Terra firma-forme dermatosis

What is terra firma-forme dermatosis?

Terra firma-forme dermatosis is a benign skin disorder that manifests as raised, dirty-appearing areas of skin. It is also called Duncan’s dirty dermatosis in honour of the dermatologist who first described it [1].

Terra firma is Latin for ‘dry land,’ reflecting the appearance of affected areas as islands of dirty skin. These areas are not truly dirty but give this impression due to abnormal keratinisation.

Who gets terra firma-forme dermatosis?

Terra firma-forme dermatosis mainly affects children and adolescents [2]. However, there have not been any large epidemiological studies to determine how prevalent it is. It may be more common than reports suggest as it is asymptomatic, and many people are not likely to seek medical attention for it.

What causes terra firma-forme dermatosis?

Terra firma-forme dermatosis is thought to be a disorder of keratinisation caused by abnormal and delayed development of skin cells. Adhesion between keratinocytes is prolonged, with retention of melanin, sebum, and microorganisms, causing the formation brown patches and thickened plaques [3].

It is possible that terra firma-forme dermatosis is a ‘forme fruste’ (an atypical or incomplete manifestation of another condition) of confluent and reticulated papillomatosis of Gougerot and Carteaud [4].

What are the clinical features of terra firma-forme dermatosis?

Terra firma-forme dermatosis presents with thickened, brown-black papules and plaques on the neck, trunk, face, and abdomen including the umbilicus. The plaques can have a papillomatous surface. The patient has normal washing habits.

Terra firma-forme dermatosis has been reported around surgical sites, such as after total knee replacement or median sternotomy [5]. Rarely, it can present with extensive areas of involvement over the chest, abdomen, and thighs [6].

What are the complications of terra firma-forme dermatosis?

Psychosocial distress can result from the dirty appearance which may be perceived to reflect poor personal hygiene.

How is terra firma-forme dermatosis diagnosed?

Terra firma-forme dermatosis is a clinical diagnosis that should be suspected in children and adolescents with dirty-appearing areas, particularly on the neck, which cannot be removed with soap.

• Wiping affected areas with a pad soaked in 70% isopropyl alcohol will completely remove the lesions and is simultaneously diagnostic and therapeutic [7].
• Large brown polygonal areas in a mosaic-like pattern are seen on dermoscopy.
• Further testing is typically not necessary, but laboratory tests for fungal infection may sometimes be performed to rule out pityriasis versicolor.
• A skin biopsy is rarely performed. The findings are mostly nonspecific and include papillomatosis, acanthosis, and orthohyperkeratosis. Keratin whorls are sometimes seen and may be specific for the diagnosis [4].

What is the differential diagnosis for terra firma-forme dermatosis?

There are several other ‘dirty dermatoses’ which mimic terra firma-forme dermatosis, including:

• Dermatitis neglecta, results from inadequate cleansing of the skin and responds to washing with normal soap and water [8]
• Acanthosis nigricans, presents as velvety lesions distributed symmetrically in intertriginous areas and is associated with obesity, metabolic syndrome, and occasionally, internal malignancy [9]
• Confluent and reticulated papillomatosis of Gougerot and Carteaud, presents with brown papules which coalesce into plaques, distinguished by their reticular or net-like periphery and confluent centre, as well as their resistance to even vigorous rubbing with alcohol [10]
• Atopic dirty neck, is a dirty-appearing acquired hyperpigmentation of the anterior and lateral neck seen in patients with atopic dermatitis, particularly in Asian populations [11]
• Symmetrical acrokeratoderma, presents with dirt-like hyperkeratotic areas distributed symmetrically on the upper and lower limbs, particularly the wrists, ankles, and dorsa of the hands and feet [12]
• Seborrhoeic keratosis, is a benign growth of keratinocytes. Multiple superficial seborrhoeic keratoses may appear similar to terra firma-forme dermatosis, but do not disappear after rubbing with alcohol.

Other conditions to consider include:

• Pityriasis versicolor
• Epidermal naevus
• Postinflammatory hyperpigmentation.

What is the treatment for terra firma-forme dermatosis?

Rubbing with ethyl or isopropyl alcohol-soaked wipes completely removes the dirty-appearing areas.

Mild topical keratolytic agents (eg, 5% salicylic acid in petrolatum ointment) can be used in extensive cases to avoid aggressive rubbing of large areas [13].

Chemical peels (eg, 20% salicylic acid in alcohol) are reported to lead to rapid resolution of the condition [14].

What is the outcome for terra firma-forme dermatosis?

Terra firma-forme dermatosis usually responds completely to treatment and rarely recurs. Patients can be reassured this is a benign condition, and they can use alcohol wipes to remove any recurrent lesions.