Psoriatic Arthritis Imaging 

According to the National Psoriasis Foundation's 2001 Benchmark Survey,^[1] psoriatic arthritis (PA) is a specific type of arthritis that has been diagnosed in approximately 23% of those who have psoriasis.^[2] PA can occur in any age group; however, in most patients, it manifests itself in patients from 30-50 years of age. On average, PA appears about 10 years after the first signs of psoriasis occur, but in about 1 of 7 people with PA, arthritis symptoms occur before any skin lesions appear. Most patients with PA also have psoriasis; patients rarely have PA without psoriasis.

See the psoriatic arthritis images below.

Stamm et al used the following 9 instruments in their analysis: Arthritis Impact Measurement Scale Short Form; Bath Ankylosing Spondylitis Disease Activity Index; Disabilities of the Arm, Shoulder, and Hand Questionnaire; Dermatology Quality of Life Index; Dougados Functional Index; Health Assessment Questionnaire (HAQ); HAQ-S (HAQ adapted for spondylarthropathies); PsA-specific Quality of Life Instrument; and Short Form 36 Health Survey.^[3] They found that the impact of environmental factors, attitudes toward individuals with health problems, and loss of leisure time may represent important aspects that are not addressed through the instruments currently used to assess function in PA.

Prasad et al studied 472 psoriatic patients, 40 of whom had PA. They found that the duration of skin lesions and duration of arthropathy correlate with each other.^[4] PASI scores also correlate with arthropathy.

Setty et al noted that the quality of life of patients with PA appeared similar to that of patients with rheumatoid arthritis (RA).^[5] The available data on mortality in PA are conflicting, leaving unanswered questions concerning mortality. Rohekar et al found that overall, 10.2% of patients in the Toronto PA cohort developed cancer.^[6] The most frequent cancers were cancers of the breast, lung, and prostate. The incidence of malignancy in the large PA cohort did not differ from that in the general population. This finding differs from that reported in Gelfand's study, in which the odds ratio of psoriasis patients developing lymphoma was higher.^[7]

Mullan et al noted that early changes in serum type II collagen biomarkers predicted radiographic progression at 1 year in patients with inflammatory PA after biologic treatments.^[8]

Types of PA

Five types of PA have been defined; these types can coexist, but they tend to occur separately in most cases:

• Arthritis involving primarily the small joints of the fingers or toes (asymmetrical oligoarthritis) — 55-70%
• Asymmetrical arthritis, which involves the joints of the extremities — 30-50%
• Symmetrical polyarthritis, which resembles RA — 15-70%
• Arthritis mutilans, which is a rare but deforming and destructive condition — 3-5%
• Arthritis of the sacroiliac joints and spine (psoriatic spondylitis) — 5-33%. Gladman et al noted that the definition of axial disease in PA ranges from isolated unilateral grade 2 sacroiliitis to criteria similar to the criteria used for making a diagnosis of ankylosing spondylitis.^[9] Depending on the criteria used, the prevalence of axial disease varies from 25% to 70% of patients with psoriatic arthritis.

Preferred examination

PA is diagnosed and assessed with radiography, which is the cornerstone in assessing and monitoring inflammatory arthritides such as PA. Radiographic findings are reproducible and allow for the serial monitoring of patients. Although magnetic resonance imaging (MRI) is more sensitive, the cost of this modality makes it a second-line means for monitoring patients with PA.^{[10, 11, 12, 13]}

Classic presentation

Classic, but not wholly pathognomonic, associations of PA include the following:

• Nail involvement such as pitting and separation from the nail bed (onycholysis), as well as yellow-pink discoloration (the oil-drop sign)
• Sausage digits (dactylitis)
• Inflammation at the sites of ligamentous and tendinous insertions (enthesopathy)
• An absence of rheumatoid factor (RF)

Psoriasis is mostly a disease of the skin wherein some stimulus leads to a hyperproliferation of skin cells, which then results in an increase in the turnover of such cells. Patients develop papules and plaques with micalike scales; such plaques have a predilection for the sacrum, elbows, and scalp. PA can lead to joint swelling and, in the most severe case, sausagelike digits. Perioperative management of tumor necrosis factor antagonists in patients with psoriasis and other inflammatory disorders has been recorded.^[14] Synovitis can manifest as edema, joint effusion, or tendinitis (see Images below).

