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Approach to diagnosis

A skin rash generally poses more problems in diagnosis than a single, well defined skin lesion such as a wart or tumour. As in all branches of medicine a reasonable diagnosis is more likely to be reached by thinking firstly in terms of broad diagnostic categories rather than specific conditions.

There may have been previous episodes because it is a constitutional condition, such as atopic eczema. In the case of contact dermatitis, regular exposure to a causative agent leads to recurrences that fit with the times of exposure and this is usually apparent from the history. Endogenous conditions such as psoriasis can appear in adults who have had no previous episodes. If there is no family history and several members of the household are affected, a contagious condition, such as scabies, should be considered. A common condition with a familial tendency, such as atopic eczema, may affect several family members at different times.

A simplistic approach to rashes is to clarify them as being from "inside" or "outside". Examples of "inside" or endogenous rashes are atopic eczema or drug rashes, whereas fungal infection or contact dermatitis are "outside" rashes.


Most endogenous rashes affect both sides of the body, as in the atopic child or a man with psoriasis on his knees. Of course, not all exogenous rashes are asymmetrical. A seamstress who uses scissors in her right hand may develop an allergy to metal in this one hand, but a hairdresser or nurse can develop contact dermatitis on both hands.

Diagnosis of rash

• Previous episodes of the rash, particularly in childhood, suggest a constitutional condition such as atopic eczema

• Recurrences of the rash, particularly in specific situations, suggests a contact dermatitis. Similarly a rash that only occurs in the summer months may well have a photosensitive basis

• If other members of the family are affected, particularly without any previous history, there may well be a transmissible condition such as scabies

Contact dermatitis as a response to Irritant dermatitis mascara


It is useful to be aware of the usual sites of common skin conditions. These are shown in the appropriate chapters. Eruptions that appear only on areas exposed to sun may be entirely or partially due to sunlight. Some are due to a sensitivity to sunlight alone, such as polymorphous light eruption, or a photosensitive allergy to topically applied substances or drugs taken internally.


The appearance of the skin lesion may give clues to the underlying pathological process.

The surface may consist of normal epidermis overlying a lesion in the deeper tissues. This is characteristic of many types of erythema in which there is dilatation of the dermal blood vessels associated with inflammation. The skin overlying cysts or tumours in the dermis and deeper tissues is usually normal. Conditions affecting the epidermis will produce several visible changes such as thickening of the keratin layer and scales in psoriasis or a more uniform thickening of the epidermis in areas lichenified by rubbing. An eczematous process is characterised by small vesicles in the epidermis with crusting or fine scaling.

The margin of some lesions is very well defined, as in psoriasis or lichen planus, but in eczema it merges into normal skin.

Blisters or vesicles occur as a result of (a) oedema between the epidermal cells or (b) destruction of epidermal cells or (c) the result of separation of the epidermis from the deeper tissues. Of course, more than one mechanism may occur in the same lesion. Oedema within the epidermis is seen in endogenous eczema, although it may not be apparent clinically, particularly if it is overshadowed by inflammation and crusts. It is also a feature of contact dermatitis.

Allergic reaction producing photosensitivity

Lesion in deeper tissue with normal epidermis

Small vesicles of eczema

Lesion in deeper tissue with normal epidermis

Small vesicles of eczema

Blisters occur in:

viral diseases such as chickenpox, hand, foot and mouth disease, and herpes simplex bacterial infections such as impetigo eczema and contact dermatitis primary blistering disorders such as dermatitis herpetiformis, pemphigus and pemphigoid as well as metabolic disorders such as porphyria.

Herpes simplex






Bullae, blisters over 0.5 cm in diameter, may occur in congenital conditions (such as epidermolysis bullosa), lichen planus, and pemphigoid without much inflammation. However, those forming as a result of vasculitis, sunburn, or an allergic reaction may be associated with pronounced inflammation. In pustular psoriasis there are deeper pustules, which contain polymorphs but are sterile and show little inflammation. Drug rashes can appear as a bullous eruption.

Induration is thickening of the skin due to infiltration of cells, granuloma formation, or deposits of mucin, fat, or amyloid.

Inflammation is indicated by erythema, which may be accompanied by increased temperature if acute—for example, in cellulitis or erythema nodosum. There may be a chronic inflammatory infiltrate in, for example, conditions such as lichen planus or lupus erythematosus.

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Curing Eczema Naturally

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