A ring-shaped patch is a clinical shape, not one histological disease. The historical category included several named scalp and skin affections whose relationship was disputed. In particular, old claims that alopecia areata was a fungal disease cannot be carried into modern pathology. A contemporary review actually records competing classifications of favus, scalp ringworm, circinate eruptions, beard disease and patchy baldness, and disagreement about whether their fungi were identical.[ref]
On the Parasitic Affections of the Skin (review, 1864). Discussion of proposed parasites of favus, herpes tonsurans, herpes circinatus, sycosis, tinea decalvans and pityriasis versicolor
The microscopic distinctions should be organized by the structure infected or injured:
- Ordinary scalp ringworm, or tinea capitis. The fungus invades keratinized hair and adjacent surface material. Hyphae and arthroconidia can occur within the shaft, the endothrix pattern, or around its exterior, the ectothrix pattern. The weakened shaft breaks, producing short stumps or dark dots. Inflammation varies from limited scale to a severe follicular reaction. Modern species cannot reliably be assigned from the shape of the bald patch alone.[ref]Tinea capitis by DermNet. Classification by invasion of the hair shaft; clinical variants and microscopy
- Favus. The characteristic yellow cups, or scutula, consist of a mass of fungal filaments and keratinous debris around a hair follicle. Hyphae may also involve the hair. Longstanding destructive disease can scar the follicle and cause permanent hair loss. The historical name of its organism was Achorion schoenleinii; the usual modern designation is Trichophyton schoenleinii. Those names describe the history of classification, not two different appearances inferred from an unseen specimen.[ref]
- Tinea corporis and fungal beard disease, tinea barbae. On smooth skin, superficial hyphae occupy the stratum corneum, with variable epidermal reaction; the advancing margin can be the most productive site for scrapings. In beard infection the hair-bearing hair follicles are involved, and the reaction may be follicular, pustular or granulomatous. The historical word sycosis also covered nonfungal follicular inflammation, so its name alone does not establish a dermatophyte.[ref]Tinea corporis pathology by DermNet. Histology and special stains[ref]
- Pityriasis versicolor. This is associated with Malassezia, not the usual dermatophytes of true ringworm. Numerous rounded yeast forms and short fungal filaments occur predominantly in the stratum corneum; a hair shaft filled by arthroconidia is not the defining lesion. This explains why the older name “tinea versicolor” should not imply identity with scalp ringworm.[ref]Pityriasis versicolor pathology by DermNet. Histology, yeast and short hyphae in the stratum corneum
- Alopecia areata, historically entangled with “tinea decalvans”. The modern lesion is immune-mediated disturbance of hair follicles, with characteristic lymphocytic inflammation around and sometimes within the bulb in active disease and changes in the hair-growth cycle. The inflammatory infiltrate may be inconspicuous in chronic disease. Finding short or altered hairs is not proof of fungi.[ref]S3 guideline diagnostics and therapy of alopecia areata, Part 1. Histopathological diagnostics, active peribulbar inflammation and chronic-stage variation
Annular eczema or psoriasis supplies a further clinical mimic: an inflammatory epidermal pattern alone is not evidence of an invading fungus. For an actual microscopic investigation, collect scale from an active margin and affected hairs when hair is involved, mount the material on a microscope slide with potassium hydroxide to clear keratin, and examine under a microscope. A modern biopsy can be examined with Periodic acid-Schiff staining to highlight fungal structures. Distribution within the surface layer, hair shaft, follicle or bulb matters as much as the presence of particles. Fungal culture or molecular identification can resolve species when morphology cannot.[ref]
Investigation of Dermatological Specimens for Superficial Mycoses by the UK Standards for Microbiology Investigations. B 39, issue 3.2, Microscopy and specimen examination
Distinguish the organism, its anatomical location and the host reaction; do not diagnose all circular lesions as one infection. Articles by others on the same topic
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