Psoriatic arthritis (PsA) is a relapsing inflammatory joint condition associated with psoriasis that affects about 1-3% of the world overall, but with widely varying incidences in different populations. It is a severe and sometimes debilitating complication of psoriasis.
The term PsA was introduced by Pierre Bazin in 1860. At present, however, the disease classification put forward by Moll and Wright (1973) is followed. They defined the disease as a seronegative inflammatory arthritis associated with psoriasis.
Research has provided further evidence that PsA is more than just a combination of arthritis with psoriasis, and that it can be caused due to the influence of genetic, immunologic, and population-specific factors.
Broadly, two types of psoriasis exist. Type I appears before the age of 40 years, and is the most common type, occurring in about 85% of PsA cases.
The most common age of onset is 18-22 years. It tends to be more severe and is the type most frequently associated with PsA.
Genetic factors are clearly associated with this type, especially with human leukocyte antigens (HLAs) Cw602 alleles. Type II psoriasis usually appears at a much later stage in life, i.e., after 40 years of age.
Types of Classification for PsA
Different types of PsA classification criteria that have been suggested. Some of them are as follows:
Moll and Wright Classification
The following are the five clinical forms of PsA as described by Moll and Wright:
- Classic distal interphalangeal (DIP) joint involvement, accounting for 5% of cases
- Destructive arthritis or arthritis mutilans, accounting for about 5% of cases
- Symmetric polyarthritis closely resembling rheumatoid arthritis but seronegative, accounting for about 15% cases
- Asymmetric involvement of some DIJ and metacarpophalangeal joints, accounting for about 70% of PsA cases. This can be oligo- or monoarthritis and occurs in almost two-thirds of early cases of PsA, later changing to polyarthritis that is symmetrical in over half of patients.
- PsA which behaves like ankylosing spondylitis, accounting for 5% of cases
The Moll and Wright PsA system of classification was modified and expanded by Gladman to include the following seven categories:
- Only DIP involvement
- Oligoarthritis affecting four or less joints
- Spondylitis alone
- Distal involvement with spondylitis
- Oligoarthritis with spondylitis
- Polyarthritis with spondylitis
In this classification, arthritis mutilans was considered a marker of disease severity and not a type of disease in itself.
Wright’s Revised Classification
Wright suggested a simpler classification based on scintigraphic evaluation to identify clinical and subclinical PsA. The criteria were as follows:
- Peripheral polyarthritis (symmetrical or asymmetrical, oligo- or polyarthritis)
- SAPHO – Synovitis, acne pustulosi, hyperostosis, osteomyelitis
The classification of psoriatic arthritis (CASPAR) criteria is as follows:
- Evidence of current psoriasis, or personal history or family history of psoriasis
- Presence of nail dystrophy
- Current dactylitis or history of dactylitis
- Juxta-articular new bone formation as confirmed by radiography
- Absence of rheumatoid factor by using laboratory testing methods such as ELISA or nephelometry
Of late, researchers have classified PsA into three groups only:
- Asymmetrical oligoarthritis
- Symmetrical polyarthritis
- Predominant spondylitis
Are Classifications Useful?
The clinical usefulness of each classification depends upon its reproducibility. Many studies show that joint involvement is not usually completely identified by clinical assessment.
The more the investigations were used (X-rays, isotope scans, color Doppler, and MRI); the greater was the final incidence of polyarticular disease.
Some of the classification criteria may not be able to capture the actual status of PsA. For instance, by the classical modified New York criteria, spondylitis in the PsA group cannot always be diagnosed because of the frequent absence of sacroiliac disease.
In addition, the damages to the axial bones mostly start at a later stage of the disease and these changes can only be observed radiologically.
The functional utility of these classifications also fails in relation to treatment with biological agents as these drugs work well across all categories of diseases.
Classification criteria can, however, be put to good use especially when the number of joints involved at initial presentation is higher. Based on this classification criteria, the patient can be treated more aggressively to prevent long-term disability.
Among all classifications, Moll and Wright’s 1973 criteria is the most commonly used criteria in epidemiological and clinical trials of PsA. The main advantage of this classification is that it is easy to understand and use.
Going forward, it would be necessary to gather more data on this heterogenous disease by developing biomarkers as well as by performing more imaging studies to provide a wholesome and effective treatment strategy.