Psoriatic arthritis (PSa) is one of the most common forms of inflammatory arthritis.

It is defined as a unique inflammatory arthritis associated with psoriasis." Estimates of its prevalence in the general population vary from 0.3% to 1%.

Initially, PsA typically presents as an inflammatory arthritis affecting one or perhaps a few joints. However, over time PsA may begin to involve many joints and becomes very severe in at least 20% of patients.

So how does a patient know when they have psoriatic arthritis?

Unfortunately, few solid diagnostic clinical criteria are present.

And... there are no clear cut research criteria for the diagnosis of PsA- even the experts have not formulated widely accepted parameters. The diagnosis has been based on the presence of inflammatory arthritis which may affect either peripheral joints such as the fingers, wrists, elbows, shoulders, hips, knees, ankles, or toes. Or the disease may affect the sacroiliac joints (joints that joint the spine to the pelvis) plus the presence of psoriasis and the absence of a positive rheumatoid factor in the blood.

PsA does often cause a peculiar type of joint swelling called dactylitis." This is often referred to as a sausage digit" because.... well, because the affected swollen finger or toe looks like a sausage!

Experienced clinicians can make the diagnosis usually although this is not a diagnosis that can be made easily... particularly at the beginning.

The erythrocyte sedimentation rate (ESR or sed rate") is elevated in 40% to 60% of patients. X-ray findings may be absent in early disease. Magnetic resonance imaging and diagnostic ultrasound may be more useful in early disease.

To make things more confusing, a patient may have concurrent psoriasis and rheumatoid arthritis, psoriasis and osteoarthritis, or no known psoriasis, which can really complicate the diagnosis of PsA. Sometimes, patients will have a family history of psoriasis or psoriatic arthritis.

Since it is a systemic form of arthritis, much like rheumatoid arthritis, patients with psoriatic arthritis can develop complications such as inflammatory eye disease.

The Classification criteria for Psoriatic Arthritis (CASPAR) are diagnostic parameters that were recently formulated by researchers to standardize the diagnosis of PsA. The system may be used in the future for clinical trials involving patients with PsA.

Classification criteria for Psoriatic Arthritis
(CASPAR) Criteria:

Presence of inflammatory joint disease (joint, spine or enthesis [where the tendon inserts into bone]) with at least 3 points from the following 5 categories:

1. Evidence of current psoriasis, personal history of psoriasis, or family history of psoriasis.

2. Typical psoriatic nail changes, including onycholysis [separation of the nail from the nailbed], pitting, and hyperkeratosis (thickening and discoloration) on examination.

3. Negative test result for rheumatoid factor

4. Current dactylitis, defined as swelling of an entire digit (finger or toe), or history of dactylitis recorded by a rheumatologist.

5. X-ray evidence of juxta-articular (next to the joint") new bone formation appearing as ill-defined ossification (calcium deposits") near joint margins on plain x-rays of hand or foot.

Once the diagnosis has been established, treatment can be instituted. The topic of treatment for psoriatic arthritis will be discussed in another article. Stay tuned!