Psoriatic arthritis mutilans (PAM) in the Nordic countries: demographics and disease status. The Nordic PAM study

Objective: To determine the prevalence and clinical characteristics of psoriatic arthritis mutilans (PAM) in the Nordic countries. Method: Patients with putative PAM aged ≥ 18 years were recruited. Fifty-nine patients were included after clinical examination. Results: The prevalence of PAM in the adult Nordic population was estimated to be 3.69 per million inhabitants [95% confidence interval (CI) 2.75-4.63]. The female to male ratio was close to 1:1. The mean age of skin disease onset was 25 years and the mean age of onset of joint disease was 30 years. The onset of skin disease was 2 years earlier among female patients. At inclusion, the mean duration of arthritis was 27 ± 11 years for male patients and 33 ± 11 years for female patients. PAM was most frequently seen in the distal interphalangeal (DIP) joints of the toes, followed by the IP joint of the thumb and the DIP joint of the little finger on the left hand. Female and male patients had similar numbers of painful and swollen joints. Enthesitis was found in 19 patients (32%), while 38 patients (64%) had a history of dactylitis. Twenty-three of these 38 patients (61%) had a history of dactylitis in the same finger/toe as they had PAM. At the time of inclusion, 45% of the patients were found to have clear or almost clear skin. Conclusions: PAM in the Nordic countries has a low prevalence, with only three to five cases per million inhabitants. The majority of the patients present with mild skin disease.