Epilepsy & Behavior 31 (2014) 219
Contents lists available at ScienceDirect
Epilepsy & Behavior journal homepage: www.elsevier.com/locate/yebeh
Editorial
When less is more. Data reduction in the prediction of postoperative outcome
In this edition of Epilepsy & Behavior, St-Laurent et al. examine the ways in which test scores from multiple neuropsychological tests can be reduced via principal component analyses into easily interpretable latent components reflecting IQ and verbal and visual memory functions. This approach has considerable intuitive appeal. While some success has previously been reported using individual scores from memory tests to predict cognitive outcomes following epilepsy surgery, potentially valuable data can be lost with the problem of multicollinearity in multiple regression analyses. Small sample sizes can also limit the number of variables that can be entered into these equations. While many experienced clinicians get a sense of visual memory deficits in people with nondominant temporal lobe foci, the definitive test, sensitive to right hippocampal dysfunction, remains elusive. Indeed, the traditional material-specific model of memory function in people with epilepsy has been challenged on a number of counts in recent years [1,2], probably rendering the quest for such a test redundant. By using principal components in their analyses, St-Laurent et al. have maximized the value of the rich database of neuropsychological scores a typical presurgical assessment generates. They demonstrate the clinical value of their statistically generated components in terms of both the presurgical lateralization of temporal lobe foci and the prediction of postoperative change. Consistent with the previous literature on the prediction of postoperative outcome, the authors demonstrate that people with good preoperative verbal memory function are at higher risk of a postoperative decline in function, while good preoperative visual function may offer some protection against verbal memory decline following a left temporal lobe resection. The majority of studies that have attempted to identify predictors of postoperative decline have used reliable change indices or standardized regression-based change scores to identify patients who have experienced a significant postoperative decline. By definition, these approaches tend to be based on pre- to postoperative change on a single test score that exceeds a statistical cutoff. The authors point out that by treating memory as a continuous variable, the sensitivity of predictive outcome models may be increased, albeit at the expense of specificity. This approach is also a pragmatic solution to the lack of test–retest data in the relevant population at the appropriate intervals needed to generate accurate reliable change indices. These are reasonable arguments.
1525-5050/$ – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.yebeh.2013.11.004
While it is important that we continue to strive to improve the accuracy of clinical predictions of postoperative cognitive outcomes in epilepsy surgery, it is imperative that we do not lose sight of the patient perspective in these endeavors. Statistical models that accurately predict neuropsychological test performance are meaningless for the patient if the tests bear little relation to their everyday experience of memory difficulties. Some patients complain bitterly of memory problems following surgery; this is not always reflected in their formal test scores [3]. Others may demonstrate clear behavioral signs of memory decline such as difficulties in returning to work, or relative reports of changed behavior, but the patient themselves may not complain. These important aspects of postoperative memory function are rarely the focus of predictive studies. It is only when we gain a better understanding of both the objective measures and subjective experience of postoperative memory change that we will be able to use predictive models to their full effect in the counseling of presurgical candidates to ensure that they are able to make a truly informed decision regarding epilepsy surgery. References [1] Saling MM. Verbal memory in mesial temporal lobe epilepsy: beyond material specificity. Brain 2009;132(Pt 3):570–82. [2] Baxendale S, Thompson P. Beyond localization: the role of traditional neuropsychological tests in an age of imaging. Epilepsia 2010;51(11):2225–30. [3] Baxendale S, Thompson P. Defining meaningful postoperative change in epilepsy surgery patients: measuring the unmeasurable? Epilepsy Behav 2005;6(2):207–11.