Symmetrical polyarthritis with joint pain and joint swelling furnishes a single pattern of clinical manifestations that often indicates erosive progressive disease.^[15] Unlike RA, PA regularly involves distal interphalangeal joints.

A haplotype epidemiologic association with PA involves the expression of both class I and class II human leukocyte antigen (HLA) alleles, including HLA-B13, HLA-B17, HLA-B27, HLA-B38, HLA-B39, HLA-Cw6, HLA-DR4, and HLA-DR7. HLA-27 is present in 60% of individuals with the disease, as compared with 8% of the general population.

Prognosis

Factors that portend a worse prognosis for patients with PA include the following:

• A strong family history of psoriasis
• Disease onset younger than age 20 years
• Expression of HLA-B27, HLA-Cw6, or HLA-DR4 alleles
• Polyarticular disease
• Erosive disease
• Extensive skin involvement

Systemic involvement can occur with ocular changes (30%), conjunctivitis, episcleritis, and keratoconjunctivitis sicca. Aortic valve disease has also been described. Because of the high skin turnover, hyperuricemia and gout can coincide with psoriasis. An association with human immunodeficiency virus (HIV) infection has also been identified; in such cases, both the psoriatic eruption and PA can be severe. Celiac disease has also been reported.^{[16, 17, 18, 19, 20]}

Differential diagnosis

Differential diagnosis includes ankylosing spondylitis and rheumatoid arthritis of the hands and spine. Enteropathic arthritis (arthritis of inflammatory bowel disease) should also be considered, and spotted bone disease has been reported in a patient with psoriatic arthritis.^[21]

Patient education

For excellent patient education resources, visit eMedicine's Psoriasis Center. Also, see eMedicine's patient education articles Types of Psoriasis, Psoriatic Arthritis, and Understanding Psoriasis Medications.

Plain radiography is the main imaging modality for assessing PA, although early in PA, there may be no radiographic findings. Soft tissue swelling often precedes osseous findings.

See the radiographic images below.

Fingers

Fournie et al prospectively compared power Doppler ultrasound findings in 25 fingers of patients who had RA with findings in 25 fingers of patients who had PA.^[22] In both RA and PA, patients had erosive synovitis and tenosynovitis.

Extrasynovial changes occurred in 21 of 25 patients (84%) with PA, whereas no patients with RA had extrasynovial changes. Of the 21 patients with PA whose fingers showed extrasynovial changes, synovial changes were apparent in 60%.

The extrasynovial changes in patients with PA reflected the following:

• Enthesitis
• Soft tissue inflammation

The main patterns of the extrasynovial changes were the following:

• Capsular enthesophyte
• Juxta-articular periosteal reaction
• Enthesopathy at the site of deep flexor tendon insertion on the distal phalanx
• Subcutaneous soft tissue thickening of the finger pad
• Subcutaneous soft tissue thickening of the entire finger

Pseudotenosynovitis, which is marked by diffuse inflammation of the digital soft tissues, occurred rarely.

Hands and feet

Findings of PA of the hands and feet include the following:

• No or minimal juxta-articular osteoporosis: In RA, this is more prominent.
• Bony proliferation near joints and ligament and/or tendon insertion sites (ie, entheses): Enthesis sites include the calcaneus, ischial tuberosities, femoral trochanters, ankle malleoli, anterior patella, ulnar olecranon, and condyles of the distal femur and proximal tibia. This is not a feature of RA.
• Bone erosion beginning in the periarticular region and progressing to more central areas
• Asymmetrical destruction of the DIP with bony ankylosis: RA affects the metacarpal phalangeal joint and proximal interphalangeal joint [PIP] to a great extent.
• Resorption of terminal phalangeal tufts (ie, the morning-star appearance)
• Osteolysis of the bone with telescoping of the digits (ie, the pencil-in-cup deformity)
• Periostitis along the shaft of a bone, often accompanied by soft tissue swelling
• Ivory phalanx
• Destruction/resorption of the interphalangeal joint of the first toe, with periosteal reaction and bony proliferation at the distal phalangeal base: This finding is highly suggestive of PA.
• Diffuse soft tissue swelling of an entire digit (ie, sausage digit)
• General symmetrical joint involvement
• Charcot-like arthropathy is a newly recognized subset of PA.^[23]
• Bond et al concluded that the number of actively inflamed joints — specifically, the number of swollen joints — could be predictive of damage progression, as visualized radiologically.^[24] A higher risk of damage progression was related to a higher number of previously inflamed and damaged joints.

