We present a case of a 73-year old patient with polycythaemia vera in whom cyclic thrombocytopenia was diagnosed. Strong fluctuations in platelet count, ranging from 31 to 1334 × 103/µL, were noticed after onset of hydroxyurea therapy. We did a literature search to find possible underlying causes of cyclic thrombocytopenia that could guide us towards a fast and appropriate diagnosis and an optimal treatment. In literature, provoked and unprovoked oscillations in platelet numbers have been described. Unprovoked oscillations can most likely be attributed to an unstable haematopoietic stem cell pool, as can be seen in polycythaemia vera. Provoked oscillations could be associated with myelosuppressive agents such as hydroxyurea. In both situations, a decrease in platelet count can be followed by a compensatory thrombopoietin-induced stimulation of megakaryocytes. Frequent hydroxyurea dose adjustments may be carried out in an attempt to control this cyclic pattern but, by contrast, may provoke a bouncing ball effect on platelet count. Certain patients will therefore benefit from maintaining therapy at a constant dose; while certain others require withholding or switching therapy. Cyclic thrombocytopenia is a rare finding and is frequently misdiagnosed as immune thrombocytopenia. If hydroxyurea-treated patients with a chronic myeloproliferative disorder present with thrombocytopenia, cyclic thrombocytopenia should be considered. Intensive follow-up with regular control of platelet count and personalised therapy is mandatory.

(BELG J HEMATOL 2018;9(5):188–91)