A rare case of myelofibrosis secondary to juvenile idiopathic arthritis

An 8-year-old boy was referred with polyarthralgia and intermittent fever for 1 month. On presentation, he was pale and had hepatomegaly and swelling of the left knee. A blood count showed haemoglobin concentration 80 g/l, WBC 1 5 9 10/l and platelet count 90 9 10/l. His blood film was normal. Ultrasonography of his left knee joint showed effusion with synovial thickening. Inflammatory markers were raised. Attempted bone marrow aspiration resulted in a dry tap on two occasions. Trephine biopsy histology showed massive areas of necrosis (top left) with ghostly outlines of cells and large number of histiocytes showing phagocytosis of red cells (top right). The remaining sections showed dense reticulin deposition (bottom left) with collagenisation. Immunohistochemistry showed the histiocytes to be CD68 positive (bottom right) and CD1a and S100 negative. No JAK2 V617F or mutation of MPL or CALR was detected. The findings were indicative of secondary myelofibrosis and macrophage activation syndrome on the background of juvenile idiopathic arthritis. The patient was treated with corticosteroids, non-steroidal anti-inflammatory agents and methotrexate. Gradually, he recovered and his blood count returned to normal.