Terra firma–forme dermatosis, first reported in 1987 and commonly known as Duncan dirty dermatosis, is a hyperpigmented plaque that often looks like dirt on the skin.1,2 Terra firma–forme dermatosis can occur in patients of any age but occurs more frequently in children and adolescents.3,4 Men and women are affected equally by the disease, and it has been described as having a predilection for overweight individuals.4,5 It is not known to be related to a specific gene or display familial inheritance.3,4,6
The Latin phrase terra firma means “dry land,” which well describes the hyperpigmented skin findings of terra firma–forme dermatosis caused by dirt accumulation in the epidermis.1,2 Lesions can be verrucous, reticular, or papillomatous and usually are black or brown in color.4 The symmetrical plaques can appear anywhere on the body, although they most commonly are present on the trunk and neck. Other potentially involved areas include ankles, face, legs, back, and axillary, pubic, and umbilical regions.3,4 The rash is more likely to be found in concave skin areas and skin folds and can be localized or generalized.2,4,5
The etiology of terra firma-forme dermatosis is unknown. One theory is that it results from slowed or delayed keratinocyte maturation, incomplete development of keratin squames, melanin retention, and dirt and sebum buildup.1-3 Predisposing risk factors are ill-defined; sun exposure and use of emollients containing urea have been cited in the literature as potential causes.2,6,7 In addition, terra firma–forme dermatosis has been associated with atopic dermatitis and xerosis.7,8 Applying oil, ointments, cleansers, and adhesives on eczematous, xerotic, or scaly skin may lead to inadequate exfoliation and dirt buildup characteristic of terra firma–forme dermatosis.7,9 This condition can occur even when affected patients have good hygiene.1,2
Diagnosis of terra firma–forme dermatosis can be made in the clinic. The plaque will disappear when rubbed with 70% isopropyl or ethyl alcohol.2,4 At times, aggressive rubbing will be necessary to remove hyperpigmentation completely.4,9 Biopsies rarely are performed, and further diagnostic tests, such as examination under a Wood lamp or cultures, are conducted only when infection is suspected.3 Reflectance confocal microscopy recently was recommended as an additional, noninvasive diagnostic tool.3
Histopathologic studies of terra firma–forme dermatosis have reported lamellar hyperkeratosis, increased melanin, and compact orthokeratosis with whorls.1,2,4 Acanthosis, parakeratosis, and papillomatosis may be absent, and toluidine blue staining can show keratin globules in the stratum corneum.4 Basal layer spores of the fungus Malassezia furfur also have been described.2 Dermoscopy generally displays a mosaic or tile-like pattern of polygonal black or brown scales with interrupting furrows.3,7
Although simple to diagnose, terra firma–forme dermatosis can be confused with other diseases, leading to unnecessary diagnostic tests, medications, and referrals.4 The differential diagnosis includes dermatosis neglecta and dirty dermatosis, which arise from inadequate hygiene habits. Dermatosis neglecta generally occurs in patients with disabilities and is not characterized by brown cornflake-like scales, differentiating it from terra firma–forme dermatosis. Numerous similar lesions include confluent and reticulated papillomatosis, nipple hyperkeratosis, acanthosis nigricans, idiopathic deciduous skin, ichthyosis, Darier disease, extensor surface darkening from friction, postinflammatory hyperpigmentation, and dirty neck syndrome with atopic dermatitis. Such skin conditions can be differentiated from terra firma–forme dermatosis by a negative alcohol swipe test.3
Other similar conditions include epidermal nevi, granular parakeratosis, omphalith, seborrheic keratosis, epidermolytic hyperkeratosis, pseudoacanthosis nigricans, and tinea versicolor.4 It was suggested that tinea versicolor could be distinguished from terra firma–forme dermatosis by a negative Wood lamp test, but more recent documentation suggests terra firma–forme dermatosis also may show fluorescence.3,7
The standard treatment for terra firma–forme dermatosis is rubbing with 70% isopropyl or ethyl alcohol.3,4 However, other methods, including lactic acid, 2% salicylic acid, lemon juice, urea lotion, oral retinoids, and topical and oral antifungals, also have been used successfully.3
Patients may wish to moisturize areas treated with isopropyl alcohol to avoid dryness. If lesions recur, patients may continue to rub the affected area with isopropyl alcohol and can rub weekly for prophylactic therapy.4
The patient’s plaque in this case was treated partially in clinic with 70% isopropyl alcohol. The patient and his wife continued this treatment at home, which led to complete resolution of the lesions on his back. The lesions have not recurred.
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