Morphology of skin lesions
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Morphology of skin lesions
Certain dermatological diagnoses are strongly suggested when the skin lesions have a characteristic morphology or pattern of arrangement (see Table 1 and Table.2), or are found in a particular anatomical location. For example, a single hyperkeratotic plaque on an elbow implies a diagnosis of psoriasis and a group of vesicles on a penis strongly suggests a diagnosis of herpes simplex virus infection.
A good history and assessment of the clinical features including morphology of skin lesions, pattern of the eruption, as well as symptoms (eg itch) are necessary to formulate a diagnosis. Including these details with the skin biopsy enables the histopathologist to assess the biopsy specimen as accurately as possible (see Skin biopsies).
Lesions may be single or multiple, discrete or confluent, and monomorphic (all the lesions are of the same appearance) or polymorphic (the lesions are of different appearance).
Lesions may also be grouped into four distinct patterns: linear, annular, grouped and reticular.
Linear lesions include:
· some forms of contact dermatitis, especially those involving plant contact
· dermatoses exhibiting the Koebner phenomenon (see Table 2) (eg psoriasis, lichen planus)
· linear epidermal naevi, lichen striatus and linear morphoea
· Blaschko’s lines—some linear lesions do not follow any vascular or nerve distribution and their pattern cannot be adequately explained. Blaschko’s lines may represent cutaneous lines of embryogenesis.
Annular lesions showing predominantly epidermal pathology include annular psoriasis, pityriasis rosea, discoid dermatitis and mycosis fungoides. Fungal infections (ringworm) are the best known annular lesions.
Dermal lesions include annular erythema, urticaria, erythema multiforme, granuloma annulare and sarcoidosis.
Grouped lesions are characteristic of herpes simplex, herpes zoster, insect bites, dermatitis herpetiformis, warts and molluscum contagiosum.
A dermatomal or zosteriform distribution could be regarded as a subgroup of grouped lesions.
Reticular lesions with net-like patterns are seen in some lesions of vascular origin, the pattern being generated by the inverted pyramidal structural shape of the dermal vasculature (eg livedo reticularis, cutis marmorata, erythema ab igne).
Some disorders, by virtue of their aetiology, are localised in a particular area:
· Herpes zoster occurs in a dermatomal pattern because the virus travels along sensory nerves to the skin.
· Hidradenitis suppurativa affects the axillae and groin because it is associated with apocrine glands and this is where these glands are located in densest distribution.
· Photosensitive disorders frequently affect the head, neck and forearms, since these are the sites of most intense sun exposure. The area under the chin and behind the ears is spared.
· Some disorders favour sites of skin trauma (the Koebner phenomenon) (eg vitiligo, psoriasis). The blisters of porphyria cutanea tarda affect sun-exposed skin and are induced by trauma as a result of shearing forces on the skin, which produce lesions directly.
· Rosacea occurs in the blush areas of the head and neck, as flushing is a frequent concomitant of this disorder of vascular lability.
It is useful to examine the mouth and nails either for confirmatory evidence or for clues to the disease. White streaks or patches on the buccal mucosa can be seen in lichen planus and small pits are often found in the nails in psoriasis.
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Bulla |
Similar to a vesicle but >0.5 cm in diameter. May be intraepidermal or subepidermal in their histological location. |
|
Comedo |
Plug of keratin and sebum in the opening and duct of a pilosebaceous gland (plural—comedones). Comedones may be open (blackheads) or closed (whiteheads). |
|
Cyst |
Closed cavity/sac with epithelial lining containing solids or fluids. |
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Discoid |
Disc shaped (an alternative term is nummular, from the Latin nummulus, ‘small coin’). |
|
Erythema |
Redness of the skin produced by vascular congestion or increased vascular flow. |
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Macule |
Flat alteration in colour of the skin, due to pigment (melanin or haemosiderin), or erythema from vasodilation or inflammation. If larger than several centimetres it may be referred to as a patch. |
|
Milium |
Tiny white cyst containing keratin, produced by occlusion of a pilosebaceous unit or eccrine duct. |
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Nodule |
Solid mass >0.5 cm in diameter, elevated or palpable, produced by cellular infiltration, neoplasia or deposits of fibrin, mucin or uric acid. |
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Papilloma |
Warty projection above the skin surface. |
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Papule |
Solid elevation of the skin <0.5 cm in diameter, due to hyperplasia, oedema or cellular or other infiltration. |
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Petechiae |
