Folliculotropic mycosis fungoides in a 4-year-old child mimicking atopic dermatitis: A rare case

Summary: Pediatric mycosis fungoides (MF) is rare, with an onset of 5 months to 18 years and a mean age at diagnosis of 8-to 10 years. Delayed diagnosis is 2-3 years. Hypopigmented and folliculotropic mycosis fungoides (FMF) are common subtypes, which may co-exist. In FMF, CD4+ atypical lymphocytes infiltrate hair follicles. Clonality may be negative in half of FMF biopsies. Clinically, variably pruritic patches/plaques with follicular papular components or alopecia arise on the head/neck/trunk/extremities. Pediatric FMF is indolent, with a good prognosis, although rare progression to tumors may occur. We present a case of pediatric FMF. A healthy 4-year-old girl presented with hypopigmented patches and thin plaques studded with follicular papules on bilateral legs and popliteal fossae, initially treated as atopic dermatitis. After a year of poor response to multiple treatments for eczema and progression, a biopsy was performed. A punch specimen from the flank demonstrated a mild lymphocytic infiltrate around and within the hair follicles with focal epidermotropism. Immunohistochemistry revealed a CD3+/ CD4+/CD7-/CD8- infiltrate. A clonal T-cell gene rearrangement was identified on PCR. The clinical-histopathological features were consistent with indolent subtype of FMF. To date, the patient has been responding well clinically to topical treatment. This case highlights the importance of considering FMF as a diagnosis, regardless of a patient’s young age, as it may mimic common childhood eruptions such as atopic dermatitis or pityriasis alba. Clonality may be negative, so early suspicion of pediatric FMF with critical histopathological examination and immunophenotyping will often be required to confirm the diagnosis.