The foot and ankle is easy to overlook in the assessment of patients with psoriatic arthritis despite representing some of the most frequent joints affected.
Professor Philip Helliwell, from the University of Leeds in the UK, told ACR Convergence 2023 that patients were reluctant to kick off their shoes and socks and instead, it was much easier to reach across the desk to look at their hands.
Yet the majority of patients – about 60% – will have foot pain and forefoot deformity affects about 90% of patients with established disease.
Professor Helliwell, co-chair of the OMERACT Foot and Ankle Working Group, reminded delegates that psoriatic arthritis was a complex heterogeneous disease with a myriad of clinical features.
He told the meeting that the unique phenotype of psoriatic arthritis included arthritis, dactylitis, enthesitis, tendonitis / tenosynovitis and sesamoiditis.
“So it can be very disabling just having one or two features of this disease in your foot and ankle. And let’s not forget the skin which is an important manifestation of this disease… onychopathy, plaque psoriasis, palmoplantar psoriasis and acrodermatitis continua of Hallopeau.”
Professor Helliwell said the University of Glasgow group have reported that the incidence of forefoot deformity and inflammation in psoriatic arthritis exceeds that of the rearfoot and is similar to that in patients with rheumatoid arthritis.
Inflamed DIP joints were predominant in the foot as well as inflammation in the MTP joints.
He said it was a challenge but possible for clinicians to manually palpate the foot, using the thumbs on top and fingers below, to determine the “soggy feeling” of synovitis in the MTP joints.
Professor Helliwell said enthesitis was a hallmark clinical feature of psoriatic arthritis particularly the Achilles tendon insertion, the plantar fascia insertion, where the tibialis posterior inserts into the tuberosity of the navicular, and where the peroneus brevis inserts into the base of the fifth metatarsal.
“But of course there are hundreds of ligamentous and tendon insertions in the foot and all these are potential enthesitis sites so a patient presenting with mid foot pain, who doesn’t have obvious synovitis may well be having enthesitis at some of the ligaments in the mid foot.”
Given the Achilles tendon was the major enthesis in the body, he said clinicians should at least look at this area for enthesitis if nowhere else, and use ultrasound to help confirm clinical findings.
“Although I’m often referred people who have a problem mid portion – Achilles tendinitis which isn’t really related to psoriatic arthritis and is a common degenerative condition.”
Professor Helliwell said plantar fasciitis was also a common degenerative condition.
“But if it’s bilateral and particularly if it’s associated with power Doppler signal, those are the red flags for it being associated with spondyloarthropathy and psoriatic arthritis is included in that course.”
He added that dactylitis was more common in the foot than the hand, in multiple digits than in single digits, and most commonly presented in the fourth toe.
Professor Helliwell said sesamoiditis – enlarged sesamoid bones showing periostitis or erosion on X-ray – can occur in the hand and in the foot and may be quite disabling.
He encouraged rheumatologists to work closely with podiatry colleagues.
“Because as physicians you can address the inflammation and your colleagues will address the mechanical features which is very important otherwise you’re going to treat the inflammation, the patient is going to continue causing biomechanical stress to the affected areas and thereby continuing the inflammatory process.”
OMERACT has recently published some of its work on the impact of foot and ankle disorders on patients with rheumatic and musculoskeletal diseases [link here] and a scoping review [link here] with the aim of developing a core outcome set for clinical trials.