Erythema multiforme occurs chiefly in infants and younger patients, appearing symmetrically on the extensor surfaces of the upper, more rarely of the lower, extremities. The first lesions consist of patches and papules which are at first red but later turn bluish red in the center. The bluish red centers often turn into blisters. The lesions may coalesce; in other cases a new bright red margin develops around the old one after the latter has turned brownish or bluish, resulting in lesions of concentric rings.

In some cases vesiculation is pronounced and affects chiefly the oral or genital mucous membranes ; this form has been differentiated as stevens-johnson syndrome, but is probably only a very severe form of erythema multiforme, with high fever. The skin changes are often accompanied by articular pains. Idiopathic erythema multiforme may recur, especially in the spring and autumn.

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Etiology
The etiology is not clear. Some of the changes are attributed to the herpes simplex virus, other are obviously associated with other virus diseases.

Differential Diagnosis
The differential diagnosis should exclude other bullous conditions, especially drug eruptions and  Lyell's syndrome. Unfortunately, nondermatologists sometimes diagnose erythema multiforme evem where the morphological criteria have not been met. Lesions resembling those of erythema multiforme may occur in virus infections, mycoplasma infections, histoplasmosis, syphilis, bacterial infections including typhus, diphteria and focal sepsis; after radiation treatment, in lupus erythematosus, in polyarteriitis nodosa and even in pregnancy

Complications
Complications may consist of involvement of other mucosae and organs, including the gastrointestinal tract,eyes, bronchi and lungs. The most serious complications include ulceration of the cornea, uveitis and panophthalmia in the serious forms, and scarring and adhesions, especially in the region of the eyelids.

Treatment
Serious forms may require internal administration of corticoids equivalent to 30 mg prednisone or over, possibly infusions, and control of the electrolytes.