Psoriatic Arthritis

Asymmetric
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Diagnosis and reasoning

Rheumatology is one of the more arcane medical disciplines - and it can often be confounding for new initiates. In practice though, a mix of strong clinical skills, pattern matching techniques, and careful observation for 'key' clues often allows rapid and accurate diagnosis. Consider the patient in this case. Several clinical features jump out at us immediately: - The joints affected are those in the fingers and toes, i.e. the small joints of the body; furthermore, the distal interphalangeal joints (DIP) appear to be exclusively involved. - The joints are involved in an asymmetric manner. - Only three joints are involved (for these purposes, all the small joints of the same type in a single extremity are counted only once) - i.e. this is an oligoarthritis (4 joints or less). - There is a history of joint stiffness upon waking up, which resolves afterwards. This is a cardinal sign of inflammatory arthritis. Thus, this is an asymmetrical oligoarthritis of the DIP joints which is most likely inflammatory in origin. Note also the history of an acute monoarthritis of the knee joint with spontaneous resolution. When the above factors are considered along with his age and male gender, this is strongly suspicious of a seronegative spondyloarthropathy. A vast number of diseases can give rise to a seronegative spondyloarthropathy; these include Ankylosing Spondylitis, Psoriatic arthritis, Reactive arthritis, enteropathic spondylitis secondary to inflammatory bowel disease, Behcet's disease, and Whipple's disease. However, this patient's examination findings provide us with an clue which effectively gives away the diagnosis: the presence of subungual hyperkeratosis and onycholysis. These clinical findings are characteristic of psoriatic arthritis (PsA). The absence of functional impairment leads further credence towards this diagnosis. Rheumatoid factor testing is negative, as expected (however, it is important to appreciated that 5% to 9% of patients with PsA are seropositive). In addition, X-Rays show erosive changes in the affected joints, which are typical of inflammatory arthritis. ESR and CRP levels should always be measured in these patients; their elevation here suggests ongoing active inflammation (as expected); these can subsequently be used to monitor the response to treatment. ANA levels are mainly of use in screening for conditions such as SLE, Sjogren's syndrome, scleroderma, polymyositis, rheumatoid arthritis, and mixed connective tissue disease, and are probably not indicated in this patient. NSAIDs are beneficial due to their combined analgesic and anti-inflammatory effects. Sulfasalazine may help slow the progression of joint damage, particularly in mild cases of PsA. Physiotherapy is extremely important, as this will result in a better functional outcome. Anti-TNF agents should not be commenced immediately; they may be considered in patients with severe disease unresponsive to other measures.


Discussion

Psoriatic arthritis (PsA) is a chronic inflammatory disease associated with psoriasis; it is classified as a spondyloarthropathy as it possesses the requisite clinical features. Psoriasis is estimated to affect around 7 million people in the United States alone; it is estimated that between 7% to 42% of them will go on to develop PsA. Unlike most of the other spondyloarthropathies, both genders are equally affected; the disease typically presents between 25 to 40 years of age. The pathogenesis and etiology of PsA are still unknown, although genetic, environmental and immunological factors are thought to play a role. Around 40% of patients report a first degree family history of Psoriasis or PsA; the disease is also known to be associated with certain Human leukocyte antigen (HLA) subtypes - particularly HLA-B27. In most individuals, Psoriasis precedes the onset of arthritis; however, in a minority, PsA develops before the dermatological manifestations of Psoriasis. PsA can present in a very heterogeneous manner; in particular, five major presentations of joint disease have been recognized: Asymmetrical inflammatory oligoarthritis, Symmetrical polyarthritis, Distal interphalangeal arthropathy, Psoriatic spondylitis, and Arthritis mutilans. Asymmetrical inflammatory oligoarthritis is the commonest presentation; here, the small joints of the hands and feet are involved initially; this subsequently progresses into an enthesitis and inflammation of the intervening tissue, resulting in dactylitis (sausage shaped digits). In addition, a couple of large joints may also be involved - particularly the knees. The clinical presentation of symmetrical polyarthritis is similar to that of rheumatoid arthritis (RA); however the symptoms and signs are usually milder, while nodules and the extra articular manifestations of RA are not seen. As the name suggests, distal interphalangeal arthropathy affects the DIPs. This form of the disease is more common in men and almost always associated with nail changes. Psoriatic spondylitis too has a male predominance; the presentation is similar to that of ankylosing spondylitis. Arthritis mutilans is the rarest form, occurring in only 5% of patients. Severe erosive arthritis results in shortening of the bones, producing telescoping fingers; radiography of the hands demonstrates the characteristic 'pencil in cup' appearance. Note that even though the deformities in arthritis mutilans appear severe and alarming, the pain is mild and hand function is surprisingly good. Almost any psoriatic skin manifestation may be found in PsA; note that if the diagnosis is suspected but no obvious lesions are present, it is important to look for hidden lesions in the scalp , perineum, intergluteal cleft, and umbilicus. Note that nail changes such as pitting, onycholysis, subungual hyperkeratosis, and leukonychia are common and may occasionally be the only dermatological manifestation. The diagnosis of PsA is generally made upon clinical grounds aided by the presence of characteristic radiographic features. Key radiographic findings include asymmetrical erosive changes in the small joints of hands and feet, osteolysis with a 'pencil in cup' appearance, enthesopathy related calcification, and spur formation. The absence of Rheumatoid Factor lends further credence towards the diagnosis - but it is important to note that 5% to 9% of patients are seropositive. ESR and CRP are often raised and help measure disease activity. The management of PsA depends on the severity of the disease; milder disease can be managed with NSAIDS and adjunct physical therapy alone, with local corticosteroid injections as needed. Note that certain NSAIDs may result in worsening of skin lesions in certain individuals; in such a scenario, changing the class of NSAID often works. Disease modifying antirheumatic drugs (DMARDS) are indicated in patients with moderate to severe disease or refractory cases of PsA; biological agents (i.e. TNF-Alpha antagonists) are also used in severe disease refractory to traditional drugs. Interestingly enough, some of the agents used in the management of Psoriasis (particularly systemic agents such as methotrexate, retinoic-acid derivatives, and psoralen plus ultraviolet light) have been shown to reduce both skin and joint manifestations. Physiotherapy for rehabilitation should not be forgotten, as severe PsA may result in significant disability causing marked impairment of quality of life. The prognosis of PsA is linked to a variety of factors; a male gender, involvement of fewer joints, and good preservation of function is associated with a good prognosis and disease remissions. Conversely, certain HLA types, and high inflammatory marker levels with evidence of radiological damage are adverse prognostic factors suggesting at more disabling disease.


Take home messages

  1. Rheumatology is often a pattern matching exercise; key characteristics of use include the number, symmetry and type of joint involved.
  2. Psoriatic arthritis is an important seronegative spondyloarthropathy, with most often presents as an symmetrical inflammatory oligoarthritis.
  3. In most patients, psoriatic skin manifestation precede the arthropathy, although the converse can also occur.
  4. The management depends on the severity of the disease, with milder cases requiring NSAIDs alone, and more severe cases, immunosuppression.

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