The interstitial expression of granuloma annulare and of papular mucinosis have several findings histopathologic in common, among them mesenchymal cells with abundant pink cytoplasm scattered among bundles of collagen, multinucleate cells in sections of tissue of some specimens, deposits of mucin in company with an increased number of mast cells, perivascular infiltrates of lymphocytes, eosinophils present rarely, involvement predominantly of the upper half of the dermis in most instances, and preservation of epithelial structures of adnexa.
At scanning magnification, papular mucinosis may be mistaken for granuloma annulare because oval and spindle-shaped fibrocytes in conjunction with slight deposits of mucin are scattered among collagen bundles. In granuloma annulare of the interstitial type, oval-shaped (epithelioid) histiocytes in association with deposits of mucin are distributed among collagen bundles.
Despite the implication of the name papular mucinosis and of synonyms for it, namely, scleromyxedema and lichen myxedematosus, the condition is primarily a proliferation of fibrocytes accompanied by deposits of mucin rather than a true “mucinosis” like pretibial myxedema or myxedema of hypothyroidism in which the abnormality in skin lesions consists wholly of a copious quantity of mucin. In those “mucinoses,” there is no proliferation of fibrocytes and no alteration in bundles of collagen.
The name scleromyxedema conveys a sense for one of the attributes clinical crucial for differentiation of that condition (when it is established fully) from granuloma annulare, to wit, hardness that results from fibrosis. In short, fibroplasia is an important element of papular mucinosis, but it is not an accompaniment at all of granuloma annulare. Because granuloma annulare is truly granulomatous (histiocytes predominate in foci), unlike papular mucinosis which is a fibroplasia, there may be evidence of phagocytic capability of histiocytes, to wit, ingestion of elastic or elastotic fibers, to which the misleading appellation “actinic granuloma” has been applied. In brief, the term scleromyxedema alerts to the fact that that condition, when evolved fully, is affiliated with prominent fibrosis. The term granuloma annulare, by contrast, signifies that that condition fundamentally is granulomatous.
Clinically, papular mucinosis usually is distinguished easily from granuloma annulare because it is constituted of widespread firm papules, whereas granuloma annulare usually consists of papules arranged in rings or arcs, which at times assume a polycyclic outline. Even the eruptive type of granuloma annulare is differentiated readily from papular mucinosis because the papules of that expression of granuloma annulare often have a subtle dell in the center of them, are not hard, and often are distributed on sites that are exposed to sunlight.
Patients with papular mucinosis nearly always exhibit an elevation of gamma globulins in the form of an isolated monoclonal (M-component) IgG band detectable by serum protein electrophoresis. Some patients have plasmacytosis of the bone marrow, but even then the changes there are not diagnosable as those of myeloma. Although conventional granuloma annulare is unassociated with any systemic illness, some students of the subject have claimed that granuloma annulare is affiliated more often than would be expected with an abnormal glucose tolerance test or even overt diabetes mellitus.
In the interstitial pattern of granuloma annulare, in contrast with the palisaded pattern of that condition, deposits of mucin are less abundant and histiocytes per unit area of dermis are fewer. Not uncommonly, both patterns are encountered together in the same section of tissue. Those patterns are not reflected in differences in appearance clinical. An able clinician usually recognizes granuloma annulare for what it is, as does an able histopathologist, irrespective of the pattern formed by histiocytes. On occasion, a superficial and deep perivascular infiltrate of lymphocytes in granuloma annulare may be so riveting to a histopathologist that the interstitial component of histiocytes may be overlooked and the lesion then misinterpreted as one of the “lymphocytic infiltrates.”
In sum, both granuloma annulare and papular mucinosis seem to be distinctive patterns morphologic that may be brought into being by more than a single factor.