Focal Cutaneous Mucinosis in Graves Disease: Relation to Pretibial Myxedema

To the Editor: A 73-year-old white woman with a 26-year history of Graves disease and a 24-year history of bilateral, severe, pretibial myxedema presented over the course of 4 months with 2, approximately 10 and 7 mm translucent, erythematous papules on the right elbow and right knee, respectively. Basal cell carcinoma was suspected, and shave biopsies were performed. Histological examination revealed similar findings for both specimens. There was moderate compact orthohyperkeratosis and atrophy of the epidermis. Rete ridges at both lateral margins of the specimen were elongated and vaguely cupped a collection of mucin that was filling and expanding the papillary and superficial reticular dermis. The mucin replaced the dermal collagen. There was no inflammation (Figs. 1A, B).FIGURE 1: A, Rete ridges vaguely cup a collection of mucin that fills and expands the papillary and superficial reticular dermis. Mucin replaces the dermal collagen. B, Closer view of a collection of mucin in the upper corium.Graves disease is well known to be associated with mucin deposition in the form of pretibial myxedema and is believed to be due to an immunoglobulin G antibody that stimulates proteoglycan production through human skin fibroblasts.1 Graves disease has not been reported in association with focal cutaneous mucinosis (FCM). FCM differs from pretibial myxedema both clinically and histologically. Pretibial myxedema presents as skin-colored plaques or nodules with a peau d'orange appearance and favors the anterior shins.2 Other sites of involvement have been reported. These include the preradial area on the arms,3 a thigh donor graft site,4 a scar,5 a smallpox vaccination site,6 and limited to plaques on the toes.7 The histology of pretibial myxedema includes a grenz zone of normal appearing collagen in the papillary and superficial reticular dermis overlying diffuse mucin deposition predominantly in the mid reticular dermis.8 FCM presents as a solitary lesion and is smaller, with average size in 1 series being 9.6 mm.9 The lesions are nondescript with a wide, clinical, differential diagnosis including cyst, nevus, keratosis, hemangioma,9 and in our case, due to the translucent appearance, basal cell carcinoma. The histology of FCM includes hyperkeratosis and effacement of the rete ridges, occasional presence of an epidermal collarette, and variable amounts of mucin, found in the upper corium.9 FCM is associated with Birt-Hogg-Dube syndrome10 and has been reported in canine species.11 In both settings, the lesions have been reported at multiple sites.10,11 Our patient did not have any manifestations of the Birt-Hogg-Dube syndrome. It is not known if FCM in a patient with history of thyroid disease is a precursor lesion to myxedema, a coincidental finding, or that of an additional mucin disorder that can be associated with thyroid disease. Mary Jo Kerns, MD Diya F.