DYNAMIC POSTUROGRAPHY IN PARKINSON’S DISEASE: DIAGNOSTIC UTILITY OF THE “FIRST TRIAL EFFECT” J. E. VISSER, a,b,c L. B. OUDE NIJHUIS, a,b L. JANSSEN, a,b C. M. BASTIAANSE, d G. F. BORM, e J. DUYSENS f,g AND B. R. BLOEM a,b * a Department of Neurology, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands b Donders Institute for Brain, Cognition and Behaviour, Radboud Uni- versity Nijmegen Medical Centre, Nijmegen, The Netherlands c Department of Neurology, Amphia Hospital, Breda, The Netherlands d Department of Rehabilitation Medicine, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands e Department of Epidemiology and Biostatistics, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands f Sint Maartenskliniek Research, Development & Education, Nijmegen, The Netherlands g Department of Biomedical Kinesiology, Katholieke Universiteit Leu- ven, Belgium Abstract—Previous dynamic posturography studies demon- strated clear abnormalities in balance responses in Parkin- son’s disease (PD) patients compared to controls at the group level, but its clinical value in the diagnostic process and fall risk estimation in individual patients leaves for im- provement. Therefore, we investigated whether a new ap- proach, focusing on the balance responses to the very first and fully unpractised trial rather than a pooled mean re- sponse to a series of balance perturbations, could further improve the diagnostic utility of dynamic posturography. Fol- lowing the first trial, subjects were exposed to repeated bal- ance perturbations, which also permitted us to investigate the training responses. Fourteen patients with PD and 18 age-matched controls were enrolled, who received a series of multidirectional postural perturbations, induced by support surface rotations. We measured trunk and upper arm kine- matics and electromyographic responses, and evaluated group differences at three levels: the postural response to the very first backward perturbation; pooled first and habit- uated postural responses; and habituation rates. Analysis of the first trial responses yielded similar results as evaluation of the mean response over trials: forward flexion of the trunk induced by backward perturbations was decreased in pa- tients, accompanied by increased muscle responses present. Moreover, trunk movement and muscle activity were equally present in both groups—suggesting a preserved training re- sponse in PD patients. Early masseter activity in both groups might be indicative of a startle-like component to the balance response. In terms of diagnostic utility, focusing on the first trial response or habituation rate is no better than analysis of pooled responses to a series of perturbations. The appar- ently preserved training response in PD patients suggests that balance reactions in PD can be improved by repeated exposure, and this may have implications for future exercise studies. Early masseter activity warrants further studies to evaluate a potential startle component in the pathophysiol- ogy of balance disorders. © 2010 Published by Elsevier Ltd on behalf of IBRO. Key words: postural control, habituation, clinical utility, mul- tidirectional dynamic posturography, electromyography, body kinematics. Balance impairment and falls are disabling features in Parkinson’s disease (PD). Clinical evaluation of postural stability, in order to identify patients at risk of falling, re- mains difficult, as history taking is unreliable (Cummings et al., 1988) and clinical balance tests are imprecise (Bloem et al., 1998a; Munhoz et al., 2004; Jacobs et al., 2006). Therefore, dynamic posturography methods—that is the evaluation of the postural responses to mechanical or sensory perturbations— have been used to assist clini- cians in the identification of balance disorders. Advantages of dynamic posturography include the standardized appli- cation of balance perturbations, objectiveness of results and the generation of multiple and concurrent quantitative outcome measures (Furman et al., 1993; Bloem et al., 2003). Previous dynamic posturography studies demon- strated clear abnormalities in balance responses in PD patients compared to controls at the group level, but its value in the diagnostic process and fall risk estimation in individual patients leaves for improvement (Di Fabio, 1996; Piirtola and Era, 2006; Visser et al., 2008c). One possible drawback of dynamic posturography is the common aver- aging of responses obtained during multiple successive trials, to overcome the variability in human postural re- sponses (Bloem et al., 2003). In addition, the first and unpractised trial is typically excluded from further analy- ses, because this response is significantly different from the reactions elicited during the following trials (Keshner et al., 1987; Hansen et al., 1988; Oude Nijhuis et al., 2009). It is unknown, however, to what extent this first trial con- tains valuable information about postural stability. Studying balance reactions recorded during the very first trial may in fact be ecologically more valid (Visser et al., 2008c), as falling usually occurs under singular and unpredictable situations. We therefore hypothesized that studying a “first trial effect”—rather than the average response of a series of sequential perturbations—might improve the diagnostic *Correspondence to: B. R. Bloem, Parkinson Centre Nijmegen (ParC), Institute of Neurology (935), Radboud University Nijmegen Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands. Tel: +00-31-24-3615202; fax: +00-31-24-3541122. E-mail address: b.bloem@neuro.umcn.nl (B. R. Bloem). Abbreviations: ABC, Activities-specific Balance Confidence; EMG, electromyography; PD, Parkinson’s disease; PIGD, postural instability and gait disability score of the UPDRS; UPDRS, Unified Parkinson’s Disease Rating Scale. Neuroscience 168 (2010) 387–394 0306-4522/10 $ - see front matter © 2010 Published by Elsevier Ltd on behalf of IBRO. doi:10.1016/j.neuroscience.2010.03.068 387