Psoriatic Arthritis: a Critical Review

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1 Clinic Rev Allerg Immunol (2013) 44: DOI /s Psoriatic Arthritis: a Critical Review Varun Dhir & Amita Aggarwal Published online: 1 February 2012 # Springer Science+Business Media, LLC 2012 Abstract Psoriatic arthritis is a chronic inflammatory arthritis that affects about 5 25% of patients with psoriasis. The prevalence varies from per 100,000 population across the world except in Japan where it is 1 per 100,000. Psoriatic arthritis affects both genders equally and in more than half it follows long-standing psoriasis. Psoriatic arthritis has been grouped into five subtypes: distal interphalangeal (DIP) predominant, symmetrical polyarthritis, asymmetrical oligoarthritis and monoarthritis, predominant spondylitis, and arthritis mutilans. Oligoarthritis occurs in nearly 60% during early disease but later polyarticular disease predominates mainly due to evolution of oligoarthritis to polyarthritis. In 50 60% polyarthritis is symmetrical. Dactylitis and enthesopathy are other major features seen in nearly one third of patients. The diagnosis of psoriatic arthritis is easy in the presence of typical skin lesions, however it can also be made in absence of skin lesions using Classification of Psoriatic Arthritis criteria. Though 30 40% of patients develop joint deformities at a follow-up of 5 10 years but most retain good functional status. Clinical damage has a strong relationship with number of swollen joints, erythrocyte sedimentation rate, and duration of arthritis. Radiological damage occurs early and erosions are present in nearly 50% at 10 years of disease. Spinal disease also has good outcome with maintained spinal mobility in majority of the patients. Screening of patients with psoriasis using questionnaire can help in early diagnosis. Nail dystrophy, scalp lesions, and intergluteal/perianal psoriasis are associated with higher chance of development of psoriatic V. Dhir : A. Aggarwal (*) Department of Clinical Immunology, Sanjay Gandhi Postgraduate Institute of Medical Sciences, Lucknow, India [email protected] arthritis. Early diagnosis will lead to early treatment and better outcome especially with advent of new drugs. Keywords Psoriasis. Inflammatory arthritis. Criteria. Outcome Introduction Psoriatic arthritis is an inflammatory arthritis that occurs in relation with psoriasis. Though psoriatic arthritis is known for more than two centuries, it was initially thought to be a variant of rheumatoid arthritis. The discovery of rheumatoid factor, present in a majority of patients with rheumatoid arthritis but absent in most patients with psoriatic arthritis, helped to establish the latter as a separate disease. It was classified as a separate entity by American College of Rheumatology (earlier called American Rheumatism association) in 1964 [1]. There is wide variability in reported incidence and prevalence of psoriatic arthritis. It is probably related to use of different case definitions and clinical settings, i.e., primary care versus hospital setting in different studies. A review in 2008 summarized the available data and reported that in North America and Europe the prevalence is similar and varies from 20 to 420 per 100,000, whereas the incidence varies from /100,000 individuals [2]. In contrast to Western data, studies from Japan show a very low prevalence of 1 per 100,000 and incidence of 0.1 per 100,000 [3]. The possible reason is the overall low prevalence of spondyloarthropathy in Japanese population. However in China, a country with similar ethnicity, the prevalence varies from per 100,000 [4]. In a study from Singapore, among multiethnic population, Indians had the highest prevalence [5]. The data from other countries are sparse [6].

