The Diagnosis: Terra Firma-Forme Dermatosis
Terra firma-forme dermatosis (TFFD), also known as Duncan dirty dermatosis, is an idiopathic benign cutaneous condition that is easily misdiagnosed or mismanaged. In 1987, Duncan et al1 first described the condition in children who had mothers that lamented over dirty skin spots that could not be washed off. The term terra firma translates in Latin to solid ground, which describes the characteristic dirtlike appearance of these lesions.
Terra firma-forme dermatosis most commonly affects children and young adults, though it can present in patients of any age without any known predisposing risk factors.1-4 The lesions have a predilection for the face, neck, shoulders, trunk, and ankles. Terra firma-forme dermatosis has no association with bathing and hygiene habits, and most patients describe unsuccessful removal of the lesions, even after vigorous scrubbing with soaps and detergents at home. The lesions are asymptomatic, and many patients present to dermatology for cosmetic concerns.1-8
The etiology of TFFD is not well understood and is considered a retention hyperkeratosis. Duncan et al1 postulated that TFFD is the result of partial or improper maturation of keratinocytes leading to keratinocyte and melanin retention. Hematoxylin and eosin stains demonstrate lamellar hyperkeratosis of the stratum corneum without parakeratosis as well as keratin pearls scattered throughout. Mild acanthosis and papillomatosis also have been reported.1,5-7 Fontana-Masson stain shows excess melanin in these lesions, extending from the basal layer to the stratum corneum. Fungal and bacterial stains as well as cultures often have no notable findings.1,7 Similarly, histopathologic examination of our patient's biopsy with hematoxylin and eosin stain revealed hyperorthokeratosis with scattered naked vellus hair shafts and incidental yeast forms (Figure 1).
The differential diagnosis for TFFD may include pityriasis versicolor, confluent and reticulated papillomatosis, acanthosis nigricans, ichthyosis, malignant melanoma, and seborrheic keratosis. All of these diagnoses can be ruled out by the easy removal of the lesions with isopropyl alcohol 70%, which was performed on our patient by scrubbing the lesions with soaked gauze (Figure 2). Indeed, removal with isopropyl alcohol 70% is both the therapeutic and diagnostic procedure for TFFD.1-8 Of note, dermatitis neglecta is histologically and clinically identical to TFFD, albeit with a history of uncleanly habits or exposure to dirty environments.
The diagnosis of TFFD often is discovered incidentally as physicians wipe the area with alcohol to prepare for biopsy.1 Occasionally, vigorous scrubbing is needed to completely remove the lesions, and without this effort the lesions may be easily mistaken for another cutaneous process.3 Failure to consider TFFD as a diagnosis has led to unnecessary endocrine workups and invasive biopsies.4 Therefore, physicians should have early clinical suspicion of TFFD and be aware of the bedside diagnostic procedure using isopropyl alcohol.