Differential Diagnosis

The differential diagnosis of a swollen toe in a patient with psoriasis includes PsA, as well as gout and, less commonly, sarcoidosis. On occasion, a patient may present with a swollen toe secondary to trauma. Gout should be considered, but this patient did not have an elevated serum uric acid, and the pattern of the inflammation did not follow the usual course of gout. There was no evidence to support a diagnosis of sarcoidosis, and his chest radiograph was normal. There was no history of trauma, and none was detected on radiographs. A diagnosis of PsA was based on the presence of inflammatory arthritis, psoriasis, nail lesions and dactylitis. The patient was given a higher dose of indomethacin (150 mg/day) and was to be followed in a month’s time.

Differential Diagnosis

This patient presents with an inflammatory arthritis associated with psoriasis. The most likely diagnosis is PsA. However, the differential diagnosis includes other forms of inflammatory arthritis, as well as seronegative rheumatoid arthritis (RA), reactive arthritis, and spondyloarthritis (SpA). The presence of SpA makes the diagnosis of RA unlikely ( Table 6-1 ). There is no evidence in the history to support a diagnosis of reactive arthritis. A diagnosis of ankylosing spondylitis (AS) may be entertained on the basis of the presence of sacroiliitis and thoracic syndesmophytes, but he has not had significant inflammatory axial disease and the peripheral arthritis is more than one usually expects in AS ( Table 6-2 ). Indeed, if radiographs were not taken, the limitation in his back movements might have been attributed to degenerative disease.

CLASSIFICATION CRITERIA FOR PSORIATIC ARTHRITIS

The Classification of Psoriatic Arthritis (CASPAR) group studied 588 patients with PsA and 536 patients with other forms of inflammatory arthritis collected in 30 centers according to a set protocol that included clinical evaluation, radiographs, and laboratory tests. Although the gold standard was the physician diagnosis, this was confirmed by an external team and a latent class analysis. The CASPAR criteria were derived from both logistic regression and classification and regression tree analyses, which yielded similar results. The resultant criteria can be applied to individuals with an inflammatory musculoskeletal disease including peripheral arthritis, spondylitis or enthesitis. To be classified as having PsA, a patient must score at least 3 points from the following criteria: Current psoriasis provides 2 points. If psoriasis is not present on the current assessment but the patient has a history of psoriasis, they score 1 point, or in the absence of personal psoriasis, if there is a family history of psoriasis, they may score 1 point. The following each provide 1 point: nail lesions; a negative rheumatoid factor; presence of dactylitis or history of dactylitis documented by a rheumatologist; presence of fluffy periostitis on hand or feet radiographs (excluding osteophytes) ( Table 6-3 ). According to these criteria, a patient may be classified as PsA with 91.4% sensitivity and 98.8% specificity. Using these criteria, 99% of the patients in a large PsA clinic would be classified as PsA. Moreover, the criteria were 99% sensitive in an early PsA (less than 2 years of disease). In addition, the criteria were very sensitive and specific in a family medicine clinic, as well as in an early arthritis clinic. Thus, these criteria may actually function well as diagnostic criteria.

In applying the CASPAR criteria to the patient in Case 1 who clearly had inflammatory arthritis, we can determine that the patient had dactylitis, nail lesions, a negative rheumatoid factor, psoriasis, and periosteal reaction, thus he scores 5 on the CASPAR criteria. The patient in Case 2 also clearly had an inflammatory musculoskeletal disease; he had current psoriasis, nail lesions, a negative rheumatoid factor, and dactylitis, thus scoring 5 on the CASPAR criteria.

SCREENING TOOLS FOR PSORIATIC ARTHRITIS

Because the CASPAR criteria require the recognition of inflammatory arthritis, which may not be apparent to a dermatologist or a general practitioner, a number of screening questionnaires have been developed to identify patients who might have PsA. The Psoriasis and Arthritis Questionnaire (PAQ) was developed by Peloso et al. This 12-item questionnaire was administered to 108 psoriasis patients, of whom 70 patients (37 with low PAQ scores [3/12] and 33 with high PAQ scores [>7/12]) were clinically evaluated by a rheumatologist. The PAQ score predicted PsA with a sensitivity of 0.85 and a specificity of 0.88 for a score of 7 or higher.

