Eritema Multiforme

01Pathophysiology

Erythema multiforme is an acute, immune-mediated mucocutaneous reaction driven primarily by a cell-mediated (Type IV) hypersensitivity response against foreign antigens deposited in the skin. Understanding the mechanism unlocks why the disease presents the way it does and why it recurs.

In the most common scenario, herpes simplex virus (HSV) reactivates and viral DNA fragments are transported by circulating cells to the skin, where they are deposited in keratinocytes. These keratinocytes then express HSV DNA polymerase gene fragments on their surface. CD4+ T-helper type 1 cells recognize these viral antigens, release interferon-gamma, and recruit cytotoxic CD8+ T cells that attack the antigen-expressing keratinocytes. This creates the hallmark histologic finding of interface dermatitis: lymphocytes lining up along the dermal-epidermal junction and destroying individual keratinocytes through apoptosis (so-called necrotic or apoptotic keratinocytes, also described as Civatte bodies).

The target lesion forms because of this radial gradient of immune-mediated damage. The central zone undergoes the most destruction, with epidermal necrosis and sometimes vesiculation or bullae formation. The intermediate pale zone represents edema from vascular leakage. The peripheral erythematous ring results from vasodilation and the inflammatory infiltrate spreading outward. This three-zone architecture is the pathognomonic finding of EM and distinguishes it from the flat, atypical targetoid macules of SJS/TEN.

The acral predilection (hands, feet, extensor surfaces) is thought to relate to the lower skin temperature at these sites, which may favor deposition and expression of viral antigens. This distribution pattern is a consistent and testable feature.

In Mycoplasma pneumoniae-associated cases, the pathogenesis involves immune complex deposition and cross-reactive antibodies against mucosal epithelial antigens, which is why Mycoplasma-associated EM tends to show more prominent mucosal involvement relative to skin findings. This distinct phenotype is sometimes referred to as Mycoplasma-induced rash and mucositis (MIRM).

02Classification and Clinical Manifestation

Additional clinical details tested in vignettes:

A critical distinction to internalize: true target lesions (raised, three-zone, acral) point toward EM, while flat, atypical targetoid macules (two-zone, trunk-predominant) point toward SJS/TEN. This morphologic difference is the single most tested discriminator between these entities.

03Diagnostic Workup

Erythema multiforme is fundamentally a clinical diagnosis. The best initial step when a patient presents with an acute symmetric eruption of the extremities is a thorough physical examination focusing on lesion morphology. If you identify raised, three-zone target lesions in an acral distribution, EM is your working diagnosis.

Skin biopsy is the most accurate confirmatory test and should be pursued when the morphology is atypical, the distribution is unusual, or you need to distinguish EM from other conditions such as SJS, urticarial vasculitis, or autoimmune blistering diseases. The biopsy should be taken from the edge of an active lesion and sent for both routine histopathology and direct immunofluorescence. Histopathology will show the characteristic pattern of interface dermatitis with a band-like (lichenoid) lymphocytic infiltrate at the dermal-epidermal junction, scattered necrotic keratinocytes, and papillary dermal edema. DIF in EM is nondiagnostic (granular or negative), which helps exclude bullous pemphigoid (linear IgG/C3 at the basement membrane zone) and pemphigus vulgaris (intercellular IgG in the epidermis).

Once the diagnosis of EM is established, the next step is to identify the underlying trigger. For suspected HSV, order HSV PCR from an active herpetic lesion if one is present, or from lesional skin biopsy tissue. HSV serology can support prior exposure but does not confirm causation for the current episode. For patients with a preceding cough, fever, or respiratory symptoms, obtain Mycoplasma IgM antibodies, cold agglutinins, or Mycoplasma PCR along with a chest radiograph.

04Management & Treatment

The cornerstone of management for EM Minor is supportive care, as the condition is self-limited and resolves without scarring in most cases. Topical corticosteroids help reduce local inflammation, and oral antihistamines (cetirizine 10 mg daily or hydroxyzine 25 mg TID) manage pruritus. No systemic therapy is required for uncomplicated EM Minor.

The most important management concept for the exam is the treatment of recurrent EM. When a patient has three or more episodes per year, the next best step is to initiate continuous oral antiviral prophylaxis with acyclovir 400 mg PO BID or valacyclovir 500 mg PO BID for 6 to 12 months. This is effective even when there is no clinically apparent HSV outbreak preceding each EM episode, because subclinical HSV reactivation is the trigger. The prophylaxis must be continuous, not episodic; starting antivirals after EM lesions have already appeared does not abort the immunologic cascade that has already been set in motion.

For EM Major with painful oral erosions, the priority is maintaining hydration and nutrition. Use a combination of oral antiseptic rinses, viscous lidocaine (swish and spit), and a bland diet. Consider a short course of systemic corticosteroids if mucosal involvement is severe and impairing oral intake, but be aware that the evidence base for steroids in EM is limited.

Contraindications and cautions: Acyclovir and valacyclovir require dose adjustment in renal insufficiency (). In patients who cannot tolerate antivirals, or in whom prophylaxis fails, immunomodulatory agents such as dapsone or mycophenolate mofetil may be considered under dermatology guidance. Dapsone requires checking G6PD levels before initiation to avoid hemolytic anemia.

05Differential Diagnosis & Distractors

06Traps & High-Yield Pearls

The most common way students get EM questions wrong is by confusing erythema multiforme with Stevens-Johnson syndrome. Test writers exploit this by presenting a young patient with "target lesions" and mucosal involvement, then offering both EM Major and SJS as answer choices. The discriminator is always in the details of the lesion morphology and distribution. If the vignette describes raised, three-zone target lesions on the hands, palms, or soles in a patient with a recent cold sore or upper respiratory infection, the answer is EM. If the vignette describes flat, dusky macules or atypical targetoid lesions on the trunk in a patient who recently started a new medication (sulfonamides, NSAIDs, anticonvulsants, allopurinol), the answer is SJS.

Another frequent trap involves the management of recurrent EM. Students often select "start acyclovir at the onset of the next EM episode" instead of the correct answer, which is continuous antiviral prophylaxis. Episodic treatment initiated after target lesions appear does not work because the immune cascade is already underway by the time skin lesions become visible. The correct answer is always prophylactic daily antivirals for 6 to 12 months.

A third tested concept is the association between EM and its triggers. Vignettes will embed clues like "vesicular lesion on the lip one week ago" (HSV) or "nonproductive cough with patchy infiltrate" (Mycoplasma). Missing these embedded clues leads students to select the wrong etiology or the wrong treatment. Always scan the vignette for an infectious prodrome before the skin eruption appeared.

The core competency being tested with EM is your ability to distinguish it from SJS/TEN based on lesion morphology, distribution, and trigger, and to recognize HSV as the predominant cause requiring antiviral prophylaxis for recurrent disease.