Pruritic Papules on the Scalp and Arms

Folliculotropic Mycosis Fungoides

Folliculotropic mycosis fungoides (FMF) is a variant of mycosis fungoides (MF) that occurs mostly in adults with a male predilection. The disease clinically favors the head and neck. Patients commonly present with pruritic papules that often are grouped, alopecia, and frequent secondary bacterial infections. Less commonly patients present with acneiform lesions and mucinorrhea. Patients often experience more pruritus in FMF than in classic MF, which can provide a good means of assessing disease activity. Disease-specific 5-year survival is approximately 70% to 80%, which is worse than classic plaque-stage MF and similar to tumor-stage MF.¹

Treatment of FMF differs from classic MF in that the lesions are less responsive to skin-targeted therapies due to the perifollicular nature of dermal infiltrates. Superficial skin lesions can be treated with psoralen plus UVA (PUVA) therapy. Other options include PUVA in combination with interferon alfa or retinoids and local radiotherapy for solitary thick tumors; however, in patients who have more infiltrative skin lesions or had PUVA therapy that failed, total skin electron beam therapy may be required.²

On histologic examination, there typically is perivascular and periadnexal localization of dermal infiltrates with varied involvement of the follicular epithelium and damage to hair follicles by atypical small, medium, and large hyperchromatic lymphocytes with cerebriform nuclei. Mucinous degeneration of the follicular epithelium can be seen, as highlighted on Alcian blue staining, and a mixed infiltrate of eosinophils and plasma cells often is present (quiz image and Figure 1). Frequent sparing of the epidermis is noteworthy.^2-4 In most cases, the neoplastic T lymphocytes are characterized by a CD3⁺CD4⁺CD8^-immunophenotype as is seen in classic MF. Sometimes an admixture of CD30⁺ blast cells is seen.¹

Figure 1. Follicular and perivascular dermal infiltrates in folliculotropic mycosis fungoides (H&E, original magnification ×4).

Histologic differential considerations for FMF include eosinophilic pustular folliculitis (EPF), primary follicular mucinosis, lupus erythematosus, and pityrosporum folliculitis.

Eosinophilic pustular folliculitis has several clinical subtypes, such as classic Ofuji disease and immunosuppression-associated EPF secondary to human immunodeficiency virus. Histologically, EPF is characterized by spongiosis of the hair follicle epithelium with exocytosis of a mixed infiltrate of lymphocytes and eosinophils extending from the sebaceous gland and its duct to the infundibulum with formation of hallmark eosinophilic pustules (Figure 2). Infiltration of neutrophils in inflamed lesions generally is seen. Eosinophilic pustular folliculitis is an important differential for FMF, as follicular mucinosis has been observed in lesions of EPF.⁵ Both EPF and FMF can exhibit eosinophils and lymphocytes in the upper dermis, spongiosis of the hair follicle epithelium, and mucinous degeneration of follicles,⁶ though lymphocytic atypia and relatively fewer eosinophils are suggestive of the latter.

^{Photograph courtesy of Brian Swick, MD (Iowa City, Iowa).}

Figure 2. Eosinophilic pustular folliculitis with spongiosis of the hair follicle epithelium and exocytosis of a mixed infiltrate of lymphocytes and eosinophils (H&E, original magnification ×20).