Blisters are skin swellings containing free fluid. Up to 5mm they are called vesicles, larger than 5mm they are called bullae. The fluid can be lymph, serum, extracellular fluid or blood. Some conditions cause both kinds of blister, but others mainly one or other type. Pustules are dealt with elsewhere (see ‘Purpura and petechiae’).
Erythema is a reddening of the skin due to persistent dilation of superficial blood vessels, and can be local or generalised. It is distinguished from flushing (see ‘Flushing’) by its permanence: flushing is transient.
A macule is a flat, demarcated, abnormally coloured area of skin of any size. It may be red (e.g. drug eruption), dark red (e.g. purpura), brown (e.g. a flat mole) or white (e.g. pityriasis versicolor). Purpura is described elsewhere (see ‘Purpura and petechiae’). There is some crossover between erythema (see ‘Erythema’) and red macules.
Skin nodules are larger than papules – more than 5mm diameter. However, their depth is more significant clinically than their width. Some are free within the dermis; others are fixed to skin above or subcutaneous tissue below. The causes are various; the patient is usually concerned about the cosmetic appearance or malignant potential.
Papules are solid, circumscribed skin elevations up to 5 mm in diameter. If they are larger, they are called nodules – these are dealt with elsewhere (see ‘Nodules’). (Clearly, many nodules start life as a papule; to avoid confusion, if they are generally ‘nodular’ by the time they present , then they are dealt with in that section, and not repeated here.) They are usually round but the shape, and the colour, may vary. They may be transitional lesions, e.g. becoming vesicular, or about to ulcerate. NOTE: There are more causes of papules than can be listed here. This is a sensible selection.
Purpura are reddish-purple lesions which do not blanch with pressure. When less than 1 cm in diameter, they are called petechiae; if larger, they are known as ecchymoses. The problem often presents as ‘bruising easily’ and is a common ‘while I’m here’ complaint in primary care. Most cases are normal, with causative minor trauma simply being forgotten or unnoticed.
Pustules are raised lesions less than 0.5 cm in diameter containing a yellow fluid. They signify infection to most people, and will often present in an urgent appointment, as they are likely to have appeared suddenly. Patients will often expect antibiotic treatment. This will not always be necessary, so be prepared to offer a clear explanation and an appropriate alternative.
Skin scales represent an abnormally fast piling up of keratinised epithelium. Scales and plaques are common at all ages and have a variety of causes. The presentation will centre on cosmetic appearance, itching, fears about serious disease or a combination of these.
Given that this type of rash affects inaccessible and uninviting nooks and crannies of the body, it may be dealt with in a cursory way. In most cases, that won’t be a problem, as the diagnosis may be clear. The differential is wider than many imagine, though, and the symptoms troublesome or painful enough to cause the patient real distress.