CMI is a persistent dermatosis characterized by the presence of asymptomatic and variably sized skin-colored to erythematous papules, plaques or single nodules, generally localized in the upper extremities and trunk, with benign prognosis [
4,
5]. Our case report is particularly interesting because it is the first in which joint involvement has been reported in association with CMI, a disorder that has so far been described as limited to skin involvement. Of note, joint symptoms have been described in other forms of primary cutaneous mucinosis, such as SHJCM, in association with other systemic symptoms like fever, myositis and lymphocytosis [
6,
7]. The relationship of arthritis to CMI is not yet clear. Some interesting in vitro studies have revealed immunologic functional characteristics of specific glycosaminoglycans (GAGs): low molecular weight (LMW) hyaluronan (HA) activates macrophages and dendritic cells and plays a role in immune cell recruitment to sites of inflammation [
8]; chondroitin sulphate (CS), on the other hand, promotes neutrophil activation, and chondroitin 4-sulphate (C4S) form of CS activates monocytes to produce monokines [
9]. HA and CS are increased in dermatomyositis and lupus skin lesions [
10]. It has been suggested that in rheumatic diseases the mucinosis could be related to circulating antibodies stimulating the synthesis of glycosaminoglycans by skin fibroblasts. Based on their known immunomodulatory effects, a potential role of these GAGs species in the pathophysiology of these inflammatory skin conditions has been proposed [
10]. McAdam et al. reported a case of a 58-year-old Caucasian woman with papular mucinosis who developed severe proximal myopathy, seronegative inflammatory polyarthritis, and marked eosinophilia, suggesting that popular mucinosis may include serious rheumatic manifestations, in addition to skin involvement [
11]. The woman’s non-erosive inflammatory polyarthritis, involving both large and small joints, developed 2 years after the onset of skin involvement and was successfully treated with corticosteroid and methotrexate. Synovial histology revealed inflammatory synovitis and, quite unexpectedly, no mucopolysaccharide deposition was found. The unexpected absence of mucopolysaccharide deposition in the synovial membrane makes the hypothesis of a potential role of mucin in the development of inflammatory arthritis less plausible. The result of this study could also raise the question arthritis and CMI: a casual association or a pathogenic correlation? Based on the current knowledge the pathogenic link between skin manifestations and joint involvement in primary cutaneous mucinosis remains to be clarified and therefore we can not exclude that the association described in our patient is accidental. Further histological studies are needed in order to elucidate the pathogenesis of joint involvement in primary mucinosis.