When dirt is not dirt! Duncan's dirty dermatosis put under the microscope

Duncan’s dirty dermatosis (terra firma-forme dermatosis) is a skin condition of unknown aetiology. The brownish-pigmented macules look like dirt but cannot be removed by water. The histopathological features resemble certain reticulate and ichthyosis-like patterns, which are mirrored here by dermatoscopical findings. Clinically, rubbing the lesions with isopropyl alcohol usually clears the affected skin and thus helps both treating and distinguishing Duncan’s dirty dermatosis from similar conditions.\n\nA 21-year-old woman presented to our clinic with a single symptomless 13 cm hyperpigmented tail-shaped macula on the medial side of her left knee (figure 1). The lesion had appeared a month ago and remained unchanged. Long-term sun exposure was denied. Dermatoscopic evaluation of the lesion revealed light brown scales in a mosaic pattern and white keratin dots (figure 1, inlet). Physical examination was otherwise unremarkable. The patient denied taking any medications, exposure to sun …