What type of rash is characteristic of measles?

Measles Rash Characteristics

The measles rash is a maculopapular eruption that begins on the face and spreads cephalocaudally (downward) to the trunk and extremities, appearing 2-4 days after fever onset and following the pathognomonic prodrome of fever, cough, coryza, and conjunctivitis. 1, 2

Rash Morphology and Evolution

The rash consists of flat red macules that evolve into raised papules (maculopapular), but critically never progresses to vesicles or bullae—a key distinguishing feature from other viral exanthems. 2 The lesions begin as small, blanching, pink macules that become more confluent as they spread. 3

• The rash starts on the face and becomes increasingly confluent as it spreads in a cephalocaudal pattern (head to toe). 3
• Initial lesions appear on the face, then progress to the trunk and extremities over several days. 1
• The face is typically involved, unlike Rocky Mountain spotted fever where facial involvement is usually limited. 4

Temporal Relationship to Other Symptoms

The rash appears after a characteristic prodromal phase that is essential for clinical recognition:

• Prodrome precedes rash by 2-4 days and consists of fever, cough, coryza (runny nose), and conjunctivitis ("the three Cs"). 1, 5
• Koplik spots (small white spots on a red background inside the mouth) appear during the prodrome and are pathognomonic for measles, helping distinguish it from other causes of maculopapular rash. 1, 2
• The incubation period averages 10-12 days from exposure to prodrome and 14 days from exposure to rash. 1

Critical Distinction from Vesicular Rashes

A common pitfall is confusing measles with varicella (chickenpox) or other vesicular exanthems. The presence of vesicles essentially rules out measles and should prompt consideration of varicella, herpes zoster, rickettsial infections, or other vesicular diseases. 2

• Varicella has a characteristic vesicular rash with lesions in different stages of development (macules, papules, vesicles, crusts simultaneously). 2
• Measles remains maculopapular throughout its course without vesiculation. 2
• This distinction is critical for infection control, as varicella vesicular fluid is highly contagious. 2

Pathophysiology of the Rash

The rash represents the manifestation of the adaptive immune response rather than direct viral cytopathic effect:

• The rash coincides with MeV-specific type 1 CD4+ and CD8+ T cell responses with lymphocyte infiltration into tissue sites of viral replication. 6
• Infection begins in dermal lymphoid and myeloid cells, then disseminates to epidermal leukocytes and keratinocytes before rash onset. 7
• The rash appearance correlates with clearance of infectious virus, though viral RNA persists for weeks to months. 6
• Hyperemia and edema from immune-mediated clearance of infected cells create the visible morbilliform appearance. 7