Alenius et al. evaluated the PAQ in a study of prevalence of joint disease in 202 patients with psoriasis, all of whom underwent a clinical examination. The majority (82%) of the patients with joint or axial complaints also had radiographs, as did 20 individuals without musculoskeletal complaints. Although a large proportion of their patients (48%) turned out to have joint manifestations, they did not find the PAQ to be as sensitive in identifying patients with PsA as was detected in the first study. They identified a score of 4 as the best cutoff, providing a sensitivity of 0.60 and a specificity of 0.62. They then weighted the questions, and the weighted score did not provide better sensitivity, although the specificity increased to 0.73. They concluded that the use of that questionnaire was not helpful in identifying arthritis in their group of psoriatic patients.

The Psoriatic Arthritis Screening and Evaluation (PASE) questionnaire was recently published by Husni et al. This questionnaire was developed primarily as a screening tool for dermatologists to identify patients with PsA because it was recognized that to have every patient with psoriasis evaluated by a rheumatologist would be impossible. PASE consists of two subscales, a symptom subscale and a function subscale. The instrument was tested in 69 patients, including 17 patients with PsA and 52 patients with psoriasis without arthritis. PsA was diagnosed by a rheumatologist on the basis of the Moll and Wright criteria. There were statistically significant differences in scores between patients with and without PsA, both in terms of symptoms and function components. Patients with PsA had higher scores than patients with psoriasis and osteoarthritis. Patients with more severe PsA had higher scores than those with milder disease. PASE scores ranged from 28 to 63. A cutoff of 47 proved to be optimal for differentiating patients with and without PsA. Using this cutoff, the sensitivity of PASE for PsA was 82% (95% confidence interval, 57–96) and the specificity was 73% (59–84).

The Toronto Psoriatic Arthritis Screening tool (ToPAS) was designed as a screening tool for PsA regardless of whether or not a patient was followed for psoriasis. In that respect, it is different from the other screening tools. It was applied to patients in a PsA clinic, psoriasis clinic, general dermatology clinic, general rheumatology clinic and a family medicine clinic, and it was found to be highly sensitive and specific in all groups of patients in whom it was tested. ToPAS includes pictures of psoriasis and nail lesions. The latter were a major issue for the PAQ, because the agreement between physician and patient was very poor. ToPAS addresses the issue of whether the patient has ever had joint symptoms or skin lesions. Although these screening questionnaires still require validation in other centers, they may be used to detect patients who should be seen by rheumatologists.

EPIDEMIOLOGY OF PSORIATIC ARTHRITIS

The exact prevalence and incidence of PsA are unknown. Estimates have varied from 0.04% to 0.37%. A recent systematic review of incidence and prevalence studies in PsA highlights the wide variation in estimates, as well as the importance of having widely accepted classification criteria. Now that the CASPAR criteria are available, as well as the screening questionnaires, better epidemiologic studies can be carried out that provide more accurate estimates of the incidence and prevalence of PsA. PsA occurs in men and women almost equally. The average age at diagnosis in most series in the 4th decade, usually 7 to 10 years after the onset of psoriasis. Although in almost 85% of the patients the psoriasis either precedes or has a simultaneous onset with PsA, about 15% of the patients present with PsA before the onset of psoriasis. The clinical features outlined in the following sections help identify the correct diagnosis in these patients.

CLINICAL PICTURE OF PSORIATIC ARTHRITIS

Patients with PsA may present with peripheral arthritis, with or without dactylitis and enthesis, with or without spinal involvement.

Peripheral Arthritis

The arthritis of PsA is inflammatory in nature, presenting with pain, swelling, and stiffness in the affected joints. There is often a purplish discoloration of the affected joint. About half of the patients have morning stiffness of more than 45 minutes’ duration. The onset is often oligoarticular, but over time, many patients develop polyarticular disease. Although initially the arthritis tends to be asymmetric, with the accrual of more joints, it may become more symmetric. As the number of affected joints increases, the arthritis is more likely to become symmetric. A typical feature of PsA is the “ray distribution.” This refers to the involvement of all three joints of a particular digit, as opposed to the same joints being involved in both sides ( Figs. 6-2 through 6-6 ).

Psoriatic arthritis, distal involvement.

Psoriatic arthritis, oligoarthritis.

Psoriatic arthritis, polyarthritis. This figure demonstrates the shortened digit, which is typical for psoriatic arthritis, as well as the “ray” phenomenon.

Psoriatic arthritis, arthritis mutilans, demonstrating severe deformities.

Psoriatic spondylitis. A, The figure demonstrates the inability of the patient to bend down. B, Radiographs demonstrate why the patient cannot bend, with evidence of both classic and paramarginal syndesmophytes.

Any joint may be affected in PsA, including both small and large joints. Distal joint disease is common, occurring in more than 50% of the patients. The distal joints of the hands and feet may be affected. At the same time, knees, hips, shoulders, elbows, and ankles may be affected, with or without more distal joint disease ( Table 6-4 ).