2 142 Clinic Rev Allerg Immunol (2013) 44: In a study from Mayo clinic, the annual incidence of psoriatic arthritis had increased nearly three times from 3.6 per 100,000 population in 1970s to 9.8 in 1990s [7]. This is most likely related to increasing incidence of psoriasis in the same population [8] and both may be linked to change in environmental factors as genetic factors are unlikely to change in such a short span of time. Demographic Features Psoriatic arthritis usually occurs in the fourth and fifth decade but no age is exempted with cases occurring in young children and elderly. Both genders are equally affected with studies showing a male to female ratio varying from 0.7:1 to 2.1:1. In more than half of the patients (49 75%), arthritis follows long-standing psoriasis of about 7 12 years duration. This is followed by simultaneous onset of skin and joint disease in 10 37% patients and lastly it can precede psoriasis in 6 18% (Table 1) [9 17]. Psoriasis vulgaris is the commonest form of psoriasis associated with psoriatic arthritis. About 5% cases of psoriatic arthritis are associated with guttate and pustular psoriasis. Isolated nail involvement without skin involvement is seen in 1 2% of cases (Table 1) [9 17]. Classification Criteria Since psoriatic arthritis has varied clinical features and there is no confirmatory laboratory test, many different classification and diagnostic criteria have been proposed (Table 2). The initial criteria by Moll and Wright in 1973 [18] are still the simplest and have good sensitivity but are not too specific. These were later modified by Gladman et al., with addition of certain exclusions like: rheumatoid nodules, rheumatoid arthritis, crystal-induced arthritis, grade IV osteoarthritis, Reiter's syndrome, and obvious inflammatory bowel disease [9]. These exclusions were intended to probably take care of coincidental rheumatoid arthritis, which may occur in a patient with psoriasis, and to enable the inclusion of even rheumatoid factor-positive patient (if characteristic features of RA are not present) as psoriatic arthritis. The Vasey and Espinoza criteria emphasized the fact that psoriatic arthritis could be peripheral or axial [19]. These initial criteria failed to recognize disease in absence of clinical psoriasis, which was remedied by later criteria. The European Spondyloarthropathy Study Group criteria [20] and modified McGonagle [21] criteria took the family history of psoriasis in absence of skin disease as one of the clinical features to facilitate diagnosis of psoriatic arthritis before development of skin disease. Further McGonagle et al. for the first time emphasized the presence of enthesitis, either clinical or radiological as a major feature of psoriatic arthritis [21]. The most recent criteria, Fournié's [22] or the Classification of Psoriatic Arthritis (CASPAR) [16] have tried to identify psoriatic arthritis based on a scoring system even in the absence of psoriasis or family history of psoriasis thus facilitating early diagnosis. CASPAR criteria were formulated by an international collaboration, for which patient data were collected prospectively by multiple centers on psoriatic arthritis and other inflammatory diseases (rheumatoid arthritis, ankylosing spondylitis, undifferentiated arthritis, etc.) [16]. These criteria have a specificity of 98.7% and a sensitivity of 91.4%. They perform well both in early [23] and late disease [16] and can be used to classify patient with previously collected data and are thus very useful for cohort studies. Table 1 Demographic and general features in different series of PsA Reference Age of onset (years) M/F Duration after psoriasis (years) Onset of Arthritis in relation to psoriasis After psoriasis (%) Simultaneous (%) Before (%) Psoriasis type (%) Only nail (%) Gladman et al. [9] : V94, G 4 2 Jones et al. [10] : V89 G4 P3 0 Rajendran et al. [11] :1 NA V81 G1 E9 P4 4.3 Torre Alonso et al. [12] : V94 G1 E3 P2 2 Zisman et al. [13] : NA NA Madland et al. [14] NA 1.1:1 NA NA NA NA V94 P6 NA Michet et al. [15] NA NA Noosent and Gran [17] :1 8 NA NA 13.8 NA NA Figures are rounded off to nearest whole number V vulgaris, G gutate, E erythrodermic, P pustular; NA not available

