Ocular Tremor in Parkinson’s Disease Is Due to Head Oscillation Diego Kaski, PhD, 1 Tabish A. Saifee, MBBS, 2 David Buckwell, PhD 1 and Adolfo M. Bronstein, MD 1 * 1 Department of Neurosciences, Imperial College London, Charing Cross Hospital, London, United Kingdom 2 Sobell Department of Motor Neuroscience and Movement Disorders, UCL Institute of Neurology, 33 Queen Square, London, United Kingdom ABSTRACT Background: We investigated the origin of a recently reported ocular microtremor in patients with Parkin- son’s disease (PD). Methods: Eye movements were recorded in 2 unselected patients with PD. Two recording techniques were used to control for artifacts: infrared video-oculography and infra- red scleral reflection techniques. Head movements were also recorded with 2 different accelerometers. Results: We recorded ocular oscillations in both patients (microtremor). Ocular tremor was accompa- nied by a recordable (but clinically nonvisible) head tremor of equal fundamental frequency and high coher- ence with both the eye oscillation and a recordable limb tremor. The eye movements were in the opposite direction to the head oscillation (ie, compensatory) and were suppressed by head restraint. There was no sub- jective oscillopsia, nor ocular tremor on fundoscopy. Conclusions: The “ocular tremor” observed in patients with PD disease is a compensatory eye movement secondary to transmitted head tremor, in agreement with clinical wisdom that these patients do not report oscillopsia. V C 2013 Movement Disorder Society Key Words: ocular tremor; Parkinson’s disease; eye oscillation; vestibulo-ocular reflex; head oscillation The wide availability of modern eye-tracking devices has led to a growth of research describing clinical ocu- lomotor findings. In a recent publication using one such device, Gitchel et al 1,2 reported that all 112 patients tested with Parkinson’s disease (PD) showed oscillatory fixation instability (termed “pervasive ocu- lar tremor”), with an average fundamental frequency of 5.7 Hz and a mean horizontal amplitude of 0.27 degrees. Given that patients with idiopathic PD typi- cally show only minor oculomotor abnormalities at the bedside, 3 the presence of fixation instability may have important implications for the early diagnosis of PD, although the underlying mechanism of such ocu- lar tremor remains unknown. The most parsimonious explanation is a mechanical spread of tremor from the limbs to the head and that the ocular tremor is a com- pensatory eye movement brought about by an intact vestibulo-ocular reflex (VOR). Indeed, patients with head tremor and impaired VOR show eye oscillation on fundoscopy, termed pendular pseudonystagmus, 3 of similar amplitude to that reported by Gitchel et al, 1 but with troublesome oscillopsia. Importantly, PD patients with ocular tremor appeared not to have any tremulous head activity, 1 as measured with a position recording device, which may not have sufficiently high resolution to reveal small-amplitude movements of the head. Our aim was to investigate the origin of the recently reported ocular microtremor in patients with PD. Patients and Methods We performed simultaneous eye movement and head movement recordings in 2 consenting unselected patients with established DATscan-confirmed idio- pathic PD. The study was approved by the local ethics committee. We recorded eye and head movements using 2 separate methods, with high dynamic resolu- tion to rule out possible technical confounds. For patient 1, we used the Mobile Eyebrain Tracker (e[ye]BRAIN, Ivry-sur-Seine, France; http://www.eye- brain.com), an infrared video-oculography device equivalent to that used by Gitchel et al. 1 The Eyebrain Tracker has an accuracy of 0.5  and a calibrated range of 620  with the screen at 60 cm. The device is linear in the calibrated range. We recorded simultaneous head motion using a portable triaxial android applica- tion–based accelerometer (Android SensorManager API) secured to the back of the patient’s head to assess the contribution of head movements to any potential ocular instability. Android-based accelerometers are comparable to triaxial accelerometers. 4 Sampling rate was adjusted to 300 Hz. We performed a fast high- acceleration head movement (horizontal plane) in this ------------------------------------------------------------ Correspondence to: Professor Adolfo M. Bronstein, Department of Neurosciences, Imperial College London, Charing Cross Hospital, London W6 8RF, UK; a.bronsein@imperial.ac.uk Relevant conflicts of interest/financial disclosures: Nothing to report. Full financial disclosures and author roles may be found in the online ver- sion of this article. Received: 19 August 2012; Revised: 2 November 2012; Accepted: 3 December 2012 Published online 6 February 2013 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/mds.25342 KASKI ET AL 534 Movement Disorders, Vol. 28, No. 4, 2013