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Psoriatic arthritis is a heterogeneous inflammatory disorder that affects both women and men, with musculoskeletal symptoms typically occurring at age 40 to 60. Approximately 2-4% of individuals in the general population have psoriasis, and 20-30% of these develop psoriatic arthritis, usually at a later time, but concurrently in some cases.

In the prospective , the annual incidence rate of psoriatic arthritis was 2.7 cases (95% CI 2.1-3.6) per 100 patients with psoriasis.

Affected sites include the peripheral and axial joints, entheses, skin, and nails. Dactylitis -- i.e., the uniform swelling of a finger or toe, also known as "sausage digit" -- also is often present. The wide array of symptoms poses challenges in both diagnosis and treatment.

"The main challenge in dealing with psoriatic arthritis is the delay in diagnosis," said M. Elaine Husni, MD, of the Cleveland Clinic. "There can be a 4- to 10-year lag from when patients develop psoriasis to when they develop psoriatic arthritis."

"We need to raise awareness of this among patients with psoriasis to be on the lookout for joint pain and swelling that has been present for 6 weeks or more without apparent injury or trauma," she told 番茄社区. "It's also important that the patient be referred for a thorough workup by a rheumatologist, as other conditions such as gout or osteoarthritis can mimic psoriatic arthritis."

The importance of prompt diagnosis is highlighted by the fact that despite the dramatic recent advances in treatment, once joint damage has occurred, it cannot be reversed.

And in fact, of 283 patients with psoriatic arthritis found that even a 6-month delay from the onset of psoriasis symptoms to the first visit to a rheumatologist resulted in more joint erosions, sacroiliitis, and worse scores on health assessment questionnaires.

However, not all studies have found consistent time lags between the onset of psoriasis and the development of psoriatic arthritis. For example, one from the Mayo Clinic in Rochester, Minnesota, presented at the 2020 virtual annual meeting of the American College of Rheumatology (ACR), included 157 incident cases of psoriatic arthritis. Among this group, whose mean age at the time of psoriatic arthritis diagnosis was 46, half had their diagnosis of psoriasis and psoriatic arthritis concurrently, and for those whose psoriatic arthritis developed later, the mean lag time was 8.6 months.

In some cases, the diagnostic lag results from patients being in denial, Husni said. "They say they don't want to be on harsh medications, or want to take care of it with diet or see a naturopath. They can rationalize for years. If they already have damage by the time I see them, I don't have anything to reverse that."

Features and Risk Factors

The diagnosis of psoriatic arthritis can be confirmed with the Classification Criteria for Psoriatic Arthritis criteria, which requires a score of at least three points in the following disease features:

• Current psoriasis (two points)

• History of psoriasis (one point)

• Family history of psoriasis (one)

• Dactylitis (one)

• Juxta-articular new bone formation (one)

• Rheumatoid factor negativity (one)

• Nail dystrophy (one)

The workup relies on a physical examination, blood work, and imaging of potentially involved joints. Laboratory abnormalities can include elevated markers of inflammation such as erythrocyte sedimentation rate or C-reactive protein, but there are no more specific biomarkers such as the rheumatoid factor and anti-citrullinated protein antibodies typical of rheumatoid arthritis. Patients also may be anemic.

Sonography has been shown to be useful in diagnosis of psoriatic arthritis. by the Group for Research and Assessment of Psoriasis and Psoriatic Arthritis that compared ultrasound findings for 50 patients with psoriatic arthritis and 50 controls, entheseal changes more commonly found in patients included enthesophytes, erosions, thickening, and hypoechogenicity in six specific entheseal sites such as the Achilles tendon insertion into the calcaneus.

Another at ACR's virtual annual meeting found that risk factors for progression to psoriatic arthritis included obesity, presence of arthralgia, severe psoriasis, history of uveitis, scalp psoriasis, nail psoriasis, and having a first-degree relative with psoriatic arthritis.

"The concept of prevention of psoriatic arthritis has gained increased interest given the physical limitation and poor quality of life experienced by patients coupled with low remission rates observed with the use of currently available therapies," the authors of that report stated. "The transition from psoriasis to psoriatic arthritis offers a unique window of opportunity to identify individuals at increased risk for psoriatic arthritis, to study and implement preventive strategies."

Experts agree that the severity of psoriasis and the location of the affected sites are important predictive features. In , having three or more sites affected by psoriasis was associated with a more than twofold increased risk of developing psoriasis, and having scalp lesions was associated with an almost fourfold higher risk.

Nail involvement has been associated with enthesitis, or inflammation where the nail unit is attached to the extensor tendon. Enthesitis and nail involvement are thought to develop prior to joint inflammation. Nail disease may represent increased immunoreactivity or may actually be an early phase of psoriatic arthritis, predictive of clinical disease, studies have suggested.

Nail pitting also was found to be linked with the development of psoriatic arthritis in the Toronto Psoriasis Cohort in a that included 464 patients with the skin disease, 51 of whom subsequently reported musculoskeletal symptoms. Among this cohort, the relative risk of progression with nail pitting in a multivariate analysis was 2.51 (95% CI 1.37-4.49, P=0.002).

Ever having had uveitis also was associated with the development of psoriatic arthritis (RR 31.5, 95% CI 5.06-195.8, P=0.0002).

Another feature in early disease that may be predictive is dactylitis. In of 177 patients presented in a poster session at the ACR virtual annual meeting, erosions were detected on ultrasound significantly more often with patients who had dactylitis (31.9% vs 12.8%, P=0.004).

The link between psoriatic arthritis and obesity may relate to increased systemic inflammation related to cytokine release by adipose tissue, elevated mechanical loading on the joints, and obesity-related dyslipidemia.

Looking Toward Treatment

Patients initially are classified as having mild, moderate, or severe disease, Husni explained. "For a patient with mild disease whose labs are normal, one or two joints are affected, and imaging didn't show any irreversible damage, we might offer an oral treatment such as apremilast [Otezla] or methotrexate."

"But if there are blood abnormalities, higher joint counts, and erosive changes on imaging, we tend to be more aggressive, using biologics such as [tumor necrosis factor] or [interleukin]-17 inhibitors," she said.

The myriad options for treatment -- from older conventional agents to those only recently approved -- provide a wide variety of choices for individual patients, which will be explored in a further part of this Clinical Challenge series.

Husni said: "It's a really great time to have psoriatic arthritis, because we have so many more treatment options compared with 10 years ago."

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