3 Clinic Rev Allerg Immunol (2013) 44: Table 2 Different criteria for diagnosis of psoriatic arthritis Moll and Wright Criteria [18] Inflammatory arthritis (peripheral arthritis and/or sacroiliitis or spondylitis) Presence of psoriasis Absence of serological tests for rheumatoid factor Criteria for psoriatic arthritis proposed by Vasey and Espinoza [19] Psoriatic arthritis is defined as criterion I plus one from either criterion II or III Criterion I: psoriatic skin or nail involvement Criterion II: peripheral pattern Pain and soft tissue swelling with or without limitation of movement of the distal interphalangeal joint for over four weeks Pain and soft tissue swelling with or without limitation of motion of the peripheral joints involved in an asymmetrical peripheral pattern for over four weeks. Includes a sausage digit Symmetrical peripheral arthritis for over four weeks, in the absence of rheumatoid factor or subcutaneous nodules Pencil in cup deformity, whittling of terminal phalanges, fluffy periostitis and bony ankylosis Criterion III: central pattern Spinal pain and stiffness with the restriction of motion present for over 4 weeks Grade 2 symmetric/grade 3 or 4 sacroiliitis Modified ESSG criteria for psoriatic arthritis [20] Inflammatory spinal pain or Synovitis (either asymmetrical or predominantly lower limb) and One or more of the following: positive family history of psoriasis or psoriasis Modified McGonagle criteria for psoriatic arthritis [21] Psoriasis or family history of psoriasis Plus any one of: Clinical inflammatory enthesitis Radiographic enthesitis (replaces MRI evidence of enthesitis) Distal interphalangeal joint disease Sacroiliitis/spinal inflammation Uncommon arthropathies (SAPHO, spondylodiscitis, arthritis mutilans, onycho-pachydermo-periostitis, chronic multifocal recurrent osteomyelitis) Dactylitis Monoarthritis Oligoarthritis (four or less swollen joints) Psoriatic arthritis criteria of Fournié [22] Cut-off for diagnosis of psoriatic arthritis = 11 points Criteria Arthritis of a distal interphalangeal joint (3 points) Asymmetrical monarthritis or oligoarthritis (3 point) Buttock pain, heel pain, spontaneous anterior chest wall pain, or diffuse inflammatory pain in the entheses (2 points) Radiological criterion (5 points) (any one criterion present) Erosion of distal interphalangeal joint Osteolysis Ankylosis Juxtaarticular periostitis Phalangeal tuft resorption Human leucocyte antigen (HLA)-B16 (38, 39) or B17 (6 points) Negative rheumatoid factor (4 points) CASPAR criteria [16] Inflammatory musculoskeletal disease (joint, spine or entheseal) with 3 of the following: Evidence of Psoriasis (either of three) Current: psoriatic skin or scalp disease as judged by a dermatologist or rheumatologist* (score of 2) Personal history: may be obtained from patient, family doctor, dermatologist or rheumatologist Family history: in a first-or second-degree relative according to patient report Psoriatic nail dystrophy Typical psoriatic nail dystrophy including oncholysis, pitting and hyperkeratosis observed on current physical examination Negative rheumatoid factor By any method except latex, but preferably by ELISA or nephelometry, according to the local laboratory reference range Dactylitis Current: swelling of an entire digit Personal history: recorded by a rheumatologist Radiological evidence of juxtaarticular new bone formation Ill-defined ossification near joint margins (but excluding osteophytes formation) on plain X-rays of hand or foot Clinical Subtypes Psoriatic arthritis can affect both the peripheral joints as well as the spine; thus, the joint involvement has been grouped into different subtypes. The original Moll and Wright classification divided psoriatic arthritis into five subtypes: distal interphalangeal (DIP) predominant, symmetrical polyarthritis, single or few finger or toe joints involved (asymmetrical oligoarthritis and monoarthritis), predominant spondylitis, and arthritis mutilans [18]. The University of Toronto cohort found that many patients had axial symptoms and signs although they fitted well into the Moll and Wright polyarthritis or oligoarthritis groups. They divided the patients into seven groups, four of which were the same as the Moll and Wright classification but were more precisely defined: (1) distal (DIP only affected), (2) oligoarthritis ( four joints), (3) polyarthritis ( five joints), and (4) back (radiological evidence of sacroiliitis and/or classical syndesmophytes and inflammatory back pain, but without any peripheral joint disease). In addition they added three new groups: (5) distal with back, (6) polyarthritis with back, and (7) oligoarthritis with back. Arthritis was further divided into symmetric and asymmetric [9]. However, it is the original

