Where does Psoriatic Arthritis start?
Introduction
When a patient is told that they have Psoriatic Arthritis (PsA), one of the first questions they will ask is "how did it start?" or "why did it start?"
Doctors researching PsA have asked the related question, "where does it start?" This has allowed question about "how did it start" to be posed in a new light. An increasing body of work suggests that PsA starts at the enthesis. This website is devoted to all aspects of diseases of the entheses and especially PsA and its sister conditions.
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| There are many different variants of PsA. All of these have been linked to visible or hidden or silent abnormalities of the entheses around the body. |
Laboratory models of arthritis with features of PsA start at the enthesis. Patients with psoriasis without PsA often have lots of abnormalities at their entheses when the joints are scanned [1].
As discussed here there are many different variants of PsA and it is not known whether every joint in every patient has primary abnormalities at the entheses.
Patients with psoriasis have abnormal entheses.
Early changes at the enthesis have been reported in up to 40-50% of cases of patients with psoriasis who don't have joint pain.
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| this is an ultrasound scan of a normal enthesis. The yellow arrows show the point of the Achilles enthesis attachment to bone. Abnormal blood flow related to inflammation is not evident at the normal enthesis. |
These changes are commoner in psoriasis patients with nail disease [1]. This is because the nail is a type of enthesis and is anchored directly to the skeleton.
Severe inflammation at the enthesis is often associated with an increased blood flow that can be seen on ultrasound scanning. This is visible in nearly half the cases of PsA but not seen in normal subjects. These blood vessel changes are seen in about 10% of psoriasis subjects [2].
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| About 10% of patients with psoriasis without known arthritis have abnormal blood vessels visible at the enthesis. The attachment site is thickened in the case (yellow arrows). The vessels are visible on ultrasound due to the increased blood flow through the tissue. |
It has not been proven yet that an abnormal ultrasound scan will predict who is going to get bad future arthritis but there is some supportive evidence.
Inflammation spreads from the enthesis
There are well over 1000 entheses around the body and this is why PsA can attack virtually any joint.
The enthesis is at the junction where ligament and tendon attachments meet the bone with the joint cavity on one side and the soft tissues outside the joint on the other side. Because of this, enthesis related inflammation may cause sausage digits or marked bone pain.
The enthesis attachment site has lots of nerve endings but unlike the joint lining and adjacent soft tissues the enthesis does not swell up as readily when it is inflamed. This is why the key role of the enthesis is hard to understand unless a patient has inflammation of a large enthesis such as the Achilles tendon.
Implications
There are many different variants of PsA. It is clear that disease of the spine and disease of the distal interphalangeal joint are strongly linked to enthesitis. This is harder to prove for isolated knee involvement or for generalised patterns of PsA that have a similar pattern to Rheumatoid arthritis.
The answer as to how PsA starts is addressed on this website.
References
Resources
These are good general information resources but don't focus on where disease starts.
Psoriatic Arthropathy Alliance gives general information about PsA