Lichenoid papular dermatoses
and monoclonal gammopathy

Article written and reviewed by Dr Luca Moretti

The expression lichenoid papulosis associated with monoclonal gammopathy does not identify a recognized nosologic syndrome with its own diagnostic criteria. The simultaneous presence of lichenoid papules or plaques and a monoclonal component may be coincidental or may represent a manifestation of a specific dermatosis related to a plasma cell dyscrasia. Before establishing a causal relationship, the dermatologic condition must therefore be precisely defined.

The main differential diagnoses include lichen planus, lichenoid drug eruptions, lichen myxedematosus and scleromyxedema, cutaneous amyloidosis, xanthomas and other dermatoses associated with paraproteinemia. In particular, scleromyxedema is a generalized papular mucinosis frequently associated with monoclonal gammopathy and must not be confused with lichen planus.

Etiology, pathogenesis and pathophysiology

Etiology depends on the actual cutaneous diagnosis. In lichen planus, an immune-mediated process directed against basal keratinocytes predominates; lichenoid eruptions may involve drugs or other triggers; in lichen myxedematosus and scleromyxedema, dermal mucin deposition, fibroblast proliferation and fibrosis are observed. The presence of MGUS alone does not demonstrate that the paraprotein caused the lesions. A pathogenic relationship is more plausible when the dermatosis is a recognized manifestation of a monoclonal gammopathy of clinical significance and improves in parallel with reduction of the clone.

Clinical manifestations

Lichenoid lesions may present as violaceous, flat-topped, pruritic papules, sometimes coalescing into plaques, with possible mucosal or nail involvement. Scleromyxedema papules, by contrast, are small, waxy, closely spaced and associated with progressive sclerodermoid induration of the skin. Distribution, color, consistency, pruritus, mucosal involvement and systemic signs guide the diagnosis but do not replace histologic examination.

Investigations and diagnosis

Skin biopsy is central. A lichenoid infiltrate with interface damage suggests lichen planus or a lichenoid eruption; mucin deposition between collagen bundles with fibroblast proliferation suggests lichen myxedematosus or scleromyxedema; Congo red staining may be required if amyloid is suspected. Hematologic evaluation includes a complete blood count, creatinine, calcium, electrophoresis, immunofixation, free light chains and, when indicated, bone marrow examination and imaging.

The assessment requires:

• clinical and histopathologic definition of the dermatosis;
• investigation of drugs, infections and autoimmune diseases capable of causing a lichenoid eruption;
• characterization of the monoclonal component and assessment for any organ damage;
• cautious demonstration of a causal relationship, avoiding the transformation of simple coexistence into a new syndrome.

Treatment and prognosis

Treatment follows the confirmed dermatologic diagnosis. Lichen planus may require topical or systemic corticosteroids, phototherapy or other immunomodulators according to site and severity. Scleromyxedema is frequently treated with intravenous immunoglobulin and requires specialist management. Asymptomatic MGUS is not treated solely because it coexists with a dermatosis; clone-directed therapy is reserved for myeloma, macroglobulinemia or monoclonal gammopathies of clinical significance in which the relationship between paraprotein and cutaneous damage has been adequately documented.

Complications

Complications depend on the specific diagnosis and may include chronic pruritus, excoriations, secondary infections, dyspigmentation and impaired quality of life. In systemic mucinoses, neurologic, cardiac, respiratory or gastrointestinal involvement may occur. At the same time, the gammopathy must be monitored for possible hematologic progression, without automatically attributing that risk to the dermatosis.

References
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