# Diagnostic Radiology/Musculoskeletal Imaging/Joint Disorders/Psoriatic arthritis

Psoriatic arthritis (PsA) is a type of inflammatory arthritis that affects around 20% of people suffering from the chronic skin condition psoriasis. It occurs more commonly in patients with tissue type HLA-B27. Treatment of psoriatic arthritis is similar to that of rheumatoid arthritis. More than 80% of patients with psoriatic arthritis will have psoriatic nail lesions characterised by pitting of the nails. Psoriatic arthritis is said to be a sero-negative spondyloarthropathy.
Psoriatic arthritis can develop at any age, however on average it tends to appear about 10 years after the first signs of psoriasis. For the majority of people this is between the ages of 30 and 50, but it can also affect children. Men and women are equally affected by this condition. In about one in seven cases the arthritis symptoms may occur before any skin involvement.
As well as causing joint inflammation, psoriatic arthritis can cause tendonitis and a sausage-like swelling of the digits known as dactilytis.
* 2-17% of patients with psoriasis get arthritis
* skin lesions and arthritis are often asynchronous: 80-85% of pts have skin lesions first
* synovial inflammation leading to bony proliferation at joint margins
* inflammation at the ligamentous attachments: enthesopathy
* Age of onset usually 30–45 years, no gender preference

## Prognostic Factors
### Poor prognosis

* late age of onset
* five or more effused joints
* high immunosuppressive medication use

### Good prognosis

* ESR < 15 mm/h

## Labs

* HLA B27 positive in 60-80% of psoriatic spondylitis and 20% of peripheral PsA
* CRP - usually elevated
* may be elevated: acute serum amyloid A (A-SAA)
* ANA may be mildly elevated
* Rheumatoid factor negative (or mildly elevated with titer<1/40, which is cut-off for Rheumatoid Arthritis (RA))

## Pattern of Presentation

* asymmetric oligoarthritis: > 50%
* polyarthiritis with predominantly DIP involvement (classic): 5-19%
* symmetric seronegative polyarthritis simulating RA: up to 25%
* sacroiliitis and spondylitis resembling Ankylosing Spondylitis (AS): 20-40%
* arthiritis mutilans with resorption of the phalanges: 5%
Onset may be insidious (66%) or acute (33%) mimicking gout or septic arthritis

## Radiologic Findings
### Early Findings

* normal
* soft tissue swelling: sausage fingers or fusiform swelling about finger joint
* usually no osteoporosis (˙≠ RA)
* prominent erosions in marginal areas of joints
* Perisostitis in metaphysis and diaphysis
* asymmetric paravertebral ossification of thoraco- lumbar junction (from vertebral body to body ˙≠ AS: corner to corner)
* bone scans may show hot spots prior to radiographic abnormality

### Distribution of radiographic findings

* synovial or cartilaginous joints and tendon attachment of axial and appendicular skeleton (= Reiter = AS ˙≠ RA)
* unilateral or asymmetric (˙≠ RA) at the hands and feet
* upper and lower extremity joints (˙≠ Reiter: more commonly lower extremity only)
* DIP, PIP of hand and foot commonly affected
* Abnormal phalangeal tufts and calcaneus (characteristic)
* If axial skeleton: most commonly sacroiliac (SI) and spinal joints

### General findings

* Soft tissue swelling
* No osteopenia (˙≠ RA)
* Joint space may be widened or narrowed
* Severe marginal erosions gnawing away bone towards the center of the joint are typical
* Pencil-and-cup appearance of small joints of hand and feet (DDX: RA, leprosy, sarcoid)
* Bone proliferation (=other seronegative spondylarthropathies = gout): spiculated, frayed, paintbrush appearance (˙≠ RA: no bone deposition)
* Perisostitis in metaphysis and diaphysis (= Reiter= Juvenile RA =infection) sometimes accompanied by condensation of bone
* Intraarticular osseous fusion ( AKA bone ankylosis) (DDX: erosive OA, RA (carpus and tarsus), infection, other seronegative spondylarthropathies)
* Enthesopathy of calcaneus (Achilles tendon), femoral trochanters, ischial tuberosities, malleoli, olecranon, patella, femoral condyles.
* Tuft resorption of the distal phalanges of the hands and feet (characteristic) (DDX: scleroderma, thermal injury)

### Erosions

* Start at joint margins, tend to be severe
* Proceed centrally into joint
* Result in irregular osseous surfaces: lack of apposition of adjacent bone margins (˙≠ OA)
* Wrist abnormalities not so common in psoriasis

### Calcaneus

* Erosion and proliferation of posterior or inferior surface
* Radiodense area postero-inferior due to retrocalcaneal bursitis
* Achilles tendon may be thickened.

### SI joints

* 30-50% of patients with PsA
* erosions and sclerosis of SI joints
* bilateral lesions are more common than unilateral

### Spine

* asymmetric paravertebral ossification of lower thoraco- lumbar junction (from vertebral body to body ˙≠ AS: corner to corner)
* Syndesmophytes are greater in size, asymmetric distribution, away form vertebral column ˙≠ AS
* squaring of vertebral bodies, apophyseal sclerosis uncommon ˙≠ AS
* Cspine abnormalities may be extensive: facets, discovertebral, proliferations along anterior surface, atlantoaxial subluxation, dens erosions.

### Scintigraphy

* Bone scans may show abnormality prior to radiographs
* asymmetric ˙≠ RA
* DIP, PIP >> SI, calcaneus
Lumbar spine showing bilateral sacroiliitis
Fig. 1 Lumbar spine showing bilateral sacroiliitis
Psoriatic arthritis with proliferative osteophyte on heel
Fig. 2 Psoriatic arthritis with proliferative osteophyte on heel
T1-weighted sagittal MR of psoriatic involvement of retrocalcaneal bursa and distal Achilles tendon
Fig. 3 T1-weighted sagittal MR of psoriatic involvement of retrocalcaneal bursa and distal Achilles tendon
T1-weighted sagittal MR of psoriatic involvement of retrocalcaneal bursa and distal Achilles tendon
Fig. 4 T1-weighted sagittal MR of psoriatic involvement of retrocalcaneal bursa and distal Achilles tendon
Radionuclide bone scan in a patient with psoriatic arthritis and a swollen 5th finger. This delayed image shows markedly increased uptake in the 5th finger.
Fig. 5 Radionuclide bone scan in a patient with psoriatic arthritis and a swollen 5th finger. This delayed image shows markedly increased uptake in the 5th finger.
Hand film from same patient shows swollen 5th digit from psoriatic arthritis.
Fig. 6 Hand film from same patient shows swollen 5th digit from psoriatic arthritis.
Detail view of 5th finger from same patient shows erosions from psoriatic arthritis
Fig. 7 Detail view of 5th finger from same patient shows erosions from psoriatic arthritis.

## References

* Psoriatic Arthritis by Ken Linnau, M.D., University of Washington Department of Radiology.