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Ann Thorac Surg 1999;67:297
© 1999 The Society of Thoracic Surgeons

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Correspondence

Defining neuropsychological deterioration after cardiac surgery

^a Department of Cardio-Thoracic Surgery, Leiden University Medical Center, P.O. Box 9600, 2300 RC Leiden, The Netherlands

To the Editor

We read with interest the recent article by Kneebone and colleagues [1] and agree with their criticism on the frequent use of the standard deviation (SD) method for defining deterioration in a patient’s neuropsychological test performance after cardiac surgical procedures. According to this method, a decline in test performance is considered significant if a patient’s preoperative to postoperative test score decrement is at least one SD, the SD being generally determined on the distribution of the preoperative scores in the patient sample. The one SD criterion can be critized because it does not address two common aspects of neuropsychological testing: measurement error (ie, imperfect reliability of test scores) and practice effects (ie, improvement of test performance at retesting because of familiarity with the test). When these two aspects are not taken into account, both overestimation and underestimation of postoperative neuropsychological deficits may occur.

To reliably define a preoperative to postoperative change in a patient’s neuropsychological test scores, Kneebone and coworkers [1] recommend applying a method that was proposed by Chelune and colleagues [2]. In this method, measurement error is dealt with by using the standard error of the difference between the preoperative and postoperative scores of the patient sample (SE_diff). Practice effects are controlled for by subtracting the mean practice effect observed in a control group from the preoperative to postoperative change score. A score change (after correction for practice) exceeding 1.64 SE_diff is considered to exceed chance fluctuations caused by test unreliability (ie, 90% confidence interval) and is therefore said to be statistically significant. This criterion seems to be a more appropriate index than the one SD criterion. However, as we have recently discussed, there are still two cautions against using it [3].

First, the method of Chelune and associates [2] does not correct for all effects of measurement error because it examines the wrong score distribution. This method focuses on the distribution of the difference between the actual preoperative and the actual postoperative score observed for the patient. Yet, to deal with unreliability of both the preoperative and the postoperative test score, a confidence interval should refer to the difference between the estimated true preoperative and the estimated true postoperative score. In fact, both the change score and its standard error should refer to estimated true change.

Second, the method of Chelune and colleagues [2] does not adequately correct for practice effects. In healthy populations, there is ample evidence of marked differences between individuals in the magnitude of test score improvement at retesting. Therefore, it is counterintuitive to correct individual change scores for practice effects by using the mean change score of a control group. Because estimated true change scores are in general found to show a strong linear relation to initial test scores, it seems more appropriate to take into account a patient’s preoperative score and to correct for practice effects by relating the patient’s change score to the change score of control subjects with a similar pretest score as the patient.

In view of the above cautions, we have presented an index that takes into account estimated true change and a patient’s preoperative score in controlling for measurement error and practice effects [3]. Comparison of the results of our index and that of Chelune and coworkers [2] teaches us that using the method of Chelune and associates, similar to the one SD method, may lead to overestimation and underestimation of neuropsychological deficits (Table 1).

1. Kneebone A.C., Andrew M.J., Baker R.A., Knight J.L. Neuropsychologic changes after coronary artery bypass grafting: use of reliable change indices. Ann Thorac Surg 1998;65:1320-1325.[Abstract/Free Full Text]
2. Chelune G.J., Naugle R.I., Lüders H., Sedlak J., Awad I.A. Individual change after epilepsy surgery: practice effects and base-rate information. Neuropsychology 1993;7:41-52.
3. Bruggemans E.F., van de Vijver F.J.R., Huysmans H.A. Assessment of cognitive deterioration in individual patients following cardiac surgery: correcting for measurement error and practice effects. J Clin Exp Neuropsychol 1997;19:543-559.[Medline]

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Hans A. Huysmans Permission Requests Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Bruggemans, E. F. Articles by Huysmans, H. A. Search for Related Content PubMed Articles by Bruggemans, E. F. Articles by Huysmans, H. A. Related Collections Related Article