Healy et al assessed 17 patients with psoriatic dactylitis who underwent a change of treatment, usually to methotrexate.^[25] MRI scans of the affected hand or foot were performed before and after treatment; a 1.5T Siemen's scanner was used both before and after administration of contrast. MRI images demonstrate widespread abnormalities in digits of people with PA. Tender dactylitic digits have more abnormalities than other digits, but the relationship between clinical findings and MRI scores is not strong.

Healy et al used the following tools^[25] :

1. The Leeds Dactylitis Index
2. The assessment tool used in the Infliximab in Psoriatic Arthritis Clinical Trial (IMPACT)
3. A simple count of tender dactylitic digits
4. A count of all dactylitic digits, both tender and nontender

They reported the following results:

1. All patients improved clinically; improvement was also seen in the patients' dactylitis scores and in MRI images.
2. The findings on MRI in both dactylitic and nondactylitic digits were profound and widespread.
3. The difference between tender and nontender dactylitis was quantitative rather than qualitative.
4. Synovitis and soft tissue edema were the most frequent abnormalities, being present in 69% of tender dactylitic digits; bone edema and flexor tenosynovitis were also frequently seen.
5. Soft tissue edema was circumferential and enhancing and was not limited to the flexor or extensor tendons.
6. There was not a close relationship between the extent of abnormalities seen on MRI and any of the clinical indices of dactylitis.

Spine

Findings of PA of the spine include the following:

• Asymmetrical paravertebral ossification from the lower thoracic spine to the upper lumbar spine: This may be vertically directed and may not always be attached to the vertical margins, as in ankylosing spondylitis.
• Bony proliferation along anterior cervical spine and apophyseal joint-space narrowing
• Squaring of the lumbar vertebrae in L-spine: This occurs in ankylosing spondylitis but is uncommon in PA.
• Paravertebral soft tissue calcifications
• Synovitis: This may cause atlantoaxial subluxation in a few patients.

Sacroiliac joints

Findings of PA of the sacroiliac joints include the following:

• Bilateral, often asymmetrical, sacroiliitis: However, joints do not usually fuse, as in ankylosing spondylitis.
• Marginal erosions
• Widening of the joint margin and sclerosis: This is less common in PA than in ankylosing spondylitis.

Other sites

Findings of PA of other sites include the following:

• Erosions in the temporomandibular joints
• Sternal synchondrosis

The radiographic appearance of PA can be similar to that of RA. The distinguishing features of PA include the following:

• The presence of bony ankylosis
• The presence of bony proliferation in periarticular and enthesis sites
• The absence of osteopenia

A pattern of predominant DIP involvement can also distinguish PA from RA. In the axial skeleton, cervical spinal involvement can be prominent in RA, but thoracic and lumbar changes are unusual. Sacroiliac involvement is a minor feature of RA, but large joints are more involved in RA than in PA.

The distinction between PA and other seronegative spondyloarthropathies is largely made on the basis of distribution. PA involves both the upper and lower extremities, whereas Reiter syndrome predominantly involves the lower extremities. Ankylosing spondylitis causes changes mainly in the axial skeleton, with bilateral and symmetrical changes being the rule. The spinal bony excrescences of PA and Reiter syndrome are larger and broader than the typical thin, linear syndesmophytes of ankylosing spondylitis.

