We must begin with a note on terminology. There is confusion because the term erythema multiforme (EM) describes a spectrum of disease whose boundaries seem to shift by author. Authors are either “splitters” or “lumpers.”

Splitters divide the EM spectrum into several entities, beginning with mild EM and severe EM. Mild EM has mild skin involvement and sometimes mild mucosal involvement, usually the mouth. Severe EM has more advanced skin involvement and prominent mucosal involvement, including the mouth, labia, and conjunctiva. To confuse things further, some splitters label the severe form of EM the Stevens–Johnson syndrome (SJS). Furthermore, toxic epidermal necrolysis (TEN), with its massive skin sloughing, is considered in this scheme to be yet another, separate entity.

Lumpers divide the spectrum into only two groups, EM and TEN/SJS. For this book, we will use the simpler scheme of the lumpers (see Chapter 31).

The symptoms of EM may be minimal—a feeling of warmth or pruritus of involved skin—or severe. Bullous lesions may be very painful. Pain on the palms is a good clue to the diagnosis. The primary sign of EM is erythema; the classic lesions are targets (Fig. 16-1; see also Fig. 31-2). Target (or iris) lesions are round with a central dusky red bull’s-eye surrounded by edematous concentric pink rings. EM may also be arcuate like common
urticaria or bullous (Fig. 16-2). The distribution and extent of involvement can be variable. However, EM usually begins as a symmetric eruption on the extensor extremities and may progress to trunk or facial involvement.