DIFFERENTIAL DIAGNOSIS.

Considerations in the differential diagnosis of RA are numerous (Table 286-3) . Early RA, especially that of acute onset, is more difficult to diagnose than is the typical established case. The finding of subcutaneous nodules and the presence of rheumatoid factor are useful but not absolutely specific differential features. Therefore, a complete medical evaluation, often including synovial fluid analysis, is indicated in all patients with significant joint manifestations.

ARTICULAR MANIFESTATIONS.

RA can affect any diarthrodial joint. Those most commonly involved are the small joints of the hands, wrists, knees, and feet. With time, the disease may also affect the elbows, shoulders, sternoclavicular joints, hips, and ankles. The temporomandibular and cricoarytenoid joints are less frequently involved. Spinal involvement in RA is generally limited to the upper cervical articulations. In contrast to the spondyloarthropathies, RA does not cause sacroiliitis or clinically significant disease in the lumbar or thoracic spinal areas.

HANDS.

Swelling of the PIP joints with a fusiform or spindle-shaped appearance of the fingers is one of the most common early signs. Bilateral and symmetrical swelling of the MCP joints is also frequent (see Fig. 286-2) . The DIP joints are usually spared, which is a useful sign in discriminating RA from osteoarthritis and psoriatic arthritis. Soft tissue laxity gives rise to ulnar deviation of the fingers at the MCP joints (Fig. 286-3 A). Swan neck deformities develop from hyperextension of the PIP joints in conjunction with flexion of the DIP joints (Fig. 286-3 B). Boutonniere (buttonhole) deformities result from flexion contractures of the PIP joints in association with hyperextension of the DIP joints. These changes result in a loss of strength and dexterity in the hands, as well as the ability to maintain a good pinch. Synovial erosions of extensor tendons, usually at the dorsum of the wrist, may lead to sudden rupture and loss of the ability to extend one or more fingers.

WRISTS.

The wrists are almost invariably involved in RA and frequently demonstrate easily palpable, boggy synovium, especially over the ulnar styloid. Loss of wrist motion, both flexion and extension, usually occurs to some degree. The median nerve on the volar side often becomes compressed by proliferating synovium, the result being carpal tunnel syndrome (Fig. 286-4) . The patient notes paresthesias or pain in the thumb, 2nd and 3rd digits, and radial side of the 4th digit. Symptoms are typically worse at night or with other activities associated with sustained flexion of the wrist. Tinel's (Fig. 286-4) and Phalen's (Fig. 286-5) signs can usually be elicited, and thenar muscle wasting may be evident.

KNEES.

Synovial proliferation and effusion are common in these weight-bearing joints. Effusions may be detected by performing ballottement on the patella or by observing a "bulge sign" along the medial aspect of the patella when fluid is pushed into the suprapatellar pouch and then expressed back into the joint. Quadriceps atrophy may occur, and a flexion contracture of the knee may compromise walking. Eventually, destruction of soft tissue around the knee can produce marked joint instability and valgus deformity. Popliteal (Baker's) cysts may form as a result of effusion or synovial

proliferation into the semimembranous bursa (Fig. 286-6) . Such synovial cysts may dissect or rupture into the calf and produce symptoms and signs mimicking those of thrombophlebitis. Ultrasonography and Doppler studies of the popliteal fossa and calf are useful