Table 6-4

Joints Involved at Presentation to the Psoriatic Arthritis Clinic

	Actively Inflamed Joints (%)	Clinical Damage (%)
Hands	71	10
Feet	64	8
Knees	33	1
Elbows/shoulders	32	1
Wrists	31	2
Ankles	17	0
Hips	9	1

 

Dactylitis

Dactylitis, or sausage digit, reflects inflammation of the whole digit ( Figs. 6-1 and 6-7 ). It likely results from inflammation in both tendons and the individual joints of the digit, although magnetic resonance imaging studies suggested that the major component is tenosynovitis. Dactylitis occurred at least once in 48% of the patients, whereas almost a third had dactylitis at presentation to clinic. Digits with dactylitis had more radiographic progression than those without dactylitis. Perhaps an extreme case of dactylitis may be the distal limb edema, which was identified in 21% of patients with PsA and only 4.9% of patients with other forms of arthritis.

Dactylitis in a finger.

Enthesitis

Enthesitis is also a typical feature of PsA ( Fig. 6-8 ). Fernández-Sueiro et al. detected a higher prevalence of enthesitis in PsA than in AS (42.4% versus 12.1%, respectively, P < 0.001). Enthesitis was more common in patients with PsA who had axial disease, and the condition was associated with the activity of peripheral and axial disease. The Achilles and superior posterior iliac crest insertions are more common in PsA.

Enthesitis seen on radiographs with both Achilles and plantar fascia spurs.

ASSESSMENT OF PATIENTS WITH PSORIATIC ARTHRITIS

In order to evaluate therapeutic intervention, proper assessment tools must be developed. In PsA, the peripheral joints, dactylitis, enthesitis, spinal disease, as well as skin and nail assessments must be carried out to fully evaluate the patient. Joint inflammation is assessed by scoring 68 joints for tenderness, and 66 for swelling. These measures have been found to be sensitive to change in clinical trials. However, whereas the tender joint count is reliable, the swollen joint count is less reliable both among experts and physicians less experienced in joint assessment. The assessment of dactylitis has been proven reliable by experts but not by nonexperts, whereas the assessment of enthesitis was found to be reliable among experts. Spinal assessment tools used in AS have been found to be reliable in PsA. Several tools have been developed for the assessment of skin and nail disease. The body surface area, physician global assessment of skin, and the PASI have been most commonly used in drug trials. The nails can be evaluated using the Nail Psoriasis Severity Index (NAPSI) or the modified NAPSI. Patient-reported outcomes are important in PsA becasue they provide additional information not included in the clinical assessment. These include the Medical Outcome Study Short Form 36 (SF-36); the Dermatology Life Quality Index, the Health Assessment Questionnaire (HAQ), which is a measure of function; the Fatigue Severity Scale, and the Functional Assessment of Chronic Illness Therapy-Fatigue (FACIT-F), which measure fatigue. All of these instruments have proven reliable and sensitive to change in patients with PsA.

SHOULD PATIENTS WITH PSA BE TREATED AGGRESSIVELY?

PsA had been considered a mild form of arthritis. Wright’s original description depicted a mild form of arthritis, with asymmetric oligoarticular pattern being most common. However, over the past 2 decades, it has become clear that PsA is a more severe disease (than previously thought) and is at least as severe as RA. Most recent studies indicate that PsA becomes polyarticular over time. Indeed, in a study of early PsA, which included patients presenting to clinic within 5 months of onset of symptoms, 47% of the patients demonstrated at least one radiologic erosion by the 2-year follow-up. Studies that included patients with more than 7 years of follow-up documented erosive disease in 67% of the patients at clinic entry. Progression of clinical and radiologic damage has been demonstrated, and although polyarticular presentation is predictive for disease progression, the number of inflamed joints at any visit as well as current damage, predict progression of both clinical and radiologic damage at subsequent visits. Patients with PsA are also at an increased mortality risk. Although causes of death are similar to the general population, previously active and severe disease is associated with increased mortality risk.

In addition to the physical aspects of the disease, patients with PsA also suffer from reduced quality of life and function, which result from the inflammation and fatigue associated with PsA, as well as the discomfort associated with psoriasis.

Thus, patients with PsA should be diagnosed and treated early in the course of their disease before there is any clinical or radiologic damage, to prevent progression of joint damage and reduce the mortality risk. Because the majority of patients with PsA have psoriasis, it is important for dermatologists and general physicians to recognize the symptoms and signs of arthritis and refer patients for rheumatologic consultation and management.