Postural responses to multidirectional stance perturbations in cerebellar ataxia Maaike Bakker a,b,c , John H.J. Allum a, ⁎ , Jasper E. Visser b , Christian Grüneberg a , Bart P. van de Warrenburg b , Berry H.P. Kremer a , Bastiaan R. Bloem b a Department of ORL, University Hospital, Basel, Switzerland b Department of Neurology, Radboud University Nijmegen Medical Centre, The Netherlands c F.C. Donders Centre for Cognitive Neuroimaging, Radboud University Nijmegen, The Netherlands Received 2 March 2006; revised 25 April 2006; accepted 6 May 2006 Available online 30 June 2006 Abstract Previous studies of patients with focal cerebellar damage underscored the importance of the cerebellum for balance control. These studies were restricted to postural control in the pitch plane, and focused mainly on leg muscle responses. Here, we examined the effect of degenerative cerebellar lesions on postural control in multiple directions, and studied how such lesions affect intersegmental coordination of the legs, trunk and arms. We formulated two main questions. (a) Do patients with cerebellar ataxia predominantly have balance problems in the sagittal or frontal planes? (b) Is instability in cerebellar ataxia associated with increased joint motion or with reduced joint motion? We selected nine patients with autosomal dominant spinocerebellar ataxia (SCA) – three with pure ataxia and six with mild extra-cerebellar features – and 12 matched controls. Upright standing subjects received support surface rotations (7.5° at 60°/s) that were randomly delivered in eight different directions of pitch or roll. We used full body kinematics to determine displacements of the center of mass (COM) and of individual body segments. We also collected surface EMG from 10 leg, trunk and arm muscles. Primary variables of interest were COM displacement and trunk control (angles and muscle responses). Secondary analyses focused on angles and muscle responses of the legs and arms. COM analysis demonstrated that SCA patients had greatest instability following backward and laterally directed perturbations. Major factors in causing this instability were, first, a marked reduction of stimulus-induced knee flexion and, second, excessive “hypermetric” motion of the pelvis (in roll) and trunk (in pitch). Muscle responses of SCA patients were characterized by increased late balance correcting activity. Responses of patients with pure ataxia were comparable to those of patients with mild extra-cerebellar features. A main underlying cause of postural instability in SCA patients appears to be “locking” of the knees, which may reflect compensation (by reducing interaction between body links) or reduced vestibulocerebellar control over leg muscles. The observed pathophysiology is very different from that seen in other patient populations. © 2006 Elsevier Inc. All rights reserved. Keywords: Cerebellum; Balance; Postural control; Ataxia; Cerebellar disorder; Posturography Introduction Re-establishing upright posture when balance is perturbed is a complex process. One approach to gain more insight into the underlying physiology of normal balance control is to examine mechanisms of instability in patients with selective lesions. This approach assumes that by examining the clinical effects of a focal lesion, one can estimate the normal function of the lesioned brain area, provided there is no compensation through other central nervous system regions. Following this approach, several studies have examined patients with ataxia caused by lesions in the cerebellum. One conclusion reached was that cerebellar damage causes increased postural sway during stance (Mauritz et al., 1979; Diener and Dichgans, 1992). A second conclu- sion was that cerebellar ataxia patients cannot scale the size of their postural responses to perturbations of stance, as they tend to over-respond (Diener et al., 1984; Horak and Diener, 1994; Timmann and Horak, 1997). Cerebellar ataxia patients had no problems with timing of their postural responses (Horak and Experimental Neurology 202 (2006) 21 – 35 www.elsevier.com/locate/yexnr ⁎ Corresponding author. University HNO-Klinik, Petersgraben 4, CH-4031 Basel, Switzerland. Fax: +41 61 265 2750. E-mail address: jallum@uhbs.ch (J.H.J. Allum). URL: http://www.unibas.ch/hno/neurooto (J.H.J. Allum). 0014-4886/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.expneurol.2006.05.008