What are serum-sickness-like reactions and Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS) syndrome?

Serum Sickness-Like Reactions and DRESS Syndrome: Definitions and Clinical Features

Serum Sickness-Like Reactions

Serum sickness-like reactions are immune-mediated hypersensitivity responses that mimic classic serum sickness but occur without immune complex formation. These reactions typically present with fever, rash, and arthralgias developing 1-3 weeks after drug exposure 1. Unlike true serum sickness (which involves antibodies to foreign proteins), serum sickness-like reactions are triggered by medications and represent a distinct clinical entity.

Key distinguishing features include:

• Onset typically 7-21 days after drug initiation
• Fever, urticarial or maculopapular rash, and joint pain
• Generally self-limited after drug discontinuation
• Less severe systemic involvement compared to DRESS
• No eosinophilia or significant organ dysfunction

DRESS Syndrome (Drug Reaction with Eosinophilia and Systemic Symptoms)

DRESS syndrome is a severe, potentially life-threatening T-cell-mediated hypersensitivity reaction characterized by fever, widespread rash with facial edema, eosinophilia, atypical lymphocytosis, and multiorgan involvement 2. The mortality rate reaches up to 10%, often related to unrecognized myocarditis and cytomegalovirus complications 2.

Clinical Presentation

The hallmark features of DRESS include:

• Cutaneous manifestations: Widespread maculopapular rash with facial edema (though rare cases can present without skin involvement) 3
• Fever: Persistent high-grade fever
• Hematologic abnormalities:
  • Eosinophilia (often >1,500 cells/μL)
  • Atypical lymphocytosis
• Multiorgan involvement: Hepatitis (most common), nephritis, pneumonitis, myocarditis, lymphadenopathy, hepatosplenomegaly 2, 4

Timing and Pathogenesis

DRESS typically develops 2-8 weeks after drug initiation 2, 5, distinguishing it from immediate hypersensitivity reactions. The majority of delayed T-cell-mediated reactions occur early in the second week after starting therapy 6.

Critical pathogenic features:

• T-cell-mediated delayed hypersensitivity reaction
• Associated with reactivation of human herpesviruses (HHV-6, EBV, CMV) 2
• Relapses occur 2-4 weeks after acute symptoms, often coinciding with viral reactivation 2
• Strong HLA associations exist (HLA-B13:01 with dapsone, HLA-B58:01 with allopurinol, HLA-B*32:01 with vancomycin) 2

Common Culprit Drugs

The most frequently implicated medications include:

• Aromatic anticonvulsants (phenytoin, carbamazepine, phenobarbital) 7
• Allopurinol
• Sulfonamides (dapsone, sulfasalazine) 3
• Minocycline 1, 5
• Vancomycin 5
• Lamotrigine

Diagnostic Approach

Use the RegiSCAR (Registry of Severe Cutaneous Adverse Reactions) scoring system to establish the diagnosis 6, 7. An online calculator is available for standardized assessment 6.

Essential diagnostic elements:

1. Complete medication history covering the 8 weeks preceding symptom onset 2
2. Peripheral blood smear showing atypical lymphocytes and eosinophilia
3. Liver function tests, renal function, and cardiac enzymes
4. Skin biopsy when diagnosis is uncertain 6
5. Bone marrow examination may show increased eosinophil precursors 3

Critical pitfall: Atypical lymphocytes can mimic viral infection or hematological malignancy, potentially delaying diagnosis 3. The temporal relationship with drug initiation is paramount.

Long-Term Complications

DRESS can lead to serious sequelae that require ongoing monitoring:

• Autoimmune diseases (particularly thyroiditis) 2
• Chronic organ dysfunction from initial injury 8
• Persistent or relapsing disease course even after drug withdrawal 8
• CMV reactivation complications requiring specific surveillance 2

Testing Considerations

For diagnostic testing after DRESS resolution 6:

• Wait at least 6 months after complete healing before performing patch testing or delayed intradermal testing
• Wait at least 4 weeks after discontinuing systemic corticosteroids (>10 mg prednisone equivalent)
• Patch testing sensitivity for DRESS is approximately 64% 6
• Never rechallenge patients with the culprit drug, even with negative skin testing 6

Key distinction from serum sickness-like reactions: DRESS requires prolonged follow-up due to risk of viral reactivation, relapse (12% incidence), and development of autoimmune complications 6, 8, whereas serum sickness-like reactions typically resolve completely after drug discontinuation without long-term sequelae.