Tinea Nigra
Tinea nigra is an uncommon infection with a pigmented (dematiaceous) fungus that stays confined to the stratum corneum, the outermost layer of the skin. Although it is benign, never penetrates deeply and causes no systemic complications, it matters clinically because the pigmented patch it produces on the palm or sole is easily mistaken for acral lentiginous melanoma (ALM), a junctional nevus or post-inflammatory hyperpigmentation. Limited experience with dermoscopy, or skipping a simple fungal test, can lead to a biopsy or wide excision that the patient never needed.
At Haute-Derma, we pair high-resolution digital dermoscopy with 20% KOH microscopy and fluorescent staining to confirm that the surface pigment comes from the fungus. The infection can then be cured within 2–4 weeks without any invasive procedure.
Causative Organism and Pathogenesis
Tinea nigra is caused mainly by the salt-tolerant (halophilic/halotolerant) dematiaceous fungus Hortaea werneckii (formerly known as Phaeoannellomyces werneckii or Exophiala werneckii).
- How infection occurs and risk factors: H. werneckii is common in soil, seawater and decaying vegetation in tropical and subtropical climates. It enters the stratum corneum through tiny scrapes (micro-abrasions) in the skin. Excessive sweating of the palms or soles (hyperhidrosis) raises surface moisture and salt levels, creating ideal conditions for the spores to attach and multiply.
- Why the patch is dark: H. werneckii makes and stores melanin within its own cell walls. As the hyphae and budding cells spread sideways through the stratum corneum, they form a dark brown, olive-gray or black patch on the skin surface.
- Non-invasive behavior: Because the fungus lives only in the most superficial layer and never reaches the granular or basal layers, the condition causes no pain or itch and does not provoke an inflammatory immune response.
Clinical Features and Differential Diagnosis
Tinea nigra typically appears as a single flat patch level with the skin. Its border is well defined or slightly ragged, and its color varies from light brown to dark brown or olive-black. Up to 90% of cases occur on the palm (usually the non-dominant hand) and 10% on the sole or elsewhere. The surface is smooth with no scale, or only very fine scale, and the patient has no symptoms at all.
Clinical Differential Diagnosis
| Feature | Tinea nigra | Exogenous staining |
|---|---|---|
| Nature of the lesion | Pigmented fungus H. werneckii in the stratum corneum | Chemicals, ink, plant sap or silver nitrate (AgNO3) on the stratum corneum |
| Dermoscopy | Scattered pigmented dots/spicules that ignore the skin-ridge pattern | Uneven pigment on top of the skin creases that fades quickly |
| Course | Spreads sideways over weeks to months | Fades and disappears within a few days to 2 weeks |
| 20% KOH microscopy | Brown/olive septate hyphae and budding cells | Negative |
| Management | Topical antifungal + keratolytic (100% cure) | No treatment needed |
Our Diagnostic Process at Haute-Derma
Digital Dermoscopy: Avoiding the Melanoma Trap
- Typical dermoscopic picture of tinea nigra: Light brown to olive-brown pigmented dots and spicules, evenly spread but discontinuous, that merge into fine bands without following the skin-ridge anatomy (non-dermatoglyphic pattern). The pigment lies over both the ridges and the furrows, favoring neither.
- Why it rules out melanoma: It lets the doctor immediately exclude the parallel ridge pattern (pigment concentrated on the raised skin ridges), the gold-standard sign of melanoma.
20% KOH Microscopy With Calcofluor White Staining:
- The doctor gently scrapes the surface layer of the patch with a rounded medical blade. It is painless and does not draw blood.
- Under the fluorescence microscope: short, irregular, light brown or pale olive hyphae that branch and have clear septa, along with oval, two-celled budding cells (blastoconidia of Hortaea werneckii).
Culture on Sabouraud Dextrose Agar (SDA): Incubated at room temperature (25°C). After 1–2 weeks, the fungus forms shiny black or olive yeast-like colonies that gradually become the characteristic dense, velvety olive-black mold.
Comprehensive Treatment at Haute-Derma
Topical Therapy with a 100% cure rate: Because Hortaea werneckii lives only in the superficial stratum corneum, topical treatment combined with a keratolytic clears it completely, with no need for oral antifungals.
- Keratolytic agents (essential): 3–5% salicylic acid solution or cream, or Whitfield ointment, applied once or twice daily. Salicylic acid dissolves the desmosome bonds between the old, fungus-laden surface cells, so the patch fades and peels away within just 7–10 days.
- Azole or allylamine antifungals: ketoconazole 2%, miconazole 2%, terbinafine 1% or ciclopirox olamine 1% cream, applied twice daily.
- Treatment length: Keep applying the creams for 2–4 weeks to make sure no spores remain hidden in the palm creases.
Managing Risk Factors and Preventing Recurrence:
- Controlling sweaty palms (hyperhidrosis): Soaking the hands in 20% aluminum chloride solution or iontophoresis at Haute-Derma keeps the palms dry and removes the salty environment the fungus thrives in.
- Hand hygiene: Patients should wash their hands well with antiseptic soap after handling soil, plants, seawater or other damp tropical environments.
