The majority of patients with psoriatic arthritis have symmetric polyarthritis or asymmetric oligoarthritis of the hands and feet. Often the DIP joints become stiff, swollen, and tender in an asymmetric fashion. When present, involvement of the DIP joints helps distinguish psoriatic arthritis from rheumatoid arthritis, but sometimes results in confusion with osteoarthritis or gout. Other joints that are affected by psoriatic arthritis include the knees, hips, and sternoclavicular joints.
Regardless of the number of symptomatic joints at disease onset, most patients progress to additional joint involvement in the absence of effective treatment. There is ongoing destruction of joints, as evidenced clinically by the appearance of joint deformities and radiographically by the development of juxta-articular erosions, joint-space narrowing, and, in some cases, bony ankylosis. Arthritis mutilans describes the end stage of the destructive process, where loss of bony architecture allows complete subluxation and telescoping of the involved digit (“doigt en lorgnette” or opera-glass finger). This phenomenon is uncommon and is associated with long-standing, poorly controlled disease.
Dactylitis, or “sausage digit,” is the complete swelling of a single digit of the hand or foot (Figure 19–1). It is a distinctive feature of the spondyloarthropathies, and it is common in psoriatic arthritis, occurring in one-third to one-half of patients at some point during the course of the disease. Toes are more frequently involved than fingers. Dactylitis is associated with more severe radiographic joint damage.
Dactylitis of the ring finger of a patient with psoriatic arthritis. (Used with permission from Dr. J. Graf, University of California, San Francisco.)
Enthesitis is an inflammatory process occurring at the site of insertion of tendons into bone. This is a feature common to other spondyloarthropathies and occurs in up to 40% of psoriatic arthritis patients. On physical examination, there is a soft tissue swelling usually accompanied by tenderness to palpation and sometimes by overlying erythema and warmth as well. Common sites for enthesitis are the Achilles tendon, plantar fascia, and pelvic bones. Entheseal inflammation may evolve to destruction of the adjacent bone and joints.
All forms of psoriasis are associated with arthritis, although classic psoriasis vulgaris is seen most frequently. Typical psoriatic lesions are erythematous plaques that produce scaling with scratching. Interestingly, many patients with psoriatic arthritis have only mild to moderate skin disease, and there has been no consistent correlation between the degree of psoriasis and the extent of joint involvement. The psoriasis may be subtle. Therefore, careful examination of the entire skin surface must be performed when psoriatic arthritis is suspected, with particular attention to the hairline, scalp, external auditory canal, periumbilical area, and gluteal cleft.
As with uncomplicated psoriasis, nail involvement is common in psoriatic arthritis (Figure 19–2). Psoriatic nail changes include ridging, pitting, onycholysis, and hyperkeratosis, and may represent the manifestation of psoriasis before the presence of more characteristic skin lesions. Nail changes on the affected finger virtually always occur when psoriatic arthritis affects a DIP joint.
Psoriatic nail changes with onycholysis and subungual debris. (Used with permission from Dr. J. Graf, University of California, San Francisco.)
Symptomatic involvement of the sacroiliac joints and axial skeleton is less common than peripheral joints. Inflammation of the sacroiliac joints (sacroiliitis) in psoriatic arthritis is usually unilateral and presents with pain and stiffness in the lower back or buttock. Tenderness can sometimes be elicited by direct palpation of the joints by applying firm pressure with the thumbs when the examiner places his or her palms over the patient’s iliac crest—the thumbs will tend to fall directly over the joints. Another maneuver that may detect sacroiliitis is the Gaenslen test, in which the patient (in either the supine or the lateral recumbent position) flexes one leg at the hip with the knee close to the chest and hyperextends the other leg over the examination table. This applies stress to the sacroiliac joints and is considered positive if pain is elicited at the sacroiliac joint. However, the reliability of physical examination findings for detection of sacroiliitis is poor, and noninflammatory processes may elicit positive findings. Plain radiography of the pelvis and Ferguson views that focus on the sacroiliac joints may aid in the detection of inflammatory disease of this joint.