Sallie Baxendale NIHR University College London Hospitals Biomedical Research Centre, Department of Clinical and Experimental Epilepsy, UCL Institute of Neurology, Queen Square, London WC1N 3BG, UK Epilepsy Society, Chalfont St Peter, Bucks. SL9 0RJ, UK Department of Neuropsychology, Box 37, National Hospital for Neurology & Neurosurgery, Queen Square, London WC1N 3BG, UK. Fax: +44 207 813 2516. E-mail address: [email protected]. 4 November 2013
Contents lists available at ScienceDirect
Epilepsy & Behavior journal homepage: www.elsevier.com/locate/yebeh
Editorial
When less is more. Data reduction in the prediction of postoperative outcome
In this edition of Epilepsy & Behavior, St-Laurent et al. examine the ways in which test scores from multiple neuropsychological tests can be reduced via principal component analyses into easily interpretable latent components reflecting IQ and verbal and visual memory functions. This approach has considerable intuitive appeal. While some success has previously been reported using individual scores from memory tests to predict cognitive outcomes following epilepsy surgery, potentially valuable data can be lost with the problem of multicollinearity in multiple regression analyses. Small sample sizes can also limit the number of variables that can be entered into these equations. While many experienced clinicians get a sense of visual memory deficits in people with nondominant temporal lobe foci, the definitive test, sensitive to right hippocampal dysfunction, remains elusive. Indeed, the traditional material-specific model of memory function in people with epilepsy has been challenged on a number of counts in recent years [1,2], probably rendering the quest for such a test redundant. By using principal components in their analyses, St-Laurent et al. have maximized the value of the rich database of neuropsychological scores a typical presurgical assessment generates. They demonstrate the clinical value of their statistically generated components in terms of both the presurgical lateralization of temporal lobe foci and the prediction of postoperative change. Consistent with the previous literature on the prediction of postoperative outcome, the authors demonstrate that people with good preoperative verbal memory function are at higher risk of a postoperative decline in function, while good preoperative visual function may offer some protection against verbal memory decline following a left temporal lobe resection. The majority of studies that have attempted to identify predictors of postoperative decline have used reliable change indices or standardized regression-based change scores to identify patients who have experienced a significant postoperative decline. By definition, these approaches tend to be based on pre- to postoperative change on a single test score that exceeds a statistical cutoff. The authors point out that by treating memory as a continuous variable, the sensitivity of predictive outcome models may be increased, albeit at the expense of specificity. This approach is also a pragmatic solution to the lack of test–retest data in the relevant population at the appropriate intervals needed to generate accurate reliable change indices. These are reasonable arguments.
1525-5050/$ – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.yebeh.2013.11.004
While it is important that we continue to strive to improve the accuracy of clinical predictions of postoperative cognitive outcomes in epilepsy surgery, it is imperative that we do not lose sight of the patient perspective in these endeavors. Statistical models that accurately predict neuropsychological test performance are meaningless for the patient if the tests bear little relation to their everyday experience of memory difficulties. Some patients complain bitterly of memory problems following surgery; this is not always reflected in their formal test scores [3]. Others may demonstrate clear behavioral signs of memory decline such as difficulties in returning to work, or relative reports of changed behavior, but the patient themselves may not complain. These important aspects of postoperative memory function are rarely the focus of predictive studies. It is only when we gain a better understanding of both the objective measures and subjective experience of postoperative memory change that we will be able to use predictive models to their full effect in the counseling of presurgical candidates to ensure that they are able to make a truly informed decision regarding epilepsy surgery. References [1] Saling MM. Verbal memory in mesial temporal lobe epilepsy: beyond material specificity. Brain 2009;132(Pt 3):570–82. [2] Baxendale S, Thompson P. Beyond localization: the role of traditional neuropsychological tests in an age of imaging. Epilepsia 2010;51(11):2225–30. [3] Baxendale S, Thompson P. Defining meaningful postoperative change in epilepsy surgery patients: measuring the unmeasurable? Epilepsy Behav 2005;6(2):207–11.
Sallie Baxendale NIHR University College London Hospitals Biomedical Research Centre, Department of Clinical and Experimental Epilepsy, UCL Institute of Neurology, Queen Square, London WC1N 3BG, UK Epilepsy Society, Chalfont St Peter, Bucks. SL9 0RJ, UK Department of Neuropsychology, Box 37, National Hospital for Neurology & Neurosurgery, Queen Square, London WC1N 3BG, UK. Fax: +44 207 813 2516. E-mail address: [email protected]. 4 November 2013
