Complex reactions occurring in the capillaries and arterioles of the skin cause erythema, which is simply redness of the skin.
This may present as flat macules or as papules, which are raised above the surrounding skin.
The lesions may be transient or last for weeks, constant or variable in distribution, with or without vesicles.
It is possible to recognise specific patterns within this plethora of clinical signs, but even the most experienced dermatologist may be reduced to making a general diagnosis of “toxic” erythema.
The best we can do therefore is to recognise the common types of erythema and list the possible causes.
It is then a matter of deciding on the most likely underlying condition or group of conditions for example, bacterial infection or autoimmune systemic disease.
Morphology and distribution
Because there can be the same cause for a variety of erythematous rashes detailed descriptions are of limited use.
None the less, there are some characteristic patterns.
Morbilliform
The presentation of measles is well known, with the appearance of Koplik’s spots on the mucosa, photophobia with conjunctivitis, and red macules behind the ears, spreading to the face, trunk, and limbs.
The prodromal symptoms and conjunctivitis are absent in drug eruptions.
Other viral conditions, including those caused by echoviruses, rubella, infectious mononucleosis, and erythema infectiosum, may have to be considered. Scarlatiniform
These rashes are similar to that in scarlet fever, when an acute erythematous eruption occurs in relation to a streptococcal infection.
Characteristically erythema is widespread on the trunk.
There is intense erythema and engorgement of the pharyngeal lymphoid tissue with an exudate and a “strawberry” tongue.
Bacterial infections can produce a similar rash, as can drug rashes, without the systemic symptoms.
Figurate erythemas
These are chronic erythematous rashes forming annular or serpiginous patterns.
There may be underlying malignancy or connective tissue disease.
Erythema multiforme
Erythema multiforme is sometimes misdiagnosed because of the variety of lesions and number of possible precipitating causes; some of these are listed in the box on the right.
Clinical picture
The usual erythematous lesions occur in crops on the limbs and trunk.
Each lesion may extend, leaving a cyanotic centre, which produces an “iris” or “target” lesion.
Bullae may develop in the lesions and on the mucous membranes.
A severe bullous form, with lesions on the mucous membranes, is known as the Stevens–Johnson syndrome.
There may be neural and bronchial changes as well.
Barbiturates, sulphonamides, and other drugs, are the most common cause.
Histologically there are inflammatory changes, vasodilatation, and degeneration of the epidermis.
A condition that may be confused is Sweet’s syndrome, which presents as acute plum coloured raised painful lesions on the limbs sometimes the face and neck with fever.
It is more common in women.
The alternative name, “acute febrile neutrophilic dermatosis”, describes the presentation and the pathological findings of a florid neutrophilic infiltrate.
There is often a preceding upper respiratory infection.
Treatment with steroids produces a rapid response but recurrences are common.
Erythema induratum
Erythema induraturn occurs on the lower legs posteriorly, usually in women, with diffuse, indurated dusky red lesions that may ulcerate.
It is more common in patients with poor cutaneous circulation.
Epithelioid cell granulomas may form.
This erythema was originally described in association with tuberculous infection elsewhere in the body (Bazin’s disease).
It represents a vasculitic reaction to the infection, and when there is no tuberculous infection another chronic infection may be responsible.
Erythema nodosum
Erythema nodosum occurs as firm, gradually developing lesions, predominantly on the extensor aspect of the legs.
They are tender and progress over four to eight weeks from an acute erythematous stage to residual lesions resembling bruises.
Single or multiple lesions occur, varying in size from 1 to 5 cm.
The lesions are often preceded by an upper respiratory tract infection and may be associated with fever and arthralgia.
Infections (streptococcal, tuberculous, viral, and fungal) and sarcoidosis are the commonest underlying conditions.
Drugs can precipitate erythema nodosum, the contraceptive pill and the sulphonamides being the commonest cause.
Ulcerative colitis, Crohn’s disease, and lymphoma may also be associated with the condition.
