An unexpected finding after a fall from a horse

A 37 year old jockey was admitted with left sided abdominal pain and fullness one week after a fall from a horse. Before this he had been fit and well. On examination, he was tender in the left upper outer quadrant of his abdomen and his blood pressure was 90/55 mm Hg. A computed tomography scan showed an area of active bleeding in the parenchyma of the spleen and a large subcapsular haematoma.

Blood tests showed a white blood cell count of 259×10⁹/L (reference range 4-11) with the following differential: blasts 3%, promyelocytes 11%, myelocytes 28%, metamyelocytes 19%, neutrophils 21%, basophils 10%, lymphocytes 4%, and eosinophils 4%. His haemoglobin was 109 g/L (130-180), mean cell volume was 80 fL (76-96), and platelets were 201×10⁹/L (150-400). Lactate dehydrogenase was 563 IU/L (70-250; 1 mmol/L=9.01 mg/dL) and uric acid was 600 µmol/L (300-470; 1 µmol/L=0.03 mg/dL).

He underwent emergency splenectomy. Macroscopically, the spleen was greatly enlarged, at 22×13×10 cm, and microscopically it showed a dense infiltrate of left shifted immature myeloid cells.