Tinea Nigra

Tinea nigra is a superficial mycosis of the stratum corneum caused by the melanised, yeast like fungus Hortaea werneckii. It produces a brown to black, sharply demarcated, asymptomatic macule, usually on the palm. The name is misleading: it is not a dermatophytosis, and the organism does not digest keratin as dermatophytes do. The infection is confined to the outermost keratin layer, with no inflammatory response and no invasion of living tissue.

Its clinical importance is diagnostic. The lesion mimics acral and junctional melanocytic lesions, including melanoma, and a potassium hydroxide preparation resolves the question in minutes. Misdiagnosis leads to unnecessary biopsy or excision.

Epidemiology:

• Tropical and subtropical coastal regions: Central and South America, the Caribbean, West and Central Africa, South and Southeast Asia, and the Gulf and south east coasts of North America.
• Children and young adults, with a female predominance in most series.
• Imported cases occur in travellers returning from endemic coasts.
• Person to person transmission is rare. Infection follows contact with the environmental source.

Hortaea werneckii belongs to the Ascomycota (order Capnodiales). Older literature uses Exophiala werneckii, Cladosporium werneckii and Phaeoannellomyces werneckii for the same organism.

Key microbiological features:

• Dematiaceous (melanised) yeast like fungus with a polymorphic life cycle.
• Extremely halotolerant. It grows over a very wide range of sodium chloride concentrations, including near saturated brine, and is isolated from salt marshes, saline soil, beach sand, sea water, salted fish and decaying vegetation near the coast.
• Colonies on Sabouraud dextrose agar at 25 to 30 degrees Celsius begin as moist, shiny, black, yeast like colonies, which become velvety as aerial hyphae develop. The reverse is black. Growth takes about 1 to 2 weeks.
• Microscopy of a young culture shows brown, oval, two celled yeast cells that bud and produce annellidic conidia. Older cultures show septate brown hyphae and chlamydoconidia.
• The cell wall contains melanin, which stains with Fontana Masson and gives the lesion its colour.

The fungus is acquired by contact of skin with contaminated soil, sand, plants or water, with minor trauma assisting inoculation into the stratum corneum. Palmar hyperhidrosis is the commonest predisposing factor. Sweat concentrates salt on the skin surface and creates a saline niche for a halophilic organism.

The organism grows in the superficial keratin and produces melanin. Pigment in the fungal cell wall, not melanocyte activity, accounts for the dark colour. Because the infection is superficial and the host response negligible, there is no scale or erythema in most cases, no itch, and no spread beyond the epidermal surface. Immunosuppression is not a recognised risk factor for tinea nigra.

• Solitary, unilateral macule or patch, sharply demarcated, with irregular or geographic borders. Colour ranges from light brown to greenish brown to black and is typically darker at the edge.
• Appearance resembles a stain (silver nitrate, potassium permanganate or ink).
• Surface is smooth with no scale, or has fine scale only.
• Palm is the commonest site, followed by the sole. Neck, trunk and other sites are rare.
• Bilateral or multiple lesions are uncommon.
• No symptoms, and lesions enlarge slowly over weeks to months.
• No nodularity, ulceration or surface elevation.

Dermoscopy shows light to dark brown pigment arranged as fine filaments or spicules, with pigment following skin furrows without the parallel ridge pattern of acral melanoma. A suspected melanoma still needs excision biopsy if features are atypical. Microscopy should come first to avoid unnecessary surgery.

• Specimen: superficial skin scraping from the edge of the lesion, taken with a blade. Skin without scale needs a firm scrape because the organism lies in the stratum corneum.
• Potassium hydroxide preparation (10 to 20 percent): olive green to brown, septate, branching hyphae with budding yeast cells, 1.5 to 5 micrometres wide, within the keratin. The pigmented elements are visible without stain.
• Culture: Sabouraud dextrose agar with chloramphenicol at 25 to 30 degrees Celsius, with growth in 1 to 2 weeks. Culture confirms the diagnosis and is not required if microscopy is typical.
• Histopathology: rarely needed. Brown hyphae and yeast cells are confined to the stratum corneum with no inflammation, and stain with Fontana Masson as well as periodic acid Schiff.
• Molecular identification: internal transcribed spacer sequencing identifies the species where culture is atypical.

Treatment is topical. Systemic therapy is not needed.

Keratolytics remove the infected stratum corneum, and azoles, ciclopirox and terbinafine act on the fungus. Treatment is continued for 1 to 2 weeks after the lesion clears. Recurrence follows continued exposure and hyperhidrosis. Hyperhidrosis can be treated with topical aluminium chloride when it contributes. Pregnant and breastfeeding patients can use topical agents.