Pustulotic arthro-osteitis complicated by polyarteritis nodosa.

Pustulotic arthro-osteitis (PAO) is a chronic inflammatory disorder associated with palmoplantar pustulosis (PPP) and characterised by osteoarticular manifestations. Although several immune-mediated comorbidities have been reported in association with PPP/PAO, polyarteritis nodosa occurring in patients with PPP/PAO has not been described previously. We report a case of a 56-year-old woman with a long-standing history of PPP and PAO who developed worsening osteoarticular symptoms in close temporal proximity to purpura, lower-extremity pain, and sensory disturbance, followed by paraesthesia in the fingers. Neurological examination revealed mild weakness of the left tibialis anterior muscle. Laboratory testing showed elevated inflammatory markers, whereas myeloperoxidase- and proteinase 3-antineutrophil cytoplasmic antibodies were negative. Musculoskeletal imaging revealed bilateral sternoclavicular joint synovitis, inflammatory changes adjacent to the right patellar tendon insertion with infrapatellar bursitis, and right peroneal tenosynovitis. Nerve conduction studies in the lower extremities suggested mononeuritis multiplex. A skin biopsy of a purpuric lesion demonstrated fibrinoid necrosis and inflammatory infiltration in medium-sized arteries within the subcutaneous tissue. The patient was diagnosed with an exacerbation of PAO complicated by polyarteritis nodosa and was treated with oral prednisolone followed by methotrexate, resulting in rapid normalisation of inflammatory markers and improvement in osteoarticular symptoms, purpura, and finger paraesthesia, although lower-extremity neurological symptoms persisted at discharge. When patients with PAO develop atypical features such as purpura or neuropathic symptoms suggestive of mononeuritis multiplex, concomitant vasculitis, including polyarteritis nodosa, should be considered in the differential diagnosis.