Diagnosis of psoriatic arthritis

The ClASsification criteria for Psoriatic ARthritis (CASPAR) are newly developed criteria for the diagnosis of PsA. They are simple to use, have a high specificity of 98.7%, and a sensitivity of 91.4% for the diagnosis of PsA.

CASPAR Criteria, A patient must have inflammatory articular disease (joint, spine, or entheseal) with 3 or more of the following 5 criteria:

1-      Current OR personal history of psoriasis, OR family history of psoriasis (1st or 2nd degree relative). Psoriasis is defined as skin or scalp disease.

2-      Psoriatic nail disease including: onycholysis, pitting, hyperkeratosis on current physical exam

3-       Negative for rheumatoid factor (by any method except latex)

4-        History of or current dactylitis recorded by a rheumatologist

5-       Radiographic evidence of juxta-articular new bone formation, appearing as ill defined ossification near joint margins (but excluding osteophyte formation) on plain radiographs of the hand or foot.  

Based on these criteria and established clinical features of PsA, so diagnosis include a history, physical examination, laboratory investigations, and review of treatment options such as :

History includes : Current OR personal history of psoriasis, OR family history of psoriasis, Swelling of joints, Pain or tenderness in joints, Morning stiffness >30 minutes and Functional capacity in activities of daily living (changes in ability to function at home and at work and impact on quality of life), etc.

Musculoskeletal :  Signs of joint inflammation such as swelling, effusion, synovial thickening, erythema, decrease in range of movement and Other manifestations such as  DIP joint involvement, enthesis, dactylitis, spondylitis and sacroiliitis, eye symptoms (i.e., iritis), etc .

Physical Examination : Nails: evidence of onycholysis, pitting, hyperkeratosis, oil-drop sign, and nail crumbling . Skin and joints which have forms like :

Form of PsA	Frequency	Joint Distribution	Other Features
Oligoarticular asymmetrical arthritis	70%	≤ 4 joints	dactylitis and/or monoarthritis
Symmetrical polyarthritis	15%	5 joints	erosive and metacarpophalangeal joint involvement
Predominantly DIP arthritis	5%		severe nail psoriasis
Arthritis mutilans	5%	May not have severe general disease	telescoping of fingers and toes, often associated with sacroiliitis
Spinal form	5%	Sacroiliitis	can be asymptomatic, uni/bilateral sacroiliitis, ± peripheral joints

 Diagnostic Investigations :

Laboratory tests should include: complete blood count, erythrocyte sedimentation rate, C-reactive protein, Rh factor, and routine renal and liver function tests.

The most important serological feature is the negative test for rheumatoid factor, particularly in patients with distal and mutilating arthritis. In patients with psoriatic arthritis of the rheumatoid type, about one-quarter had a positive test or a test that fluctuated between positive and negative . Some of these patients probably represent the coincidental association of psoriasis and rheumatoid arthritis, but a weakly or intermittently positive rheumatoid factor occurs in approximately 5% of the normal population and should not necessarily lead to the diagnosis of rheumatoid arthritis. Other reported laboratory abnormalities, such as anaemia, raised ESR (Erythrocyte sedimentation rate) and C-reactive protein, transient leukocytosis, and raised immunoglobulin levels, most consistently IgA, have been reviewed. It has been concluded that none of these is sufficiently specific to contribute to diagnosis, management or prognostic evaluation .

Comparison of Expected Laboratory Values in Psoriatic Arthritis and Rheumatoid Arthritis

Laboratory Studies	Psoriatic Arthritis	Rheumatoid Arthritis
Erythrocyte sedimentation rate	Elevated (<100)	Elevated (<100)
Rheumatoid factor	Negative	Positive (85% of patients)
Antinuclear antibody	Negative	Positive (30% of patients)
C-reactive protein	Elevated	Elevated
Synovium	WBC count 5-15,000/µL, 50% polymorphonuclear leukocytes	WBC count 2,000/µL

Histopathology. The histopathologies of psoriatic and rheumatoid arthritis appear to be similar, except that characteristic rheumatoid granulomas have not been found and there may be more fibrosis and vascular changes in the psoriatics. In spite of these possible differences, synovial biopsy usually has no place in the routine clinical management of psoriatic arthritis .

Radiological changes. The changes may be indistinguishable from those of rheumatoid arthritis: local demineralization, narrowing of joint spaces, articular erosion of varying degree and soft-tissue swellings. Atypical features include destructive changes in the terminal interphalangeal joints, a tendency to hypertrophic changes and absence of generalized demineralization. In the distal type of psoriatic arthropathy, early changes may consist only of minimal ‘fluffiness’ and osteoporosis of the distal phalanx, but gros destruction eventually occurs. Four characteristic signs of psoriatic arthropathy, seen in one controlled series .

1-      Adestructive distal interphalangeal arthropathy—bony ankylosis of the interphalangeal joints

2-      Destruction of the interphalangeal joints with abnormally wide joint spaces and sharply demarcated adjacent bony surfaces

3-      Destruction of the interphalangeal joint of the great toe with bony proliferation of the distal phalanx

4-      Resorption of tufts of the distal phalanges of hands and feet (uncommon).

In arthritis mutilans, the joint changes are widespread. The ‘opera-glass hand’, in which the fingers can be pulled in and out, results from gross destruction and absorption of the bones. The heads of metacarpals and metatarsals may completely disappear leaving a tapered bone looking like a sharpened pencil. Such gross osteolysis may be followed by bony fusion . Intermittent hydrarthrosis may occur. Sacroiliac changes similar to those of ankylosing spondylitis are common . Syndesmophytes and calcification of the interspinous ligaments are also seen Cervical changes including apophyseal sclerosis or joint narrowing, and calcification of the anterior ligament, are common , and there is a tendency to posterior fusion of the cervical vertebrae   Paravertebral ossification of the lumbar and thoracic regions may occur more laterally in psoriasis than in spondylitis . Magnetic resonance imaging has enhanced the ability to identify early changes of psoriatic arthritis

Differential diagnosis. This has been reviewed in detail elsewhere One of the subgroups of psoriatic arthritis may be clinically indistinguishable from rheumatoid arthritis. As a weakly or intermittently positive rheumatoid factor occurs in 5% of the normal population , this should not necessarily lead to the diagnosis of rheumatoid arthritis. Gout may be simulated, especially if the onset is acute and monoarticular, or if widespread psoriasis is associated with hyperuricaemia. Ankylosing spondylitis may be similar, but the onset tends to be later, peripheral involvement is more common, and the arthritis does not necessarily begin in the spine.
  
MRI in the Differential Diagnosis of Psoriatic and Rheumatoid Arthritis: Subjects and Methods