4 144 Clinic Rev Allerg Immunol (2013) 44: classification of five major groups, which has stood the test of time and continues to be widely used. There have been differing interpretations and definitions of the five groups, with some studies following the University of Toronto definitions. The CASPAR group while retaining the original five groups has tried to define the groups by Moll and Wright more precisely DIP predominant defined as more than 50% of total joint count being DIP joints; polyarthritis as five joints involved; oligoarthritis as < five joints involved; arthritis mutilans as such; and spine predominant as inflammatory spinal pain, reduced spinal movements, and radiographic sacroiliitis. Symmetry was removed from the polyarthritis group, but analyzed separately, defined by half or more of joints showing symmetrical involvement [30]. Prevalence of Various Subtypes The proportions of different subset are different at onset and during the course of disease (Table 3). At the onset (or near onset), the major subgroup is oligoarthritis including monoarthritis, seen in up to 60% of patients. Monoarthritis is present in 16 30% of patients, in fact in one series, monoarthritis was seen in 39% of patients at onset [10]. As expected, with longer duration of disease many patients with oligoarthritis evolve into polyarthritis. One series found a change to polyarthritis in 67% of the patients with oligoarthritis [10]. The predominant pattern of involvement in patients followed up longitudinally is polyarthritis seen in 48 69% of patients. Symmetry was present in more than 80% of patients in CASPAR series [30], whereas, in the initial University of Toronto series [9] only half the patients had symmetric involvement. Oligoarthritis is the second most common pattern, with 10 37% of the patients showing this pattern. Spondylitis alone was present in 6 14% and DIP predominant subtype in 1 4% among different series (Table 3). Torre Alonso et al. found that females predominantly had polyarthritis, whereas males tend to have oligoarthritis [12]. Similar observation has been made by the CASPAR group [30]. Table 3 Distribution of subtypes of psoriatic arthritis in different series (%) References Country N Polyarthritis Oligoarthritis (oligo + mono) a Spondylitis DIP Mutilans Pattern at onset Gladman et al. [9] b Canada NA Pattern at presentation Gladman et al. [9] c Canada Veale et al. [24] Ireland Jones et al. [10] UK (24+39) 10 2 NA Michet et al. [15] USA (49+16) <1 NA NA Wilson et al. [7] USA NA NA Noosent and Gran [17] Norway (48+7) Cumulative pattern Roberts et al. [25] UK NA NA Torre Alonso et al. [12] d Spain NA 4.4 Jones et al. [10] UK (22+4) Rajendran et al. [11] India Michet et al. [15] USA NA 18.6 NA NA Madland et al. [14] Norway ( ) CASPAR et al. [16] e Multiple Reich et al. [26] Germany NA NA 4.9 Zisman et al. [13] f Israel NA not available a Figures in parenthesis represent figures for oligoarthritis (two to four joints) + monoarthritis b Polyarthritis includes only polyarthritis (41%) + polyarthritis and back (7%); oligoarthritis includes only oligoarthritis (22%) + oligoarthritis and back (3%); spondylitis means only back; DIP includes only DIP (20%) + DIP and back (4%) c Polyarthritis includes only polyarthritis (40%) + polyarthritis and back (21%); oligoarthritis includes only oligoarthritis (14%) + oligoarthritis and back (7%); DIP includes only DIP (12%) + DIP and back (4%) d Spondylitis group includes patients with isolated sacroiliitis and/or spondylitis (7.2%), sacroiliitis and/or spondylitis with peripheral joint involved oligoarthritis (7.2%), polyarthritis (5.6%), or DIP arthritis (3.3%) e Undefined in 3% f Overlapping subtypes present in 25%

5 Clinic Rev Allerg Immunol (2013) 44: Major Clinical Features Axial Disease (Spondylitis) Inflammatory backache is present in 18 46% of patients. Inflammatory neck pain has been reported in 23 39% and some series have found it to be more common than back pain [9, 16]. The CASPAR study found thoracic inflammatory pain in 17% [16]. On longitudinal follow-up, only 25 50% of patients continue to have axial symptoms by 5 10 years, the rest become asymptomatic. Also, of those patients who do not have axial symptoms at onset, only 10 25% of patients develop these over the next 10 years. Despite axial symptoms, spinal mobility is maintained in a majority of the patients with no reduction in the spinal flexion or chest expansion over 10 years [24, 27]. However, one study did show an increase in cervical spine limitation [27]. Axial involvement can often be clinically silent, and symptoms may be present in only about half of patients with radiological spinal involvement [27]. Radiological sacroiliitis is present in 11 37% of patients [9, 12, 14, 15, 24]. An Italian study found prevalence of 32% using bone scan to detect active sacroiliitis [28]. However, a multicenter study from USA on 202 patients with a disease duration of 12 years found sacroiliitis in 78% patients [29]. The high figures in this study may be due to the long disease duration. Indeed, one study showed that among patients who did not have any radiographic sacroiliitis at baseline, one third developed it at 5 years and half by 10 years. Similarly, among those with lesser grades, nearly half progressed to higher grades over 10 years [27]. In addition males have been found to have three times higher prevalence of sacroiliitis as compared to females [12]. As compared to ankylosing spondylitis, the