A case of hypereosinophilic syndrome with STAT5b N642H mutation.

Hypereosinophilia is defined as persistent eosinophilia (>1.5 × 10/L). Hypereosinophilic syndrome (HES) is a term used to describe a group of disorders characterized by sustained hypereosinophilia associated with end-organ damage. Based on underlying molecular mechanism of eosinophilia, there are different subtypes of HES. Diagnosis of HES subtype can be challenging, especially in the absence of overt lymphoid/myeloid neoplasms or discernable secondary causes. Long-term outpatient follow-up with periodic complete blood count and repeated bone marrow biopsy may be needed to monitor disease activity. Somatic signal transducer and activation transcription 5b (STAT5b) N642H mutation was recently found to be associated with myeloid neoplasms with eosinophilia. We report a case of HES who presented with pulmonary embolism and acute eosinophilic pneumonia, found to have recurrent STAT5b N642H mutation by next-generation sequencing, suggesting possible underlying myeloid neoplasm.