Rashes due to drugs
There is an almost infinite variety of types of drug reaction.
External contact with drugs can cause a contact dermatitis presenting with eczematous changes.
This occurs commonly with neomycin and bacitracin.
Chloramphenicol and sulphonamides from ophthalmic preparations can also cause dermatitis around the eyes.
Penicillin is a potent sensitiser so is not used for topical treatment.
Drugs used systemically can cause a localised fixed drug eruption or a more diffuse macular or papular erythema, symmetrically distributed.
In the later stages exfoliation, with shedding scales of skin, may develop. Antibiotics, particularly penicillins, are the most common cause.
They also cause erythema multiforme as already mentioned.
Penicillins are the most common cause of drug rashes, which range from acute anaphylaxis to persistent diffuse erythematous lesions.
Joint pains, fever, and proteinuria may be associated, as in serum sickness.
Ampicillin often produces a characteristic erythematous maculopapular rash on the limbs seven to 20 days after the start of treatment.
Such rashes occur in nearly all patients with infectious mononucleosis who are given ampicillin. Vasculitis
Inflammation associated with immune complexes in the capillaries and small blood vessels is part of the pathological changes of many of the conditions described above.
The term vasculitis is also used clinically to describe a variable clinical picture with red macules and papules and with necrosis and bruising in severe cases.
In children purpura is more prominent and these cases are classified as Henoch–Schönlein purpura.
The legs and arms are usually affected. Skin signs are preceded by malaise and fever with arthropathy and there may be associated urticaria.
As a high proportion of cases are associated with systemic lesions, it is essential to check for renal, skeletal, gastrointestinal, and central nervous system disease.
In children with Henoch–Schönlein purpura nephritis is common.
Urticaria
In this condition itching red weals develop; they resemble the effects of stinging nettle (Urtica dioica) on the skin.
The condition may be associated with allergic reactions, infection, or physical stimuli, but in most patients no cause can be found.
Similar lesions may precede, or be associated with, vasculitis (urticarial vasculitis), pemphigoid, or dermatitis herpetiformis.
The histological changes may be very slight but usually there is oedema, vasodilatation, and a cellular infiltrate of lymphocytes, polymorphs, and histiocytes.
Various vasoactive substances are thought to be involved, including histamine, kinins, leukotrienes, prostaglandins, and complement.
Angio-oedema is due to oedema of the subcutaneous tissues; it can occur rapidly and may involve the mucous membranes.
Hereditary angio-oedema is a rare form with recurrent severe episodes of subcutaneous oedema, swelling of the mucous membranes, and systemic symptoms.
Laryngeal oedema is the most serious complication.
The physical urticarias, which account for about 25% of cases, include dermatographism and the pressure, cold, heat, solar, cholinergic, and aquagenic urticarias.
Dermatographism is an exaggerated release of histamine from stroking the skin firmly with a hard object, such as the end of a pencil.
Pressure urticaria is caused by sustained pressure from clothing, hard seats, and footwear; it may last some hours.
Cold urticaria varies in severity and is induced by cold, particuarly by cold winds or by the severe shock of bathing in cold water.
It appears early in life in infancy in the rare familial form.
In a few cases abnormal serum proteins may be found. Heat urticaria is rare, but warm environments often make physical urticaria worse. Solar urticaria is a rare condition in which sunlight causes an acute urticarial eruption.
Tolerance to sun exposure may develop in areas of the body normally exposed to sun.
There is sensitivity to a wide spectrum of ultraviolet light.
Cholinergic urticaria is characterised by the onset of itching urticarial papules after exertion, stress, or exposure to heat.
The injection of cholinergic drugs induces similar lesions in some patients.
Aquagenic urticaria occurs on contact with water, regardless of the temperature.
Non-physical urticaria may be acute in association with allergic reactions to insect bites, drugs, and other factors.
Chronic recurrent urticaria is fairly common.
Innumerable causes have been suggested but, to the frustration of patient and doctor alike, it is often impossible to identify any specific factor.