An entity often grouped with psoriasis is pustulosis palmaris et plantaris, or PPP. Mejjad et al reported that palmaris et plantaris and PA can be distinguished radiologically.^[26] The anterior chest wall, especially the sternocostoclavicular joints, is more frequently involved in pustulotic arthritis than in psoriasis, both clinically and radiologically. Sternocostoclavicular joints generally appear with erosive lesions in psoriasis but with large ossifications in palmaris et plantaris.

Peripheral joint involvement may be monoarticular or oligoarticular and affect the proximal joints in palmaris et plantaris (74% vs 21%); or it may be polyarticular and involve small distal joints in psoriasis (60% vs 0%).^[26] In PA, peripheral joint involvement is more often erosive (43% vs 8%). The frequency rates of sacroiliitis and spinal involvement are similar in palmaris et plantaris and psoriasis. Biologic features and bone scans do not help in distinguishing these 2 conditions.

Achilles tendon

Ozcakar et al noted that the psoriatic group of patients in their study had thicker tendon measurements than the control group and that those patients with radiologically proven enthesopathy had even thicker tendon measurements.^[27]

Differentiation of PA from RA

Differentiating PA from RA can be difficult; however, the following may aid in the diagnostic effort:

• PA maintains bone mineralization and has periosteal reaction and new-bone formation.
• PA is RF negative, whereas RA is RF positive.
• PA does not manifest with rheumatoid nodules.
• Sausage digit and spontaneous joint fusion are common in PA and Reiter syndrome but not in RA.
• In addition, spondylitic changes are similar in PA and Reiter syndrome, but PA affects the hands more than Reiter syndrome does.

A study by Fournie et al is promising in that it helps distinguish PA from RA.^[22]

Although plain radiography is the primary modality in assessing PA, computed tomography (CT) scanning can provide further information about the extent and severity of disease. This is particularly true in areas that are difficult to evaluate with radiographs; such areas include the sacroiliac joint, the temporomandibular joint, and the sternum/manubrium.

In particular, CT scanning is useful in identifying inflammatory lesions, even when preexisting degenerative disease is present; in demonstrating the articular surfaces of bone in an exact fashion; and, in some cases of sacroiliitis, in clearly demonstrating erosive changes that can appear equivocal or negative with radiography.

See the CT image of PA below.

Furthermore, CT scans may demonstrate a variety of findings in PA, including right sternal bone sclerosis with surrounding subchondral erosion and a clavicular bone sclerosis, and can show bilateral clavicular osteophytes and discrete surrounding subchondral cysts and erosions.^[28]

Findings on magnetic resonance imaging (MRI) are the most sensitive and specific for sacroiliitis and for other changes in the axial skeleton and in the hands and feet. With MRI, it is possible to identify the early inflammatory phase of enthesitis before the development of erosion, as seen on radiographs. MRI can depict early cortical erosion, inflammatory granulation tissue, and bone marrow edema.^{[29, 30, 31, 32]}

Marzo-Ortega et al found infliximab to be efficacious in reducing bone edema in psoriatic arthritis, as assessed with MRI.^[33]

In patients with PA, MRIs of the temporomandibular joint can show slight anterior displacement of the disk, as well as large condylar erosions and joint effusion, either with or without the use of contrast agents.

Morrison et al summarized psoriatic findings in the ankle and foot.^[34] The sausage digit is a typical clinical finding in PA and can be seen as diffuse soft tissue edema of the digits. On MRIs, tenosynovitis in association with an increase in fluid in the tendon sheath, as well as enhancement related to synovial proliferation, may be seen.

The terminal tuft may show edema and enhancement with psoriatic involvement of the nail bed. The bursae may demonstrate fluid and enhancing synovial tissue; this is true for the anatomic bursae—in particular, the retrocalcaneal bursa and the adventitial bursa—as a result of friction.

Enthesis inflammation manifests itself as plantar fasciitis, with edema and enhancement of the proximal plantar fascia and the adjacent calcaneus. Reactive edema in the adjacent tissue beneath the tendon is linked to involvement of the tendon sheath.

