Followup Of Important Events

Analysis and evaluation of pharmacoepidemiolo-gical data should include medical assessment, both to improve the understanding of signals raised by epidemiological techniques and to raise (and evaluate) new signals or hypotheses by using medical judgement with appropriate systems for causal inference.

Medical evaluation of individual case reports and clusters of reports is an important part of PEM. Important safety signals have been generated in this way. In the PEM study of the anti-epileptic drug vigabatrin, following published case reports of visual field defects associated with the use of the product, four cases of visual field defects were identified initially in the PEM cohort. In view of the importance of the signal, 7228 patients who were reported to be taking the product by the end of the study were followed up by sending a simple questionnaire to the GP to ask whether any serious adverse events or changes in vision had been reported since the initial green form had been returned. In addition, if the patient has been seen by an ophthalmologist for visual problems, the ophthalmologist was asked to complete a questionnaire giving details of visual field testing before and during treatment with vigabatrin. The follow-up information revealed an additional 29 cases of visual field defects which were considered by the ophthalmologist to be probably or possibly related to vigabatrin, giving an incidence of risk of 7.00 per 1000 patients (Wilton et al., 1999). The follow-up exercise in the PEM study of vigabatrin contributed to the understanding of this important adverse reaction and provided a method to compute the reported rate of the adverse reaction in real clinical use which was not possible with spontaneous reporting or in clinical trials.

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