sacroiliitis is more often likely to be unilateral and can become bilateral in later disease. Syndesmophytes have been reported in 5 26% [9, 11, 12, 30]. Most of the syndesmophytes are non-marginal, and in some patients they may be present without sacroiliitis [9, 12]. Symptomatic hip joint involvement was seen in 6.3% of 504 patients in the Mayo series, a majority having bilateral disease. Majority of them also had sacroiliitis suggesting a strong association of symptomatic hip disease with psoriatic spondylitis. Younger age of onset of psoriatic arthritis was a risk factor for hip joint disease; however, enthesitis, dactylitis, or pattern of peripheral arthritis had no association with hip joint disease [15]. Peripheral Arthritis Peripheral arthritis can vary from monoarthritis to polyarthritis. In early disease, asymmetrical oligoarthritis which commonly involves knee or a large joint along with a few small joints in finger or toes is seen more often. Oligoarthritis can be associated with dactylitis. Isolated DIP joint involvement associated with nail involvement is typical of psoriatic arthritis; however, it is uncommon [31]. DIP joint is often involved along with other joints. Polyarthritis usually involves small joints of hands and feet and thus can simulate rheumatoid arthritis (Fig. 1). Almost 40% of patients with polyarthritis subtype have dactylitis and enthesitis and [28, 30] this may help in differentiating psoriatic arthritis from rheumatoid arthritis. The polyarthritis is generally symmetrical. Shortening of finger due to pencil in cup deformity may also be seen. Dactylitis Dactylitis or sausage digit is a combination of enthesitis, tenosynovitis, and arthritis leading to diffuse swelling of a digit (Fig. 2). It is present in 32 48% of patients with psoriatic arthritis in various series [13, 15, 24, 28, 30 34]. In three fourths of the patients, dactylitis involves the toes and in almost half of the patients multiple digits are involved simultaneously [32]. As expected, the prevalence of dactylitis increase with disease duration, in the University of Toronto cohort, 32.5% patients had dactylitis at presentation which increased to 49.1% over follow-up [33]. Also, in a patient who has dactylitis, half will have a recurrence [32]. Enthesitis Enthesitis is present in 25 53% over the course of the disease [15, 16, 33, 34]. In the University of Toronto cohort, enthesitis was present in only 14.8% at onset of treatment and increased to 35.9% over the course of the disease [33]. Common sites affected are tendoachillies, plantar fascia, greater trochanter, etc. Ultrasonography is a sensitive tool to diagnose enthesitis. Uveitis Acute anterior uveitis similar to that seen in other spondyloarthropathies is seen in 4 18% of patients with psoriatic arthritis [9, 16, 35, 36]. Uveitis in psoriatic arthritis can also be bilateral, chronic, and rarely posterior. It is more common in the patients with spondylitis subtype of psoriatic arthritis with or without peripheral joint involvement [9]. However, it seems to be distinctly uncommon in some areas, as series from Spain and Israel found a prevalence of only 1 3% [12, 13]. Fig. 1 Hand photograph showing peripheral hand joint involvement along with psoriatic skin lesions and nail changes. Note the ray sign and complete sparing of index finger on both hands

6 146 Clinic Rev Allerg Immunol (2013) 44: Fig. 2 Dactylitis involving third finger of right hand Diagnosis When a patient with preexistent psoriasis has arthritis, the first diagnosis that is thought of is psoriatic arthritis. However, if a patient has features like classical erosions of rheumatoid arthritis, high titer rheumatoid factor, and anticyclic citrullinated antibodies, then possibility of coexistent rheumatoid arthritis needs to be considered. If a patient presents with oligoarthritis, especially lower limb asymmetrical arthritis along with dactylitis, a possibility of reactive arthritis should also be considered and a history of preceding gastrointestinal or genitourinary infections enquired into. Gout must be considered in cases with acute onset of monoarthritis or oligoarthritis, whereas an insidious onset of arthritis will be more suggestive of psoriatic arthritis (Table 4). Table 4 Features differentiating three common type of arthritis Feature Psoriatic arthritis Rheumatoid arthritis Reactive arthritis Fig. 3 X-ray of hands showing ankylosis of DIP and PIP joint of little finger on both sides, pencil in cup deformity in the left first IP joint and DIP joint involvement in multiple fingers In a patient lacking psoriatic skin lesions, a combination of spondylitis with peripheral arthritis, the presence of dactylitis, involvement of distal interphalangeal joints, shortening of finger, ray sign, i.e., involvement of multiple joints of one finger