Effusions and synovial proliferation can be seen as a result of joint involvement. Synovial proliferation within joints is seen as "dirty fluid" on T2-weighted images. Inflamed synovial tissue is strongly enhancing on gadolinium-enhanced images. Chronic synovial proliferation can demonstrate a high T1 signal, representing hyperplasia of subsynovial fibrofatty connective tissue without a pannus.

Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans.

NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness.

Thickened nails with low signal intensity on T1-weighted MRIs are typical findings of nail involvement. MRI can show the underlying histopathologic changes of edematous and/or fibrotic synovial tissue, which can cause erosions of cortical bone and can eventually lead to complete destruction of the joint.

Offidani et al compared the use of MRI with the use of standard x-rays in the evaluation of patients with PA.^[35] With MRI, 68% of patients had positive findings of PA, as well as at least 1 arthritic sign. By contrast, with standard x-rays, only 32% of the same patients had positive findings of PA. Abnormal signal intensity in the subchondral focal areas was seen in 9 PA patients but in none of the control subjects. In a high percentage of psoriatic patients who had no apparent arthritic signs and symptoms, MRIs revealed hand articular involvement, particularly distention of the capsule and periarticular edema.

McQueen et al described the initial steps in forming an Outcome Measures in Rheumatology (OMERACT) MRI scoring system for peripheral PA.^{[36, 37]} Four readers scored a preexisting MRI dataset (finger joints) from 10 patients with peripheral PA. The physical findings that were assessed included the following:

• Bone erosion
• Bone edema
• Synovitis
• Tendinopathy
• Extracapsular features of inflammation (including enthesitis)

The results of McQueen et al's OMERACT MRI scoring system demonstrated the following:

• Bone edema and erosion showed moderate to high scoring reliability
• Soft tissue inflammation showed lower scoring reliability

Ultrasonography is useful for assessing the extent of disease, but it is not the method of choice for monitoring bone involvement in PA.^[38]

In PA, joint effusion and synovitis result in hypoechoic sonograms. In most cases, the joint space is widened and appears hypoechoic; in some cases, a pseudocystic image connected with the joint space can be observed. Increased thickness of the joint capsule can also be present. The joint capsule is usually clearly distinct from the condylar head, which appears as a linear hyperechoic line with posterior shadowing.

Today, ultrasonography is considered a highly specific technique, not only for demonstrating soft tissue alterations but also for visualizing bone abnormalities, especially in the examination of the small joints. Thus, alterations of the condyle are observed more frequently with ultrasonography than with MRI.

The ultrasonographic features of dactylitis in PA have been investigated, and flexor tenosynovitis was detected in more than 90% of the tested patients.^[39] An association between dactylitis and articular synovitis was noted in more than 50% of patients.

Bone scanning is a useful technique for assessing the inflammatory nature and extent of PA. Whole-body scintigraphy shows the distribution of active joint disease. Abnormal radiotracer uptake precedes findings on plain radiographs.

Bone scanning is highly sensitive but not specific; bone scans may show that inflammation exists, but a positive finding is not diagnostic of PA and must be correlated with clinical findings.

Enthesopathies may be readily detected on bone scans and show hyperactive foci. This is the case when technetium-99m (^99m Tc) bisphosphonate bone scans are centered on the chest wall, where the jugular notch, costal notches, and xiphoid are most commonly involved. Enthesopathy manifests itself with a radiologic triad: hyperostosis, osteitis, and periostitis syndrome.

Bone scans are also useful for assessing palmaris et plantaris and a related condition, pustulotic arthrosteitis (PAO). The bone scan can show characteristic, bullheadlike, high tracer uptake in the sternocostoclavicular region; in the bullhead sign, the manubrium sterni represents the upper skull of the bull, and the inflamed sternoclavicular joints correspond to the horns.^[40]

Other types of inflammatory arthritis, as well as infection and trauma, can sometimes mimic PA when a bone scan is used.^99m Tc phosphate scans show the location and distribution of active lesions. Scintigraphy enables the inexpensive and simple assessment of joints in the body, but the anatomic detail is poor and the specificity is low.