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A drug hypersensitivity reaction is an adverse, allergic response
to an ingested or parenterally administered drug characterized by
a cutaneous eruption.
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There are several different immune mechanisms thought to play
a role:
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1. Type I: IgE-dependent drug reactions urticaria,
angioedema, and anaphylaxis.
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2. Type II: Cytotoxic drug-induced
reactions petechiae from drug-induced thrombocytopenia.
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3. Type III: Immune complex-mediated
drug reactions vasculitis, serum sickness, urticaria.
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4. Type IV: Delayed-type, cell-mediated
drug reactions exanthematous, fixed drug eruptions, Steven–Johnson
syndrome (SJS), and toxic epidermal necrolysis (TEN).
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Exanthematous
Drug Reaction
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An exanthematous drug reaction is an adverse, allergic response
to an ingested or parenterally administered drug characterized by
a morbilliform cutaneous eruption that mimics a viral exanthem.
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In the appropriate clinical setting, an exanthematous drug reaction,
a viral exanthem, and acute graft-vs-host disease (GVHD) are both
clinically and histologically indistinguishable.
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Synonyms Morbilliform drug eruption,
maculopapular drug eruption, drug rash.
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Age Children < adolescents < adults.
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Incidence 1% of population
on a systemic medication.
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Etiology Drugs with high probability
of reaction (10%–20%): penicillin and
related antibiotics, carbamazepine, allopurinol, gold salts. Medium
probability: sulfonamides (bacteriostatic, antidiabetic, diuretic),
nitrofurantoin, hydantoin derivatives, isoniazid, chloramphenicol,
erythromycin, streptomycin. Low probability (1% or less):
pyrazolone derivatives, barbiturates, benzodiazepines, phenothiazines,
tetracyclines.
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Exanthematous drug hypersensitivity reactions are likely type
IV, cell-mediated immune responses. Viral infections may increase
the incidence (e.g., aminopenicillin causes a morbilliform rash
in 100% of patients concurrently infected with EBV).
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The exanthematous rash typically appears 7 to 14 days (peak incidence
ninth day) after drug administration; however, skin lesions can
appear anytime between day 1 though 21 after drug exposure. The
rash starts on the trunk and spreads to the face and extremities.
It can be quite pruritic and distressing. Fever and malaise may
or may not be present.
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Type of Lesion Macules, papules,
plaques (Fig. 15-1).
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Color Pink/red to purple/brown.
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Distribution of Lesions Trunk,
spreads to face and extremities. Confluent in intertriginous areas
(axilla, groin, inframammary area). Palms and soles may be involved.
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Mucous Membranes ± Exanthem on
buccal mucosa.
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Differential
Diagnosis
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