Pinpoint, nonraised, perfectly round, purplish red spots caused by intradermal or submucous haemorrhage. |
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Plaque |
Flat, solid, elevated lesion. May be formed by extension or coalescence of papules. A circumscribed palpable area of skin. |
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Poikiloderma |
The combination of telangiectasia, hypopigmentation, hyperpigmentation and atrophy. |
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Prurigo |
Rash composed of discrete irritable papular or nodular lesions. |
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Prurigo Papule |
Itchy dome-shaped papule with a small transient vesicle on top, followed by crusting or lichenification. |
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Purpura |
Bleeding into the dermis. May be macular or papular. |
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Pustule |
Circumscribed collection of pus (neutrophils and necrotic debris), commonly staphylococcal, but may be sterile in dermatoses, eg pustular psoriasis. |
|
Telangiectasis |
Tiny visible blood vessels (capillaries, venules or arterioles) in the upper dermis. May be accompanied by inflammation. |
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Verrucous |
Rough, warty. |
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Vesicle |
Visible accumulation of fluid within or beneath the epidermis, <0.5 cm. |
|
Wheal |
Transient area of dermal oedema, pale and compressible, which may be papular or plaque-like. |
Morphological definitions of secondary skin lesions (Table 2)
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Atrophy |
Loss of tissue from the epidermis, dermis or subcutis, characterised by loss of normal markings and pattern, often with fine wrinkling and increased translucency of the skin. |
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Callus |
Localised hypertrophy of the stratum corneum. |
|
Crust (scab) |
Dried exudate of necrotic cells, serum, fibrin and often bacteria. |
|
Erosion |
Loss of epidermis which heals without scarring. |
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Excoriation |
Loss of epidermis and sometimes dermis produced by scratching. Deep excoriations with dermal loss will heal with scarring. |
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Exfoliation |
Loss of epidermal keratin in abnormally large sheets or scales. |
|
Fissure |
Linear split in the skin, extending into the dermis. |
|
Hyperkeratosis (scale) |
Cornified cells that have become visible on the skin surface. Histological differentiation into parakeratosis (cornification with nuclear retention) and orthokeratosis (cornification without nuclear retention) can be crucial in determining aetiology. |
|
Keratoderma |
Thickening of the skin, particularly the stratum corneum. |
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Koebner (isomorphic) phenomenon |
Lesions of a dermatosis produced by and at the site of nonspecific trauma, particularly in psoriasis and lichen planus. May be an important factor in the localisation and morphology of dermatoses. |
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Lichen |
The derivation of this word is from the Greek verb ‘to lick’. The connection between this and the botanical lichens is unclear. Lichen is the term used in dermatology to signify a clinical picture resembling these flora. |
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Lichenification |
Papular or plaque- or bark-like thickening of the skin (epidermis and dermis) with accentuation of normal skin markings, hyperkeratosis and hyperpigmentation. It is caused by prolonged rubbing of the skin. |
|
Lichenoid |
Describes a rash with a clinical similarity to lichen planus, typically closely packed flat-topped papules. Also used by histopathologists to describe a histological pattern, similar to that seen in lichen planus, consisting of interface dermatitis and a band-like upper dermal lymphohistiocytic inflammation. This histology pattern may be seen in many disorders not resembling lichen planus clinically. |
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Scar |
Fibrosis, which replaces normal dermal collagen that has been destroyed by injury or disease. |
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Sclerosis |
Localised area of diffuse induration of dermal and/or subcutaneous tissues. |
|
Ulcer |
Loss of epidermis and dermis and often some subcutaneous tissue as well. Ulcers heal with scarring. |
|
Atrophy |
Loss of tissue from the epidermis, dermis or subcutis, characterised by loss of normal markings and pattern, often with fine wrinkling and increased translucency of the skin. |
|
Callus |
Localised hypertrophy of the stratum corneum. |
|
Crust (scab) |
Dried exudate of necrotic cells, serum, fibrin and often bacteria. |
|
Erosion |
Loss of epidermis which heals without scarring. |
|
Excoriation |
Loss of epidermis and sometimes dermis produced by scratching. Deep excoriations with dermal loss will heal with scarring. |
|
Exfoliation |