with sparing of other fingers should alert one to suspect psoriatic arthritis. On radiographs, features like fluffy periostitis, small joint ankylosis, osteolysis of small bones, and pencil in cup deformity on radiographs should make one consider a possibility of psoriatic arthritis (Fig. 3). A diligent search for psoriatic lesions in the hidden areas like scalp (Fig. 4), umbilicus, and natal cleft should be made. A detailed family history should also be taken. Skin rash Psoriasis Keratoderma M/F 1:1 1:3 3:1 Age Third to fifth decade Third to sixth decade Second to fourth decade Onset, course Insidious, Insidious, Sudden, acute chronic chronic Enthesitis + + Axial involvement + + Dactylitis + + Presence of RF/ rare + anti-ccp antibodies Periosteal reaction + ± DIP joints + +/ Joints involved UL > LL UL and LL LL > UL RF rheumatoid factor, CCP cyclic citrullinated peptide, DIP distal interphalangeal, UL upper limb, LL lower limb Fig. 4 Scalp psoriasis in a patient with psoriatic arthritis

7 Clinic Rev Allerg Immunol (2013) 44: Outcome Clinical In the initial study from the University of Toronto, it was reported that at a median disease duration of 9 years, 43% patients had at least one deformed joint and 16% had more than five deformed joints. Majority of these patients had received only NSAIDs [9]. In another study of 100 patients at a median disease duration of 4 years, deformity was present in 43% and more than five deformed joints in 9% of patients [24]. Clinical damage had a strong relationship with swollen joints, erythrocyte sedimentation rate, and duration of arthritis [37]. Radiographic Damage Radiographic damage occurs early and within 2 years of disease onset; almost half of patients had some evidence of radiographic damage [38]. At 7 years of disease two thirds of the patients had either clinical or radiological damage. Generally radiographic damage is detected before clinical damage is detected [38]. In another study, erosive disease developed in 56% over 11 years of follow-up and was most common in the polyarticular group [17]. Duration of arthritis at clinic entry, arthritis duration at time of damage detection, duration of psoriasis, and recent history of joint effusion were associated with increased risk of radiological damage [38]. Elderly onset patients had higher radiological damage as compared to young patients [28]. Function and Quality of Life Earlier studies found a good outcome with most patients requiring little time away from work because of arthritis [25]. Similarly, at entry into the University of Toronto cohort, with mean disease duration of 9 years, majority of the patients had a good to medium functional status, with only 11% having marked functional disability [9]. Thus, psoriatic arthritis was earlier considered a milder disease as compared to rheumatoid arthritis causing little disability. However, later studies have found that disability in psoriatic arthritis to be similar to rheumatoid arthritis [39]. Patients with psoriatic arthritis had reduced quality of life and functional capacity compared with psoriasis patients or healthy controls [40, 41]. The functional disability is highest in arthritis mutilans followed by polyarthritis [9, 10, 24]. Amyloidosis Amyloidosis is a rare complication and is seen in nearly 2% after 10 years of disease [17]. Screening for Psoriatic Arthritis Observation that psoriatic arthritis causes significant joint damage and most of it occurs early in disease and early treatment in rheumatoid arthritis improves outcome suggesting that early diagnosis and treatment of psoriatic arthritis may also improve outcome in psoriatic arthritis. Many questionnaires have been developed for screening patients, with psoriasis for psoriatic arthritis as in majority of patients, psoriasis precedes psoriatic arthritis [42]. Recently Toronto Psoriatic arthritis screen and psoriasis epidemiology screening tool questionnaire have been found useful to screen for psoriatic arthritis [43, 44]. Nail dystrophy, scalp lesions, and intergluteal/perianal psoriasis are associated with higher chance of development of psoriatic arthritis [8]; thus, such patients need to be followed closely. In future soluble biomarkers, ultrasound and MRI may also help in screening patient for psoriatic arthritis. The future seems to be bright for patients with psoriatic arthritis with rapid advances in pathogenesis, early diagnosis, better therapeutics, and assessment of response. All these should ultimately result in better outcome of psoriatic arthritis. References 1. Neil T, Silman AJ (1994) Psoriatic arthritis. Historical background and epidemiology. Ballieres Clin Rheumatol 8: Alamanos Y, Voulgari PV, Drosos AA (2008) Incidence and prevalence of psoriatic arthritis. A systematic review. 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