Post penicillin rash (388/1700)

A 7yo girl has been treated with penicillin after sore throat, fever and cough. Then she develops
skin rash and itching. What is the most probable dx?

a. Erythema nodosum
b. Erythema multiforme
c. SJS
d. Erythema marginatum
e. Erythema gangernosum

answer: B (most likely triggered by her lung infection not antibiotics, no prodromal stage in erythema multiforme minor…SJS would have a prodromal flu like stage before skin eruptions)

Erythema multiforme is a hypersensitivity reaction usually triggered by infections, most commonly herpes simplex virus (HSV). It presents with a skin eruption characterised by a typical target lesion. There may be mucous membrane involvement. It is acute and self-limiting, usually resolving without complications.

What triggers erythema multiforme?
Infections are probably associated with at least 90% of cases of erythema multiforme.
The single most common trigger for developing erythema multiforme is herpes simplex virus (HSV) infection, usually herpes labialis (cold sore on the lip) and less often genital herpes. HSV type 1 is more commonly associated than type 2. The herpes infection usually precedes the skin eruption by 3–14 days.
Mycoplasma pneumonia (a lung infection caused by the bacteria Mycoplasma pneumoniae) is the next most common trigger.

Many different virus infections have been reported to trigger erythema multiforme including:

Parapoxvirus (orf and milkers' nodules)
Herpes varicella zoster (chickenpox, shingles)
Hepatitis viruses
Human immunodeficiency virus (HIV)
Viral vaccines

Medications are probably an uncommon cause (<10%) of erythema multiforme.

There are usually no prodromal symptoms in erythema multiforme minor. However, erythema multiforme major may be preceded by mild symptoms such as fever or chills, weakness or painful joints

SJS usually develops within the first week of antibiotic therapy but up to 2 months after starting an anticonvulsant. For most drugs the onset is within a few days up to 1 month.

Before the rash appears, there is usually a prodromal illness of several days’ duration resembling an upper respiratory tract infection or 'flu-like illness. Symptoms may include:

Fever > 39 C
Sore throat, difficulty swallowing
Runny nose and cough
Sore red eyes, conjunctivitis
General aches and pains.

There is then an abrupt onset of a tender/painful red skin rash starting on the trunk and extending rapidly over hours to days onto the face and limbs (but rarely affecting scalp, palms or soles). The maximum extent is usually reached by 4 days.

The skin lesions may be:

Macules – flat, red and diffuse (measles-like spots) or purple (purpuric) spots
Diffuse erythema
Targetoid – as in erythema multiforme
Blisters – flaccid (ie not tense)

The blisters then merge to form sheets of skin detachment, exposing red, oozing dermis. The Nikolsky sign is positive in areas of skin redness. This means that blisters and erosions appear when the skin is rubbed gently.