Loss of epidermal keratin in abnormally large sheets or scales. |
|
Fissure |
Linear split in the skin, extending into the dermis. |
|
Hyperkeratosis (scale) |
Cornified cells that have become visible on the skin surface. Histological differentiation into parakeratosis (cornification with nuclear retention) and orthokeratosis (cornification without nuclear retention) can be crucial in determining aetiology. |
|
Keratoderma |
Thickening of the skin, particularly the stratum corneum. |
|
Koebner (isomorphic) phenomenon |
Lesions of a dermatosis produced by and at the site of nonspecific trauma, particularly in psoriasis and lichen planus. May be an important factor in the localisation and morphology of dermatoses. |
|
Lichen |
The derivation of this word is from the Greek verb ‘to lick’. The connection between this and the botanical lichens is unclear. Lichen is the term used in dermatology to signify a clinical picture resembling these flora. |
|
Lichenification |
Papular or plaque- or bark-like thickening of the skin (epidermis and dermis) with accentuation of normal skin markings, hyperkeratosis and hyperpigmentation. It is caused by prolonged rubbing of the skin. |
|
Lichenoid |
Describes a rash with a clinical similarity to lichen planus, typically closely packed flat-topped papules. Also used by histopathologists to describe a histological pattern, similar to that seen in lichen planus, consisting of interface dermatitis and a band-like upper dermal lymphohistiocytic inflammation. This histology pattern may be seen in many disorders not resembling lichen planus clinically. |
|
Scar |
Fibrosis, which replaces normal dermal collagen that has been destroyed by injury or disease. |
|
Sclerosis |
Localised area of diffuse induration of dermal and/or subcutaneous tissues. |
|
Ulcer |
Loss of epidermis and dermis and often some subcutaneous tissue as well. Ulcers heal with scarring. |
Differential diagnosis by body region
A variety of anatomical, pathophysiological, biochemical and exogenous factors can influence and even determine the localisation of many skin diseases. Some diseases are limited to specific regions; others more commonly affect specific regions. This characteristic can assist in the diagnosis of a cutaneous eruption.
The following table does not serve as a complete differential diagnostic list but only as an aid to diagnosis when certain sites are involved.
|
Region |
Differential diagnosis |
|
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Common |
Less common |
|
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Face - erythematous patches |
· extensive actinic keratoses · seborrhoeic dermatitis · atopic dermatitis |
· contact dermatitis · lupus erythematosus · photodermatosis · erysipelas |
|
- pustular/papular |
· acne vulgaris · rosacea · perioral dermatitis |
· tinea faciei · impetigo |
|
- butterfly rash |
· seborrhoeic dermatitis · rosacea |
· lupus erythematosus |
|
Eyelids |
· seborrhoeic dermatitis · atopic dermatitis · contact dermatitis · rosacea |
· psoriasis · periorbital dermatitis · angioedema |
|
Scalp - erythematosquamous |
· seborrhoeic dermatitis · psoriasis · actinic keratoses |
· discoid lupus erythematosus · tinea capitis |
|
- vesicular/pustular |
· staphylococcal folliculitis · seborrhoeic dermatitis (infected) |
· acne necrotica · herpes zoster · dermatitis herpetiformis |
|
Trunk |
· drug exanthem · pityriasis versicolor · seborrhoeic dermatitis · guttate psoriasis · scabies |
· pityriasis rosea · tinea corporis · mycosis fungoides · miliaria |
|
Groin |
· intertrigo · seborrhoeic dermatitis · candidiasis · tinea cruris · psoriasis |
· contact dermatitis · erythrasma |
|
Axillae |
· intertrigo · seborrhoeic dermatitis · contact dermatitis |
· candidiasis · acanthosis nigricans · tinea · psoriasis |
|
Upper limbs |
· atopic dermatitis · contact dermatitis · psoriasis · actinic keratoses · photodermatitis |
· polymorphous light eruption · lichen planus · lupus erythematosus |
|
Hands |
· actinic keratoses · pompholyx · scabies · granuloma annulare · contact dermatitis · psoriasis |
· lichen planus · lupus erythematosus · tinea · palmar pustulosis · erythema multiforme |
|
Nails |
· psoriasis · tinea unguium · paronychia · dermatitis |
|
|
Genitalia |
· seborrhoeic dermatitis · scabies · candidiasis · psoriasis · genital herpes · genital warts |
· lichen planus · fixed drug eruption · syphilis · contact dermatitis · lichen sclerosus |
|
Natal cleft |
· intertrigo · candidiasis · tinea · seborrhoeic dermatitis · psoriasis · scabies |
|
|
Submammary |
· intertrigo · candidiasis · seborrhoeic dermatitis |
· psoriasis |
|
Lower legs |
· gravitational dermatitis · contact dermatitis · folliculitis · psoriasis · atopic dermatitis · lichen simplex chronicus |
· vasculitis · necrobiosis lipoidica · erythema nodosum · insect bites · lipodermatosclerosis · livedo reticularis · pretibial myxoedema |
|
Feet |
· tinea · psoriasis · pompholyx · contact dermatitis |
· juvenile forefoot dermatitis · pitted keratolysis |
|
Generalised rash |
· atopic dermatitis · drug eruptions · exanthem |
· psoriasis · mycosis fungoides |
