Alpha-methyldopa-induced immune thrombocytopenia. Report of case

Thrombocytopenia and drugs

Thrombocytopenia is a relatively common side effect of drug therapy. The pathogenetic mechanisms of drug-induced thrombocytopenia are varied and sometimes poorly understood. Immunologic and other cytotoxic effects may result in decreased production and/or increased destruction of platelets. The pathogenesis of drug-induced thrombocytopenia is reviewed. Diagnosis and treatment are also discussed.

Drug-induced thrombocytopenia

A variety of drugs may cause thrombocytopenia. Although it occurs more often than drug-induced anemia it is less well understood because techniques for studying drug-platelet-immune interactions have been unavailable until recently. The mechanisms by which drugs cause thrombocytopenia are varied. Bone marrow suppression or increased peripheral destruction of platelets could be involved. Nonimmunologic as well as immunologic mechanisms may also occur. These different mechanisms of drug-induced thrombocytopenia are reviewed. Diagnostic methods and treatment are also summarized.

Exaggeration of defective platelet function by methyldopa in hypothyroidism

Platelet function was analyzed in two hypothyroid patients who were taking methyldopa. Results from these two patients are compared to those obtained in seven others with hypothyroidism on no medications. Bleeding times in the two drug patients were 33 and 26 minutes compared to normal values (4.0 to 8.0 minutes) in the other hypothyroid individuals. In addition, platelet aggregation responses to epinephrine, collagen, and ristocetin were abnormal in these two patients; these were normal in the others with hypothyroidism. Repeat studies were done in the first drug patient when she was euthyroid and still receiving methyldopa. Her bleeding time decreased to 11 minutes, platelet retention increased from 31% to 75%, and abnormal aggregation responses corrected to normal. Factor VIII coagulant activity, VIII antigenic activity, VIII ristocetin cofactor activity, and two-dimensional immunoelectrophoresis of VIII antigen were normal. These findings suggest that methyldopa can potentiate the mild platelet function defect seen in patients with hypothyroidism.

Factor VIII inhibitor and raised platelet IgG levels associated with methyldopa therapy

An inhibitor to the plasma coagulation factor VIII and abnormally high levels of platelet associated IgG (PAIgG) were found in a patient whilst on methyldopa. Both these parameters fell on initial withdrawal, but on rechallenge with the drug only the PAIgG rose to the high presenting levels. No inhibitory activity to factor VIIIc could be detected in a concentrated eluate prepared from the patient's platelets. These results may imply that two distinct antibodies were provoked by the administration of methyldopa, but could also be explained by the action of anti VIIIc alone on the patient's platelets.