RESUMEN
It has been seen that involvement of the foot in the seronegative arthropathies forms a regular and varied part of the clinical picture. This is often quite different from that seen in rheumatoid arthritis; its components, whether in joints, periarticular structures, or as surface manifestations, may be characteristic enough to raise the diagnosis of 'spondarthritis'. The features described, though characteristic of the spondarthritides, are, however, not pathognomonic. Thus, the osteolysis in psoriatic arthritis also occurs in neuropathic arthritis (e.g. syringomyelia, leprosy), psoriatic periosteal changes may mimic osteosarcomatous proliferations, and the calcaneal enthesitis so typical of spondylitis, Reiter's disease and psoriatic arthritis, may also be seen in metabolic arthropathies. It should also be mentioned here that the severe erosive osteolytic changes leading to psoriatic arthritis mutilans may also be seen, albeit rarely, in rheumatoid arthritis. Ankylosis, too, is not totally confined to the spondarthritides, having also been reported in occasional patients with rheumatoid arthritis. Calcaneal erosions, sometimes envisaged as a spondarthritic feature, also occur in rheumatoid patients. Within the spondarthritis matrix, a striking overlap is seen in the pattern of arthritis. Thus, involvement of the feet in psoriatic arthritis and in Reiter's disease shows many similarities, particularly the tendency to involve IP joints in asymmetrical oligoarticular fashion. In the hindfoot, too, parallels can be drawn between the tendency to Achilles and plantar insertion enthesitis in ankylosing spondylitis and Reiter's disease. On the other hand, the arthropathies of the chronic inflammatory bowel diseases, ulcerative colitis, Crohn's disease, and Whipple's disease, share with Behçet's syndrome an asymmetrical involvement of knees and ankles, but relative freedom from foot involvement. Regarding the surface features in the foot of spondarthritides, there is overlap here, too. For example, the nail dystrophy of psoriasis can be indistinguishable from that of Reiter's disease, and pustular psoriasis in its severe form cannot be differentiated from keratoderma blenorrhagica, even at the histological level. Other surface manifestations affecting the lower limb in general distribution may spread to the feet and thus fall within the ambit of this discussion. Such features include the lesions of erythema nodosum, patches of pyoderma gangrenosum, and the tender cords of thrombophlebitis, all of which have a higher prevalence in seronegative arthritis than in seropositive disease.(ABSTRACT TRUNCATED AT 400 WORDS)