THE CONCEPT OF PSORIATIC ARTHRITIS (PsA)

There is no doubt that inflammatory arthritis/enthesitis and psoriasis coexist more frequently than would be expected by chance: for instance, in a study of 1285 patients with psoriasis seen in an hospital, 483 (38%) were suffering from arthritis/enthesitis, including 40 patients classified as Rheumatoid Arthritis (RA) (3%), 177 (14%) as undifferentiated arthritis (UA), and 266 (21%) as Psoriatic Arthritis (PsA) (1). Although lower percentages have been noticed in the general population with psoriasis (6% of PsA in an extensive study of 1844 patients with psoriasis) (2), they were superior to 5% (i.e. at least 5 times greater than the figures found for patients without psoriasis) (3-7). Similarly, psoriasis is slightly more frequent in patients with arthritis than in the population without arthritis (8). These observations, and the recognition of clinical or radiological features rather specific for PsA naturally led to the hypothesis that this condition did exist as an original entity, nosologically different from RA and other rheumatisms, although presenting as different subtypes (9). The reported features of PsA supporting this assumption, are: 1-frequent arthritis of distal interphalangeal joints (DIP) (10-12) (ascribed to the preferential involvement of enthesis (13) and related structures (14)), leading sometimes to dactylitis/osteitis of the whole distal phalanx (15)); 2-involvement of a single digit (inflammation in a ray pattern); 3-inflammation of the spine (including the cervical segment in most cases) (16-17), which remains often clinically silent (9) but can lead to some ankylosis, although usually less severe than Psoriatic arthritis as a mountain L’artrite psoriasica vista come una montagna