A 47-year-old man with recurrent ventricular tachycardia is on Quinidine for rhythm control. He noticed having spontaneous gum bleeds and came to the clinic for evaluation. On lab evaluation he is found to have a Hb 13.5 gm, WBC 4.8, platelet count of 15,000 per cu mm differential count is normal.
The mechanism of thrombocytopenia in this man is:
- Depression of thrombopoiesis
- Destruction of thrombocytes by the drug
- Increased activity of the spleen induced by the drug
- Destruction of the thrombocytes by antibodies stimulated by the drug.
Answer: The answer is (d). The most common mechanism of drug-induced thrombocytopenia is immune mediated. Antibodies may be directed against drugs that are covalently bound to the platelet membrane. The drugs may also stimulate the formation of antibodies that bind to platelet membrane proteins only when the drug is circulating.
There are three mechanisms by which thrombocytpenia is mediated by drugs 1. Direct toxicity (typically chemotherapy). 2. Hapten mediated 3. innocent bystander immune antibody mediated. The commonest mechanisms are the latter - i.e. 2 & 3.
The diagnosis of Quinidine induced thrombocytopenia in this case can be confirmed by:
- Remission of symptoms on discontinuation of drug
- Return of the platelet counts to normal within 1 week of discontinuing the drug
- Associated Quinidine induced QT prolongation in the ECG
- Clinical evaluation for presence of other side effects of Quinidine
Answer: The answer is (b). Drug induced thrombocytopenia is a diagnosis of exclusion.
The diagnosis is confirmed only if the platelet counts return to normal on discontinuing
the drug. Intravenous immunoglobulin administration might be necessary in severe cases.