Histopathology and the Morbilliform Reaction

In a presentation by dermatologist and dermatopathologist Dr. Chris Fisher, the histopathology of morbilliform eruptions is discussed, particularly highlighting the utility of histological evaluation in these cases. Traditionally, morbilliform eruptions, which can present with varying histological patterns, are considered less informative; however, specific histological findings can aid in differentiating between viral exanthems, drug-related eruptions, and acute graft versus host disease (GVHD). Dr. Fisher describes various clinical scenarios showcasing the role of biopsies in diagnosing morbilliform eruptions, emphasizing patterns like interface dermatitis, early spongiosis, and perivascular dermatitis. Case examples include a patient on enfortumab with a drug-related eruption where biopsy revealed significant necrosis and spongiosis, aiding in diagnosis, and another case of a melanoma patient on combination therapy exhibiting pre-pustular stages of a severe skin reaction akin to acute generalized exanthematous pustulosis (AGEP). The presentation stresses that while histological overlap exists, certain findings, like excessive spongiosis and eosinophilia, can point away from GVHD and support diagnoses of high-risk drug eruptions. Finally, Dr. Fisher notes unique histological features associated with measles, clarifying that while most viral exanthems are reactions to viruses, measles infects keratinocytes and exhibits distinctive signs on biopsies. Overall, the importance of biopsies in accurately diagnosing and managing morbilliform eruptions is clearly articulated.