential diagnosis of causes of sesamoid bone enlargement
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