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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.A symmetrical disease produces corresponding lesions in matching parts on the two sides of the body. The correspondence may involve site, form, extent and timing; it need not be mathematically exact in every patient or at every stage.
The idea concerned the distribution of pathological change. For example, similar lesions at both elbows have a stronger anatomical correspondence than two unrelated lesions anywhere on the right and left. Bilateral involvement is therefore not always the same as close mirror symmetry. Conversely, a disorder may characteristically show a symmetrical pattern while an individual case is uneven because lesions began at different times or received different local irritation.
The concept was already a subject of organized study before 1866. William Budd's paper on diseases of corresponding parts described paired articular and periarticular lesions and developed the argument that a systemic influence acts selectively on tissues with corresponding susceptibilities. The inference was that neither the circulating influence alone nor the local tissue alone explains the complete pattern; their interaction determines where disease appears. The historical account used the language of disordered nutrition and morbid material in the blood, which should not be mistaken for identification of a modern molecular cause.[ref]
On diseases which affect corresponding parts of the body in a symmetrical manner by William Budd. Opening clinical observations and discussion of constitutional disease and tissue susceptibility
Useful examples include:
- Psoriasis, whose plaques often occupy matching extensor sites such as the elbows and knees. This is a distributional tendency, not a requirement that every plaque have an identical partner.[ref]Chronic plaque psoriasis by DermNet. Definition and typical symmetrical distribution
- Chronic inflammatory polyarthritis, exemplified today by rheumatoid arthritis, with corresponding small anatomical joints of the hands or feet affected. The modern diagnosis is more specific than the historical phrase chronic rheumatism.[ref]Rheumatoid Arthritis by Merck Manual Professional Edition. Symptoms and Signs, characteristic symmetric peripheral arthritis
- A distal symmetric polyneuropathy, for example in diabetes, with similar sensory deficits beginning in both feet and later involving the hands. The matching distribution reflects a systemic process acting on similarly vulnerable long nerves, rather than one local injury at each spot.[ref]Polyneuropathy by Merck Manual Professional Edition. Clinical features, distal symmetric patterns and length-dependent axonal neuropathies
The first two also illustrate a distinction between surface lesions and deeper structural disease; the third illustrates functional deficits following anatomical correspondence of a nerve population. The symmetry is the shared descriptive property, not evidence that these disorders share one agent.
To document it, compare matching sites, the same tissue compartments and the stage of each lesion; record exceptions rather than filling in an absent partner. Symmetry is a clue to systemic influence and selective tissue susceptibility, not a diagnosis or proof of a single mechanism.