A common site of skeletal involvement in psoriatic arthritis is the cervical spine. Here, as in rheumatoid arthritis, extensive inflammation and erosion may lead to atlantoaxial (C1–C2) instability, which can produce cervical myelopathy as the odontoid process erodes. This process is often clinically silent and painless. Involvement of other levels of the spine is also seen in psoriatic arthritis with syndesmophytes, which often arise from the midpoint of a vertebral body, bridge adjacent vertebrae, and restrict motion of the spine. In contrast to the continuous ascending spinal involvement in ankylosing spondylitis, psoriatic spinal involvement is frequently discontinuous, affecting noncontiguous vertebrae or areas (Figure 19–3).
Psoriatic spondylitis. Extensive spinal involvement has led to exaggerated thoracic kyphosis and loss of cervical extension. As a result, the patient is unable to touch the occiput to the wall when standing against the wall (“occiput-to-wall” test). There is limited chest expansion leading to a protuberant abdomen and to diaphragmatic breathing. (Used with permission from Dr. J. Graf, University of California, San Francisco.)
Ocular inflammation (eg, conjunctivitis, iritis, scleritis, and episcleritis), oral ulcerations, and urethritis occur in psoriatic arthritis, but less frequently than in the other spondyloarthropathies.
There are no laboratory tests diagnostic for psoriatic arthritis. Up to 20% of patients have hyperuricemia. Because of the systemic, inflammatory nature of the disease, acute phase reactants, such as the C-reactive protein and the erythrocyte sedimentation rate, may be elevated, although typically not as high as that seen in other inflammatory arthritides, such as rheumatoid arthritis. In some patients, elevations of acute phase reactants correlate with disease activity, more commonly in patients with a higher number of affected joints.
Synovial fluid analysis reveals inflammatory fluid, with white blood cell counts usually in the 5000–50,000/mcL range.
Patients with psoriatic arthritis usually do not have rheumatoid factor, but up to 10% of patients with psoriatic arthritis may test positive. A positive rheumatoid factor is not an exclusion criterion for the diagnosis of psoriatic arthritis. Antibodies to cyclic citrullinated peptides are sensitive and specific for rheumatoid arthritis. These antibodies have only rarely been reported in patients with psoriatic arthritis, and, in most cases to date have been seen in patients with polyarticular symmetric presentations that suggest the co-occurrence of psoriasis with rheumatoid arthritis. Antinuclear antibodies are detected in 10–20% of patients, which is comparable to the prevalence of antinuclear antibody positivity in healthy control populations.
The most common radiographic findings in psoriatic arthritis are joint-space narrowing and erosions involving the DIP and proximal interphalangeal joints. Typically, these findings are asymmetric, paralleling the pattern of the clinical arthritis. The metacarpophalangeal joints and wrists are less frequently involved than in rheumatoid arthritis. In addition, periarticular osteopenia (decreased bony density adjacent to the joints) is usually absent in psoriatic arthritis, another feature that helps distinguish psoriatic arthritis from rheumatoid arthritis.
Severe destructive changes of the joints may occur with long-standing disease but may also develop rapidly in a single joint, resulting in a whittling phenomenon of the bone. When a phalanx is involved, it becomes “penciled,” thus giving rise to the classic “pencil-in-cup” deformity when it abuts the base of an adjacent phalanx. Marked osteolysis results in widening of the spaces between joints and eventual complete disorganization of the joint architecture, described as arthritis mutilans. Subluxations can occur, which give the clinical manifestation of telescoping digits.
In contrast to rheumatoid arthritis, psoriatic arthritis can produce proliferative bony changes adjacent to erosive and osteolytic changes in the same bone. This new bone formation often occurs along the shaft of the metacarpal and metatarsal bones and is seen as a fluffy periostitis. Rheumatologists and radiologists may use the term “whiskering” to describe these proliferative changes.
Although not performed routinely yet in the clinical setting, joint imaging with ultrasound and MRI has gained increasing popularity among rheumatologists and can show synovitis, entheseal inflammation, and erosions on ultrasound and MRI and subchondral bone marrow changes with MRI.