Intravascular papillary endothelial hyperplasia involving the synovium.

To our knowledge, this is the first reported case of intravascular papillary endothelial hyperplasia occurring in the synovium. The patient, a 16-year-old boy, presented with pain and hemarthrosis of the knee in the absence of known trauma. The lesion arose within a subsynovial varix and episodically bled with the intra-articular hemorrhage and reactive synovial changes producing the patient's symptomatology. The clinical, pathologic, and ultrastructural features of the lesion are summarized from a review of the literature, and the differential diagnosis of hemarthrosis with a hemosiderotic synovitis is discussed.