Synonyms
Pityriasis nigra, tinea nigra palmaris
Definition
Tinea nigra is a superficial fungal infection of the stratum corneum.
Epidemiology
Tinea nigra is a rare condition overall. One hundred ten cases had been reported up to 1989 and only 19 more cases were recovered from a Medline search up to March 2001. Tinea nigra occurs most frequently in tropical climates within Central and South America, Africa, Asia, and North America [970, 1437, 2038, 2085, 2261]. Direct inoculation onto the skin from contact with decaying vegetation, wood, or soil seems to be the form of acquisition. Incubation periods may be as long as 20 years [1447].
Clinical manifestations
Clinically, the disease is asymptomatic in most cases, but it may be associated with pruritus. Patients usually consult for the new appearance of brown to black nonscaly macules with well-defined borders that resemble silver nitrate stains. Macules may be unique or multiple, rounded or have irregular shapes. Their size varies between 1 mm to 1.5 cm [953, 2038]. The palmar surfaces are most often affected (“Tinea nigra palmaris”), but lesions may occur on the soles (“Tinea nigra plantaris”) and other surfaces of the skin [2038]. Macules showing an uneven rate of spread and/or coalescence raise the suspicion of melanocytic nevi, junctional nevi, or melanoma [150, 970, 2261, 2464]. Other differential diagnoses include Addison’s disease, pinta, stains from chemicals or dyes and syphilis [1447]
Prognosis and therapy
Spontaneous resolution is rare. However historical reports of therapies as simple as shaving or striping have reported to be successful [712]. This is in perfect correlation with the superficial location of the fungus in the stratum corneum [2038]. A more conservative approach using the combination of keratolytic preparations with N/A(L): topical antifungals are equally effective [344, 1080, 2038, 2085]. Interestingly, the topical use of the antiparasitic drug, thiabendazol, has also been reported as useful [380, 2172]]. Systemic antifungal agents like N/A(L):itraconazole and N/A(L):terbinafine can also be curative but may not be required [953, 2085].
Histopathology
Abnormal thickening of the cornified epidermis (hyperkeratosis) and separation of its layers by branched hyphae that do not reach the stratum lucidum are characteristic findings. Inflammation is usually absent.
Laboratory
Direct examination
Superficial scraping mounted in 10% KOH shows abundant branched septate fragmented hyphae measuring from 1.5 to 5 µm in diameter. As it is characteristic of dimorphic dematiaceous fungi, elongated budding cells, 3 X 10 µm, in clusters or along the length of dark hyphae are also seen [953, 2038].
Isolation
Inoculate the clinical specimens onto Sabouraud glucose agar, incubate at 30°C and discard negative cultures in 4 weeks. After a week the culture yields slow growing mucoid colonies that become progressively olive to greenish-black in color and eventually develops aerial mycelium.
Mycology
- Hortaea werneckii
Natural habitat
Plants, soil and foods with a high salt content