Cutaneous mucinosis of infancyL98.5

Very rare (<50 cases in international literature), congenital or developing in the first months of life, persistent, benign, focal dermal mucinosis characterized by the presence of asymptomatic and variably sized skin-colored to erythematous papules, plaques, or single nodules. There is no association with thyroid disorders. For associations with other systemic diseases, see below.

Cutaneous mucinosis of childhood is, in addition to

• the discrete papular mucinosis
• acral persistent papular mucinosis
• self-healing juvenile cutaneous mucinosis and
• nodular mucinosis

one of 5 subtypes of localized papular mucinosis (see alsocutaneous mucinosis)

The etiopathogenesis of childhood cutaneous mucinosis is unclear. Normally, proteoglycans (mucin) are produced in only small amounts by dermal fibroblasts. It is postulated (as in other mucinoses as well) that the accumulation of mucin occurs through activation of dermal fibroblasts. The signal for increased formation of proteoglycans is possibly by autoantibodies, by monoclonal or polyclonal immunoglobulins, by proinflammatory cytokines e.g. in viral infections, or even by a balance disturbance between production and degradation of mucins (Rongioletti F 2020). Since cases with familial accumulation have also been described, a genetic factor is also discussed.

The papular lesions usually appear in early childhood up to 2 years of age at the latest. Not infrequently, they already exist since birth (Morgado-Carrasco D et al. 2020).

Preferably, the trunk and the extremities are affected. Less frequently, the face, head and neck are also affected (Chan C et al. 2018).

Clinically rather inconspicuous, 0.2 to 3.0 cm in size, asymptomatic or pruritic, usually skin-colored to slightly translucent, smooth papules and plaques; rarely, grouped or linear arrangements have been described. Also subcutaneous, soft nodules. Symmetrical distribution patterns are possible. No evidence of other systemic changes.

Focal accumulation of alcian blue-positive proteoglycans in the upper dermis, usually clearly demarcated from the normal dermis; moderate proliferation of fibroblasts. Resulting reticular pattern of cellular accumulation. Slightly increased proliferative tendency of fibroblasts. Inflammatory infiltrates are largely absent.

Self-healing juvenile cutaneous mucinosis: The clinical presentation of childhood cutaneous mucinosis (CMI) and self-healing juvenile cutaneous mucinosis (SHJCM) differs markedly. Unlike SHJCM, the mucin in CMI is confined to the dermis, and there is no accompanying fibroblastic proliferation.

Nevus mucinosus: blaschcoid distribution pattern.

Lichen myxoedematosus: small papular appearance. Histology: proliferation of fibroblasts in addition to mucin deposits.

Fibromas: histology is diagnostic

Juvenile xanthogranulomas: usually solitary, soft, elastic, initially red, later yellow or skin-colored, usually completely asymptomatic papule, plaque or nodule (usually 1-3 cm Ø). Histology is diagnostic.

Connective tissue nevi: Histology is diagnostic.

Dermal melanocytic nevi: hyperpigmentation.

Leiomyomas: firm reddish, grouped papules.

Associated comorbidities such as monoclonal gammopathy in scleromyxedema or thyroid disease in pretibial myxedema are not described in the literature in affected young patients. The occurrence of autoimmune diseases in the extended family circle(lupus erythematosus, Graves' disease) as well as a concordant occurrence with arthritis has been documented (Morreale C et al. 2021).

Overall, the prognosis of the disease is good with a high possibility of spontaneous regression. Courses with skin lesions progressive in number and size have been described as well as cases with spontaneous regression during puberty (Podda M et al. 2001). In a long-term observation of a patient with cutaneous mucinosis of childhood on the thigh already present since birth, multiple further size-progressive plaques appeared on the trunk and extremities until the age of 2 years. Thereafter, initially the congenital plaque on the thigh regressed spontaneously, and by the age of 7 years all lesions had regressed leaving only a few atrophic papules on the trunk (Morgado-Carrasco D et al. 2020).

The relationship between arthritis and CMI is not yet clear. Some interesting in vitro studies have revealed immunological functional properties of specific glycosaminoglycans (GAGs): Low molecular weight (LMW) hyaluronan (HA) activates macrophages and dendritic cells and plays a role in the recruitment of immune cells to sites of inflammation; whereas chondroitin sulfate (CS) promotes neutrophil activation, and the chondroitin 4-sulfate (C4S) form of CS activates monocytes to produce monokines (Morreale C 2021). It has also been suggested that mucinosis in rheumatic diseases may be related to circulating antibodies that stimulate the synthesis of glycosaminoglycans by skin fibroblasts. McAdam et al reported a 58-year-old with papular mucinosis severe proximal myopathy, seronegative inflammatory polyarthritis, and marked eosinophilia. This suggests that cutaneous papular mucinosis may also include severe rheumatic manifestations. The extent to which the constellation of arthritis and CMI is a coincidental association or a pathogenic correlation is currently unclear.

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