EXPERT ANALYSIS FROM THE WINTER RHEUMATOLOGY SYMPOSIUM
SNOWMASS, COLO. (FRONTLINE MEDICAL NEWS) – Dactylitis is a common and painful extra-articular manifestation of psoriatic arthritis that takes on added clinical significance because it’s also a marker of greater disease severity, Christopher T. Ritchlin, MD, said at the Winter Rheumatology Symposium sponsored by the American College of Rheumatology.
Indeed, psoriatic arthritis (PsA) patients with dactylitis are more likely to have polyarticular disease and radiologic evidence of bony damage, noted Dr. Ritchlin , professor of medicine and chief of the allergy, immunology, and rheumatology division at the University of Rochester (N.Y.).
Dactylitis, known colloquially as ‘sausage digit,’ is a diffuse painful swelling of an entire finger or toe. It is present in one-third to one-half of PSA patients participating in clinical trials. It typically occurs asymmetrically, is more common in the feet than hands, and involves on average two digits. The fourth toe is affected in 8 of 10 patients with foot involvement. The second finger is involved in one in three patients with dactylitis of the hands, making it the most commonly involved finger.
“We have no idea why this is,” confessed Dr. Ritchlin, who is also director of the Clinical Immunology Research Center at the university.
The differential diagnosis for dactylitis includes psoriatic arthritis, other spondyloarthropathies, sickle cell disease, tuberculosis, sarcoidosis, and pyogenic flexor tenosynovitis, a closed-space infection that is the major issue in the differential. Dr. Ritchlin sees many more cases of dactylitis due to PsA that get misdiagnosed as a flexor tendon sheath infection and inappropriately subjected to surgery and/or intravenous antibiotics than vice versa.
Pyogenic flexor tenosynovitis can be identified using the four Kanavel signs: diffuse swelling of a digit, often with discoloration; intense pain over the whole length of the tendon sheath, but limited to the sheath; the involved digit being held in a semiflexed posture; and exquisite pain upon passive extension of the digit, with the pain being worst at the proximal end.
University of Toronto investigators have demonstrated that, in their large longitudinal database of PsA patients, the prevalence of radiologic damage in participants with acute dactylitis of the hands is twice as great as in PsA patients without dactylitis.
“I’ve been struck over the years by how often I see psoriatic arthritis patients with dactylitic digits who not only have erosions but who actually have a complete fusion or ankylosis of the joint. The point is, when you have a joint with diffuse inflammation, in many patients it’s associated with activation of both osteoclasts and osteoblasts,” according to the rheumatologist.
Enthesitis, another cardinal extra-articular manifestation of PsA, is defined by inflammation at the sites where tendons, ligaments, and joint capsules attach into bone. The most commonly involved sites are the Achilles tendon and plantar fascia.
“It can also involve a lot of other areas and can lead to misdiagnosis as a result. Many of these patients end up in rheumatologists’ offices with previous diagnoses ranging from fibromyalgia or other chronic pain syndromes to malingering,” Dr. Ritchlin said.
Sites to examine for enthesitis, in addition to the foot and Achilles tendon, include the patellar and quadriceps tendons, iliac crest, greater trochanter, lateral epicondyle, the small joints of the hands, and the supraspinatus tendon.
“We have a registry of several hundred psoriatic arthritis patients, and I’ve been struck by the amount of enthesopathy when we examine these points,” the rheumatologist observed.
Enthesitis is a prominent feature of both early and established PsA. Power Doppler ultrasound is more sensitive than radiographs at identifying it. Italian investigators have shown ultrasound to be useful in the differential diagnosis between early rheumatoid arthritis and early PsA in patients with hand involvement. They assessed 52 clinically involved joints in 26 patients with early PsA and 68 involved joints in 34 early-RA patients. Synovitis was detected in 91% of the joints of the RA patients, compared with only 60% of the PsA patients’ joints.
In contrast, soft tissue edema was present in 42% of the most clinically involved fingers of the early PsA patients, compared with just 3% in those with early RA. Central slip enthesitis was seen in 21% of the clinically involved proximal interphalangeal joints of the PsA patients but in none of those belonging to patients with early RA. Peritendon inflammation of the extensor digitorum tendon was noted in 54% of the joints of the PsA group, compared with less than 3% of the early RA group ( Clin Exp Rheumatol. 2016 May-Jun;34:459-65 ).
“Basically, if you do ultrasound, you see there is significantly more enthesitis in early psoriatic arthritis than early rheumatoid arthritis, which has certainly been our experience as well,” Dr. Ritchlin commented.
Enthesitis is not as simple a disease process as most physicians were taught in training. Dr. Ritchlin credits Dennis McGonagle, MD, of the University of Leeds (England) with introducing the now-accepted concept of a synovio-entheseal complex as being a key player in the expression of PsA ( Arthritis Rheum. 2007 Aug;56:2482-91 ).
“The old idea is that the enthesis inserts onto bone and that’s where the pathology is. But it’s more complicated than that,” Dr. Ritchlin explained.
Dr. McGonagle and his coworkers showed that fibrocartilagenous entheses attach to bone much more deeply than previously recognized, like a tree with deep roots. That makes for lots of intimate contact between bony cells and vascular channels. And key structures are located near the intersection of enthesis and bone, including bursae and synovial membrane. For example, the Achilles tendon synovio-entheseal complex includes sesamoid fibrocartilage, periosteal fibrocartilage, the retrocalcaneal bursa, subchondral bone, and enthesis fibrocartilage, as well as the tendon itself.
Dr. McGonagle and coworkers argued that the pathogenesis of tissue inflammation and damage in PsA involves biomechanical stress, with resultant synovial inflammation accompanied by the release of inflammatory cytokines, which in turn leads to diffuse inflammation in and around the area where the enthesis inserts.
“The purpose of the enthesis is to distribute force away from the area where the tendon inserts into bone. So when biomechanical stress pulls on that tendon, other adjacent areas are also affected. What’s come out from imaging studies is that there’s synovial inflammation, bursitis, and also inflammation in and around the fibrocartilage in areas of enthesitis,” Dr. Ritchlin said.
He reported serving as a consultant to half a dozen pharmaceutical companies.