Atypical exanthems: morphology and laboratory investigations may lead to an aetiological diagnosis in about 70% of cases

Background Besides the six classical exanthems, other exanthems may occur, differing in morphology and causative agent (atypical exanthems). Their aetiological diagnosis is difficult but crucial for both the patient and community concerning issues such as time off school, immunizations, and risks for pregnant women and immunocompromised patients. Objectives To investigate whether morphology, associated symptoms and laboratory results can help to determine the aetiology of atypical exanthems. Methods We studied 112 consecutive out-patients attending two university dermatology departments. Peripheral blood mononuclear cells (PBMC) and throat, rectal and vaginal swabs were studied to identify viral and bacterial growth. Nested polymerase chain reaction was performed on PBMC and plasma using specific primers for herpesviruses. Serology for common viruses was investigated. Results We classified the exanthems into seven morphological patterns: macular erythema (32 patients), papular erythema (eight), maculopapular erythema (42), maculopapular erythema with petechiae (seven), erythema with vesiculation (11), erythema with pustules (five) and urticaria (seven). On the basis of morphology, in concert with the associated symptoms and laboratory results, we found a causal relationship in 76 patients (68%): 25 cases due to drugs, 32 to viruses, 16 to bacteria and three to parasites. A good correspondence between morphology and aetiology was found. The erythematous-vesicular pattern was exclusive to viral infections and was often accompanied by enanthema. The erythemato-pustular and papular patterns were found only in drug-related cases and in some undiagnosed cases. In contrast, the macular and maculopapular patterns were almost evenly distributed among the various aetiologies, although their colour was duskier in the drug-related exanthems. Severe pruritus was associated with drug-related exanthems. Conclusions This is the largest series of consecutive patients with atypical exanthems reported. Their morphology and their association with pruritus or constitutional symptoms proved to be important diagnostic clues.